Is the fuel pump really immersed in fuel the whole time? Access to it is through a port hole in the trunk on my '02 Accord. How would it be immersed in fuel past even 1/2 tank if it's at the top of the tank?
Is running with the low fuel light constantly bad?
- Thread starter tripleM
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There are many types of pumps some sit at the bottom of the tank others sit further up and draw through the pickup on the bottom,some can even be mounted in line again drawing through the pickup in the tank.while its true many cars apparently have no ill effects from running low and running out a few times may not bother it much,prolonged running empty can possibly also damage the catalytic converter by causing misfires when almost completely empty.My old 69 Riviera hated running low on fuel and ate the in-tank pump a few times.
In the 35 years Ive been working as a mechanic its not something i want to risk with a possibly $250 without labor pump.Its cheaper to keep a few gallons of fuel in it
I've read the same thing Trav was talking about with the catalytic converter. My understanding of it is that as the fuel pump sucks tiny bits of air from an almost empty tank (like when cornering), the computer sees this as mis-fires and adjusts air/fuel ratio accordingly. These constant, small adjustments then cause damage to the cat, and possibly other components.
Personally, I like to get gas when I hit 1/4 tank at the lowest. This prevents a lot of condensation in the tank. In the winter, I like to keep it even fuller, which is also handy in emergency situations like power outages.
In the long run, it's going to cost me the same amount to drive xxx number of miles whether I let my tank get to empty or I fill up sooner. I'll trade my time in getting gas more often than inconvenience myself with running out of gas or potentially expensive repairs.
Dave
Personally, I like to get gas when I hit 1/4 tank at the lowest. This prevents a lot of condensation in the tank. In the winter, I like to keep it even fuller, which is also handy in emergency situations like power outages.
In the long run, it's going to cost me the same amount to drive xxx number of miles whether I let my tank get to empty or I fill up sooner. I'll trade my time in getting gas more often than inconvenience myself with running out of gas or potentially expensive repairs.
Dave
Below is a very long re-post of some information about fuel levels by Jim Hopkins. Mr. Hopkins is retired after a very long and distinguished career as a trainer and troubleshooter for Toyota of North America (I think this is correct - apologies to Mr. Hopkins if I omitted something.) What follows is a long read but may clear up some issues discussed in this thread. =======
Warning: The "low fuel" warning light was NOT put on your car so that you could avoid the embarrassment of having to be rescued by the AAA because your engine quit from fuel starvation (many people somehow think it was put there just for that very purpose). Avoid using it for this purpose. It was never intended to encourage driver neglect. It is not there to release drivers from their responsibilities and encourage bad driving habits. It is there because it was "mandated" by "corporate lawyers" to meet emission control regulations in the early 1980s. That's why it's there!
The law required that all car manufacturers, warranty all emission controls for 5 years to assure compliance with emission control regulations.
Now! The engineers were fully convinced that all the right components ( platinum spark plugs, EGR, fuel injection etc., etc.) would easily meet and surpass all the legal specifications, and warranty requirements for emission controls, with only "TWO" major conditions: #1 absolutely NO leaded fuel (poisons the catalytic converter), and #2 only very limited amounts of raw unburned) fuel should ever be permitted to reach the catalytic converter (overheats the converter).
The unleaded fuel problem was eventually resolved by the installation of a smaller diameter fuel filler inlet into the fuel tank filler neck, and appropriate "unleaded fuel only" customer warning stickers, were strategically placed, With the cooperation
of the fuel suppliers, providing smaller gas pump filler nozzles ONLY for unleaded fuel, consumer mistakes were hopefully minimized.
However, the raw fuel problem was much more difficult to resolve since "fuel starvation" (misfire) causes unburned fuel to enter the catalytic converter.
If there ever is "NO" fuel available at the fuel pump pickup inlet inside the fuel tank, the fuel pump simply cannot pump any fuel! That's just elementary! With a loss of fuel delivery, the fuel rail "residual" pressure is quickly depleted, especially if the engine's demands for fuel are high, This fuel pressure drop causes a drastic reduction in the amount of fuel delivered by the injectors by each fuel pulse. The fuel mixture then becomes very "lean" (the same amount of intake air, combined with less fuel). "Lean" fuel mixtures burn much "hotter" than rich fuel mixtures, and must therefore be avoided, especially at high engine loads, and high engine RPM (more power pulses per minute) to prevent overheating the combustion chamber, and burning the valves, etc. (also exhaust emissions can increase). Lean burning is the first stage of fuel starvation.
If the mixture becomes even "leaner" ignition suddenly becomes impossible, and "lean combustion" instantly becomes "lean misfire". If a fuel charge fails to ignite the unburned gasoline and air mixture then passes right through the engine combustion chamber completely unburned. This mixture rapidly cools the overheated combustion chamber, and continues right on through the exhaust manifold, and reaches the catalytic converter, where the mixture is then burned, likely causing the converter to overheat in it’s attempts to prevent tailpipe emissions.
Now! The if fuel pump constantly delivers several times as much fuel as the engine could possibly ever use to generate it's maximum rated horsepower, fuel starvation under high loads is virtually impossible. Only if someone has somehow outrageously tricked up the engine with larger fuel injectors, overridden the computers RPM limiter, force fed the engine (turbo-super charger), etc. etc. etc.
doubling the engine's rated horsepower, or restricted the fuel pressure line or fuel filter, installed an ultra strong drive line and tires with colossal tire grip, etc. etc., could they ever create an engine load that even approach the limits of the fuel pump
delivery capacity to supply. So, forget larger fuel pumps promising increased horsepower. I have seen mechanics quickly drain a fuel tank just by opening a fuel line and directing it into a storage tank , and then running the fuel pump (a neat trick). I will explain how to do this later on in this post. Only a drag racer
could ever consume fuel that rapidly. Don't buy a larger fuel pump to increase performance.
Since fuel starvation will ALWAYS occur when the fuel in the fuel tank becomes DEPLEATED. The design engineer is very limited in what he can do to prevent this "driver" initiated problem. A larger fuel tank will only delay the problem, as eventually all cars will run out of fuel (even solar powered cars, when the sun doesn't shine). This ultimately always becomes a driver education problem, and not really an engineering design problem. Driver education is however very, very difficult!
Fuel starvation occurs long, long before the fuel in the tank is ever actually exhausted. The fuel pump pick up cannot be located at the very bottom of the fuel tank, as it would then suck in water and sediment, etc. So, the fuel sloshes merrily around (in spite of baffles) up the sides of the fuel tank every time the vehicle is
affected by any inertia forces, leaving the fuel pump pickup pipe high and dry. Way before you ever actually "run out of fuel" your engine can suffer real fuel starvation, misfire. etc., and dump a lot of your costly fuel right down the exhaust manifold and into the catalytic converter to be converted into wasted heat (so that at least it won't pollute the environment).
An engine turning even 2000 RPM fires 1000 times in a minute, so the driver will probably never even notice 50 misfires (5%) per minute. But the driver looses 5% of the engine's horsepower, and wastes 5% of his fuel. Note: Low fuel levels in the fuel tank will "NOT" cause lean misfire at idle (when the car is stationary / no inertia forces), where engine misfire is usually easily detected. Also fuel starvation will not show up at emission control test stations because they test on a rolling road (no inertia forces) Always keep the TOP half of your fuel tank full!
If you don't believe me, just try holding an open cup of coffee (even a quarter full) while the driver weaves around through busy traffic, over potholes, rounds curves etc. (without spilling any coffee). Imagine what is happening inside the car's fuel tank, as the fuel pump pick up pipe gasps for fuel. Fuel also runs to the rear of the fuel tank when ascending hills, further aggravating fuel starvation. A lot of poor fuel economy is caused by fuel starvation. as well, as contributing to catalytic converter overheating. Always keep the TOP half of your fuel tank full!
Inertia forces from rough roads can be reduced by: smoothing the road surface (municipal problem), softening the suspension (an engineering compromise), reducing vehicle speed (driver education). These inertia forces are very difficult to predict. When driving on rough "back country" roads always keep the TOP half of your fuel tank full!
Braking can generate very large inertia forces, but when the throttle closes (very likely under hard braking), fuel injection is completely terminated by the engine computer until the engine RPM slows down to near idle speed. Then at idle, so little fuel is injected that any air entrained in the rapidly circulating fuel stream is quickly released back into the fuel tank with very little consequence. So braking inertia (although very high) is not really a fuel starvation factor.
Cornering forces can be very high and can easily cause fuel starvation (just ask NASCAR drivers). Acceleration forces are particularly nasty for fuel starvation. Acceleration forces are limited by both engine torque and tire adhesion (two wheels) , and are therefore much lower than braking forces (four wheels). Acceleration forces are usually of much shorter duration than cornering forces because they cannot be sustained, but since, acceleration forces correspond directly with a very high fuel demand by the engine, they are therefore very, very likely to cause fuel starvation and lean misfire. When both acceleration and cornering forces are combined on a rough road, chances of fuel starvation greatly increase. For aggressive driving, always keep the TOP half of your fuel tank full!
If you have installed stiffer shock absorbers, stickier tires, increased your engine's power, or drive more briskly than normal, you are very likely to experience even more fuel starvation and lean engine misfire from inertia displacement of fuel inside the fuel tank. "Savvy" street racers often challenge a competitor if they
spot a low fuel gauge reading in his car. With a full fuel tank, they likely have an advantage. Always keep the TOP half of your fuel tank full!
The design engineers ESTIMATE the fuel level at which "THEY" anticipate fuel starvation "MAY" become problematic and lead to catalytic converter overheating. They consider the vehicles intended use (family sedan, sports car, etc.), engine horsepower, tire adhesion , etc. Based on this (original equipment) prediction, they appropriately locate a fuel level sensor inside the fuel tank to monitor the "low fuel" warning level. This "low fuel sensor is located very near the fuel pump pickup as this is the point requiring monitoring.
A float gauge (as is the fuel gauge) would not work, since it would be all over the place as the fuel sloshed around in the tank, and would need very heavy damping (like a normal fuel level gauge), which would make it very slow to respond and too inaccurate.
Instead a tiny electrical resistor, encapsulated in a plastic cover is used as a "low fuel level" warning sensor. This resistor is continually (ignition on) fed a very small voltage, which causes the resistor to heat up, as the resistor heats, it's resistance increases, when it's resistance increases it's current flow decreases because the applied voltage is kept constant. Therefore it's current flow reflects it's temperature.
As long as this sensor remains submerged underneath the fuel inside the fuel tank, it's heat is continuously absorbed by the surrounding fuel, so it cannot heat up and it's current draw will remain relatively high.
Should the fuel level drop, exposing the sensor to the air inside the fuel tank, the resistor will soon heat up, causing it's resistance to increase, less current will now flow, and the computer will interpret the low current flow as an indication of a low fuel level in the fuel tank, and respond by turning on the "low fuel" warning light. This light will never come on at start up, even if the fuel level is low, because it requires several seconds for the sensor to warm up sufficiently to signal the computer to turn on the warning light. This process will take a little longer on a very cold morning.
While rounding a very long curve at high speed you may have noticed the light come on after a few seconds delay as the fuel is drained away from the sensor by inertia allowing it to heat up, and turn on the warning light, to warn the driver of impending fuel starvation (do you always heed this warning?). As the car then resumed a straighter, stable course the sloshing fuel returned to a more tranquil state, quenched the sensors heat and extinguished the warning light. Did you then forget about the warning? This process will occur much more rapidly if the fuel in the tank has been previously heated by engine heat absorbed from the engine fuel rail.
Most people assume that the owner's manual says "Warning! You can only drive about 50 km more before you run out of fuel" It DOESN'T!" Check your owner's manual. What is printed there???
It reads "Warning! "DO NOT DRIVE" when the "low fuel" warning light is on. Car manufacturers assume no legal liability whatever for any damage to the vehicle's engine or emission control components etc. caused by owners who do not heed this warning light. It is considered, driver neglect! Read your owner's manual "very"
carefully, and always keep the TOP half of your fuel tank full!
Catalytic converters are engine garbage disposals. Don't risk turning your costly fuel into garbage, and tossing it away into the catalytic converter for disposal! Always keep the TOP have of your fuel tank full!
I have actually fielded owner complaints that their "low fuel" warning light was incorrectly calibrated at the factory, because (they assured me) there was still lots of fuel remaining in the fuel tank after the light came on. They complained that they found the warning light very annoying, and distracting while they proceeded to consume all of the fuel remaining inside the fuel tank. Have you ever felt this way about your low fuel warning light? Better change!
Not only are you at great risk of inducing fuel starvation, but the fuel remaining inside the fuel tank is also at greater risk of being overheated and vaporizing etc. Also, there is much greater air space inside a fuel tank with a low fuel level for water vapors to condense, causing water to collect in the bottom of the tank to freeze in fuel lines, etc. Add a half cup of methyl hydrate once or twice a year, and keep the TOP half of your fuel tank full!
If you must continue to drive with the "low fuel" warning light on, drive at reduced speeds, avoid sudden maneuvers etc. and fuel up "just as soon" as you possibly can. If you are an exuberant driver fill up much, much sooner. In hot weather, on long fast drives, fill up much sooner. If you have modified your car to achieve more cornering "Gs" fill up much, much sooner. If you experience any loss of power during hard acceleration, fill up "immediately". If the "low fuel warning light ever comes on
during hard acceleration, your fuel is probably both too hot, and too low, so forget any racing till you fill up your fuel tank.
Many a Sunday drag racer has been defeated by much lesser cars because the fuel in his fuel tank simply flowed away from the fuel pump pick up under hard acceleration, or taking a fast corner. Many street racers experience fuel starvation and are then sold "humongous", expensive fuel pumps as an "upgrade". They then fill up their fuel tanks, and will swear that it was their trick fuel pump installation etc. that delivered a terrific performance improvement. That is, until the next time they race with a low fuel level.
Few racers like to admit that their tricked out car, lost out simply because they neglected to fill up the fuel tank, and they just ran out of gas under hard acceleration. A new key ring would probably have done as much for their performance as a the " humongous" fuel pump! The cheapest performance "upgrade you can ever make to your car is to keep the TOP half of your fuel tank full!
If you really must race with low fuel levels, change your fuel tank over to one engineered specifically for racing. It may not sound as sexy as many other highly advertised performance products as a racing fuel tank cannot promise more power, but it is the best cure for fuel starvation. Do not install racing fuel tanks with a much smaller fuel capacity, unless you install an appropriate fuel heat exchanger on the fuel pressure regulator return line to the fuel tank. Make sure your replacement fuel tank installation is safe and has the appropriate openings for fuel vapor collection, etc.
I am sure many people reading this are wondering why on earth, I have devoted so much time detailing what seemingly appears to them to be the obvious. However, when I am asked by people for a good tip on maintaining their cars I often reply: "Keep the TOP half of your fuel tank full!". They usually look amazed and just snicker, and reply: "Oh I know all about that, I once ran out of gas about 3 years ago, but lately I have been very lucky. A few close calls. But I haven't run right out of fuel!" for along. "Now! Tell me something exotic, and exciting, like: How to get more power and acceleration, better handling, better gas mileage, how to make it start faster in hot weather, what additives to use to clean my fuel injectors, etc., etc."
Then I tell them that I have just told them how to do this, but they just didn't understand why I said it, and obviously, a much more in depth and detailed explanation is required for them to fully appreciate the full significance of this advice. This advice is very well known, but very few people will ever give you this detailed an explanation, probably because it serves no commercial market, and may even adversely impact on their future sales. Yet, I think it is necessary information, in order for a drivers to FULLY appreciate the full consequences of their neglect. How many times do drivers breathe a sigh of relief as they reach a service station just in time, and then congratulate themselves for getting away with it. Truth is, they probably really didn't get away with it! They just think they did! Actually they probably unwittingly cheated themselves out of some "free "gasoline.
I can also, positively guarantee that if you always keep the "TOP" half of your fuel tank full, you will NEVER, ever "run out of gas". I can also guarantee that it will cost you much less money to keep the TOP half of your fuel tank full than the bottom half. You have my word on it! If you have any friends that you think would benefit from this information, please pass it along. I always hate to see people senselessly dumping their good fuel down their catalytic converters.
I’ll bet when you refueled on a hot day, the fuel cap went "whoosh" releasing the high pressure built up by the hot vaporizing fuel during your long drive, both by the sun’s heat and that carried away from the engine by the rapidly circulating fuel combined. Now, at a constant fuel temperature a slight vacuum would always be left behind as the fuel vacates the tank. That "whoosh" however, is a good sign that the fuel storage system is well sealed and that fuel vapors inside the tank, are being effectively controlled, but it is also a sign of very "HOT" fuel inside the fuel tank. Note: The evaporative emission control (EVAP) will allow quite a high , but limited amount of pressure to develop inside the fuel tank before fuel vapors are allowed to escape out into the charcoal canister, this residual fuel tank pressure reduces the formation of even more fuel vapors under hot fuel conditions.
Should a vacuum develop as the fuel cools or as fuel is used up, with no temperature increase, the "fuel tank cap" has a little valve in it to let atmospheric air into the fuel tank, so that the tank is not crushed by atmospheric pressure. This valve also requires a little vacuum to open it, so it is not unusual to sometimes find a slight vacuum inside the fuel tank, if fuel temperatures are fairly stable. This slight vacuum "hiss" is also the sign of a good fuel tank cap when refueling.
Remember to always tighten the fuel filler cap till it "clicks" to assure a positive fuel filler cap seal. If you never hear a "whoosh" or "hiss" replace the fuel filler cap, or cap gasket, immediately, because, you are wasting the fuel you have paid for and are just polluting the atmosphere with it.
Warning: The "low fuel" warning light was NOT put on your car so that you could avoid the embarrassment of having to be rescued by the AAA because your engine quit from fuel starvation (many people somehow think it was put there just for that very purpose). Avoid using it for this purpose. It was never intended to encourage driver neglect. It is not there to release drivers from their responsibilities and encourage bad driving habits. It is there because it was "mandated" by "corporate lawyers" to meet emission control regulations in the early 1980s. That's why it's there!
The law required that all car manufacturers, warranty all emission controls for 5 years to assure compliance with emission control regulations.
Now! The engineers were fully convinced that all the right components ( platinum spark plugs, EGR, fuel injection etc., etc.) would easily meet and surpass all the legal specifications, and warranty requirements for emission controls, with only "TWO" major conditions: #1 absolutely NO leaded fuel (poisons the catalytic converter), and #2 only very limited amounts of raw unburned) fuel should ever be permitted to reach the catalytic converter (overheats the converter).
The unleaded fuel problem was eventually resolved by the installation of a smaller diameter fuel filler inlet into the fuel tank filler neck, and appropriate "unleaded fuel only" customer warning stickers, were strategically placed, With the cooperation
of the fuel suppliers, providing smaller gas pump filler nozzles ONLY for unleaded fuel, consumer mistakes were hopefully minimized.
However, the raw fuel problem was much more difficult to resolve since "fuel starvation" (misfire) causes unburned fuel to enter the catalytic converter.
If there ever is "NO" fuel available at the fuel pump pickup inlet inside the fuel tank, the fuel pump simply cannot pump any fuel! That's just elementary! With a loss of fuel delivery, the fuel rail "residual" pressure is quickly depleted, especially if the engine's demands for fuel are high, This fuel pressure drop causes a drastic reduction in the amount of fuel delivered by the injectors by each fuel pulse. The fuel mixture then becomes very "lean" (the same amount of intake air, combined with less fuel). "Lean" fuel mixtures burn much "hotter" than rich fuel mixtures, and must therefore be avoided, especially at high engine loads, and high engine RPM (more power pulses per minute) to prevent overheating the combustion chamber, and burning the valves, etc. (also exhaust emissions can increase). Lean burning is the first stage of fuel starvation.
If the mixture becomes even "leaner" ignition suddenly becomes impossible, and "lean combustion" instantly becomes "lean misfire". If a fuel charge fails to ignite the unburned gasoline and air mixture then passes right through the engine combustion chamber completely unburned. This mixture rapidly cools the overheated combustion chamber, and continues right on through the exhaust manifold, and reaches the catalytic converter, where the mixture is then burned, likely causing the converter to overheat in it’s attempts to prevent tailpipe emissions.
Now! The if fuel pump constantly delivers several times as much fuel as the engine could possibly ever use to generate it's maximum rated horsepower, fuel starvation under high loads is virtually impossible. Only if someone has somehow outrageously tricked up the engine with larger fuel injectors, overridden the computers RPM limiter, force fed the engine (turbo-super charger), etc. etc. etc.
doubling the engine's rated horsepower, or restricted the fuel pressure line or fuel filter, installed an ultra strong drive line and tires with colossal tire grip, etc. etc., could they ever create an engine load that even approach the limits of the fuel pump
delivery capacity to supply. So, forget larger fuel pumps promising increased horsepower. I have seen mechanics quickly drain a fuel tank just by opening a fuel line and directing it into a storage tank , and then running the fuel pump (a neat trick). I will explain how to do this later on in this post. Only a drag racer
could ever consume fuel that rapidly. Don't buy a larger fuel pump to increase performance.
Since fuel starvation will ALWAYS occur when the fuel in the fuel tank becomes DEPLEATED. The design engineer is very limited in what he can do to prevent this "driver" initiated problem. A larger fuel tank will only delay the problem, as eventually all cars will run out of fuel (even solar powered cars, when the sun doesn't shine). This ultimately always becomes a driver education problem, and not really an engineering design problem. Driver education is however very, very difficult!
Fuel starvation occurs long, long before the fuel in the tank is ever actually exhausted. The fuel pump pick up cannot be located at the very bottom of the fuel tank, as it would then suck in water and sediment, etc. So, the fuel sloshes merrily around (in spite of baffles) up the sides of the fuel tank every time the vehicle is
affected by any inertia forces, leaving the fuel pump pickup pipe high and dry. Way before you ever actually "run out of fuel" your engine can suffer real fuel starvation, misfire. etc., and dump a lot of your costly fuel right down the exhaust manifold and into the catalytic converter to be converted into wasted heat (so that at least it won't pollute the environment).
An engine turning even 2000 RPM fires 1000 times in a minute, so the driver will probably never even notice 50 misfires (5%) per minute. But the driver looses 5% of the engine's horsepower, and wastes 5% of his fuel. Note: Low fuel levels in the fuel tank will "NOT" cause lean misfire at idle (when the car is stationary / no inertia forces), where engine misfire is usually easily detected. Also fuel starvation will not show up at emission control test stations because they test on a rolling road (no inertia forces) Always keep the TOP half of your fuel tank full!
If you don't believe me, just try holding an open cup of coffee (even a quarter full) while the driver weaves around through busy traffic, over potholes, rounds curves etc. (without spilling any coffee). Imagine what is happening inside the car's fuel tank, as the fuel pump pick up pipe gasps for fuel. Fuel also runs to the rear of the fuel tank when ascending hills, further aggravating fuel starvation. A lot of poor fuel economy is caused by fuel starvation. as well, as contributing to catalytic converter overheating. Always keep the TOP half of your fuel tank full!
Inertia forces from rough roads can be reduced by: smoothing the road surface (municipal problem), softening the suspension (an engineering compromise), reducing vehicle speed (driver education). These inertia forces are very difficult to predict. When driving on rough "back country" roads always keep the TOP half of your fuel tank full!
Braking can generate very large inertia forces, but when the throttle closes (very likely under hard braking), fuel injection is completely terminated by the engine computer until the engine RPM slows down to near idle speed. Then at idle, so little fuel is injected that any air entrained in the rapidly circulating fuel stream is quickly released back into the fuel tank with very little consequence. So braking inertia (although very high) is not really a fuel starvation factor.
Cornering forces can be very high and can easily cause fuel starvation (just ask NASCAR drivers). Acceleration forces are particularly nasty for fuel starvation. Acceleration forces are limited by both engine torque and tire adhesion (two wheels) , and are therefore much lower than braking forces (four wheels). Acceleration forces are usually of much shorter duration than cornering forces because they cannot be sustained, but since, acceleration forces correspond directly with a very high fuel demand by the engine, they are therefore very, very likely to cause fuel starvation and lean misfire. When both acceleration and cornering forces are combined on a rough road, chances of fuel starvation greatly increase. For aggressive driving, always keep the TOP half of your fuel tank full!
If you have installed stiffer shock absorbers, stickier tires, increased your engine's power, or drive more briskly than normal, you are very likely to experience even more fuel starvation and lean engine misfire from inertia displacement of fuel inside the fuel tank. "Savvy" street racers often challenge a competitor if they
spot a low fuel gauge reading in his car. With a full fuel tank, they likely have an advantage. Always keep the TOP half of your fuel tank full!
The design engineers ESTIMATE the fuel level at which "THEY" anticipate fuel starvation "MAY" become problematic and lead to catalytic converter overheating. They consider the vehicles intended use (family sedan, sports car, etc.), engine horsepower, tire adhesion , etc. Based on this (original equipment) prediction, they appropriately locate a fuel level sensor inside the fuel tank to monitor the "low fuel" warning level. This "low fuel sensor is located very near the fuel pump pickup as this is the point requiring monitoring.
A float gauge (as is the fuel gauge) would not work, since it would be all over the place as the fuel sloshed around in the tank, and would need very heavy damping (like a normal fuel level gauge), which would make it very slow to respond and too inaccurate.
Instead a tiny electrical resistor, encapsulated in a plastic cover is used as a "low fuel level" warning sensor. This resistor is continually (ignition on) fed a very small voltage, which causes the resistor to heat up, as the resistor heats, it's resistance increases, when it's resistance increases it's current flow decreases because the applied voltage is kept constant. Therefore it's current flow reflects it's temperature.
As long as this sensor remains submerged underneath the fuel inside the fuel tank, it's heat is continuously absorbed by the surrounding fuel, so it cannot heat up and it's current draw will remain relatively high.
Should the fuel level drop, exposing the sensor to the air inside the fuel tank, the resistor will soon heat up, causing it's resistance to increase, less current will now flow, and the computer will interpret the low current flow as an indication of a low fuel level in the fuel tank, and respond by turning on the "low fuel" warning light. This light will never come on at start up, even if the fuel level is low, because it requires several seconds for the sensor to warm up sufficiently to signal the computer to turn on the warning light. This process will take a little longer on a very cold morning.
While rounding a very long curve at high speed you may have noticed the light come on after a few seconds delay as the fuel is drained away from the sensor by inertia allowing it to heat up, and turn on the warning light, to warn the driver of impending fuel starvation (do you always heed this warning?). As the car then resumed a straighter, stable course the sloshing fuel returned to a more tranquil state, quenched the sensors heat and extinguished the warning light. Did you then forget about the warning? This process will occur much more rapidly if the fuel in the tank has been previously heated by engine heat absorbed from the engine fuel rail.
Most people assume that the owner's manual says "Warning! You can only drive about 50 km more before you run out of fuel" It DOESN'T!" Check your owner's manual. What is printed there???
It reads "Warning! "DO NOT DRIVE" when the "low fuel" warning light is on. Car manufacturers assume no legal liability whatever for any damage to the vehicle's engine or emission control components etc. caused by owners who do not heed this warning light. It is considered, driver neglect! Read your owner's manual "very"
carefully, and always keep the TOP half of your fuel tank full!
Catalytic converters are engine garbage disposals. Don't risk turning your costly fuel into garbage, and tossing it away into the catalytic converter for disposal! Always keep the TOP have of your fuel tank full!
I have actually fielded owner complaints that their "low fuel" warning light was incorrectly calibrated at the factory, because (they assured me) there was still lots of fuel remaining in the fuel tank after the light came on. They complained that they found the warning light very annoying, and distracting while they proceeded to consume all of the fuel remaining inside the fuel tank. Have you ever felt this way about your low fuel warning light? Better change!
Not only are you at great risk of inducing fuel starvation, but the fuel remaining inside the fuel tank is also at greater risk of being overheated and vaporizing etc. Also, there is much greater air space inside a fuel tank with a low fuel level for water vapors to condense, causing water to collect in the bottom of the tank to freeze in fuel lines, etc. Add a half cup of methyl hydrate once or twice a year, and keep the TOP half of your fuel tank full!
If you must continue to drive with the "low fuel" warning light on, drive at reduced speeds, avoid sudden maneuvers etc. and fuel up "just as soon" as you possibly can. If you are an exuberant driver fill up much, much sooner. In hot weather, on long fast drives, fill up much sooner. If you have modified your car to achieve more cornering "Gs" fill up much, much sooner. If you experience any loss of power during hard acceleration, fill up "immediately". If the "low fuel warning light ever comes on
during hard acceleration, your fuel is probably both too hot, and too low, so forget any racing till you fill up your fuel tank.
Many a Sunday drag racer has been defeated by much lesser cars because the fuel in his fuel tank simply flowed away from the fuel pump pick up under hard acceleration, or taking a fast corner. Many street racers experience fuel starvation and are then sold "humongous", expensive fuel pumps as an "upgrade". They then fill up their fuel tanks, and will swear that it was their trick fuel pump installation etc. that delivered a terrific performance improvement. That is, until the next time they race with a low fuel level.
Few racers like to admit that their tricked out car, lost out simply because they neglected to fill up the fuel tank, and they just ran out of gas under hard acceleration. A new key ring would probably have done as much for their performance as a the " humongous" fuel pump! The cheapest performance "upgrade you can ever make to your car is to keep the TOP half of your fuel tank full!
If you really must race with low fuel levels, change your fuel tank over to one engineered specifically for racing. It may not sound as sexy as many other highly advertised performance products as a racing fuel tank cannot promise more power, but it is the best cure for fuel starvation. Do not install racing fuel tanks with a much smaller fuel capacity, unless you install an appropriate fuel heat exchanger on the fuel pressure regulator return line to the fuel tank. Make sure your replacement fuel tank installation is safe and has the appropriate openings for fuel vapor collection, etc.
I am sure many people reading this are wondering why on earth, I have devoted so much time detailing what seemingly appears to them to be the obvious. However, when I am asked by people for a good tip on maintaining their cars I often reply: "Keep the TOP half of your fuel tank full!". They usually look amazed and just snicker, and reply: "Oh I know all about that, I once ran out of gas about 3 years ago, but lately I have been very lucky. A few close calls. But I haven't run right out of fuel!" for along. "Now! Tell me something exotic, and exciting, like: How to get more power and acceleration, better handling, better gas mileage, how to make it start faster in hot weather, what additives to use to clean my fuel injectors, etc., etc."
Then I tell them that I have just told them how to do this, but they just didn't understand why I said it, and obviously, a much more in depth and detailed explanation is required for them to fully appreciate the full significance of this advice. This advice is very well known, but very few people will ever give you this detailed an explanation, probably because it serves no commercial market, and may even adversely impact on their future sales. Yet, I think it is necessary information, in order for a drivers to FULLY appreciate the full consequences of their neglect. How many times do drivers breathe a sigh of relief as they reach a service station just in time, and then congratulate themselves for getting away with it. Truth is, they probably really didn't get away with it! They just think they did! Actually they probably unwittingly cheated themselves out of some "free "gasoline.
I can also, positively guarantee that if you always keep the "TOP" half of your fuel tank full, you will NEVER, ever "run out of gas". I can also guarantee that it will cost you much less money to keep the TOP half of your fuel tank full than the bottom half. You have my word on it! If you have any friends that you think would benefit from this information, please pass it along. I always hate to see people senselessly dumping their good fuel down their catalytic converters.
I’ll bet when you refueled on a hot day, the fuel cap went "whoosh" releasing the high pressure built up by the hot vaporizing fuel during your long drive, both by the sun’s heat and that carried away from the engine by the rapidly circulating fuel combined. Now, at a constant fuel temperature a slight vacuum would always be left behind as the fuel vacates the tank. That "whoosh" however, is a good sign that the fuel storage system is well sealed and that fuel vapors inside the tank, are being effectively controlled, but it is also a sign of very "HOT" fuel inside the fuel tank. Note: The evaporative emission control (EVAP) will allow quite a high , but limited amount of pressure to develop inside the fuel tank before fuel vapors are allowed to escape out into the charcoal canister, this residual fuel tank pressure reduces the formation of even more fuel vapors under hot fuel conditions.
Should a vacuum develop as the fuel cools or as fuel is used up, with no temperature increase, the "fuel tank cap" has a little valve in it to let atmospheric air into the fuel tank, so that the tank is not crushed by atmospheric pressure. This valve also requires a little vacuum to open it, so it is not unusual to sometimes find a slight vacuum inside the fuel tank, if fuel temperatures are fairly stable. This slight vacuum "hiss" is also the sign of a good fuel tank cap when refueling.
Remember to always tighten the fuel filler cap till it "clicks" to assure a positive fuel filler cap seal. If you never hear a "whoosh" or "hiss" replace the fuel filler cap, or cap gasket, immediately, because, you are wasting the fuel you have paid for and are just polluting the atmosphere with it.
^That's a thread killer.
I run every tank to low or light on.
205,800 miles currently. Never had a problem with this car or anyone else in my family or friends that do the same thing.
I run every tank to low or light on.
205,800 miles currently. Never had a problem with this car or anyone else in my family or friends that do the same thing.
I respect the light (even the 1/4 tank level), but out on the road, interstate speeds, straight line, I'm simply not getting off the road every 200 miles to keep the car at 1/2+. Hyundai states I have two and a half gallons left at first low fuel indicator on. Well, in this car, that's 70 or eighty miles of range. Fifty miles after low-fuel I get gas. Again, that's my practice on the interstate, long trip. On home roads, it stays a 1/4 plus. Results? 288K on my last one, never a fuel pump or cat problem. I've had a lot of cars, ran em down to the light and way beyond, and have never replaced a fuel pump..
I used to run my old 3.1 sunbird on empty all the time. At 200K I replaced the gas tank but the pump was still good!
Ugly... Thanks for posting Mr. Hopkins detailed and informative explanation.
Unfortunately, his good (but somewhat extreme) advice is repeatedly marred by the logic error (always keep the "TOP" half of the fuel take full). Of course, he means "always keep the bottom half of your fuel tank full", or more simply "keep your tank half full". Luckily, everyone knew what he meant; hopefully, it didn't detract from the good advice.
Unfortunately, his good (but somewhat extreme) advice is repeatedly marred by the logic error (always keep the "TOP" half of the fuel take full). Of course, he means "always keep the bottom half of your fuel tank full", or more simply "keep your tank half full". Luckily, everyone knew what he meant; hopefully, it didn't detract from the good advice.
I suppose when our gravity-phase-modified gas tanks with fuel pumps on the top are starved of fuel problems may occur...
The figure of keeping the gas filled beyond a certain level is going to be far less than 1/4 of a tank, even 1/8 of a tank. But of course many tanks usually have their gauges set with the Empty mark at a level that still contains a few gallons in the tank and with my 13.2 gallon tank, I have about 2.5 gallons left when the needle sweeps by the E. An 1/8th of a tank would be 1.65 gallons in the tank for me which comes after the needle sweeps past the E. I usually keep a one gallon tank of gas in my trunk that I keep fresh and I make sure it is in the trunk in case it is needed. I have never needed it even running for many miles beyond the E. I've taken a look and it is obvious that this was planned for through tank design. The tank is designed to be resistant to turning and braking as it has a 'deep trough' shaped thing on the bottom of the tank so there will be gas down there until it is very empty, less than 1/2 gallon it seems. The service manual even states that this was part of its design to prevent fuel slosh towards the bottom of the tank to prevent things from running dry. My pump is top mounted, accessible by removing the back seat and lifting the pump out. Looks to be an easy job if it needed to be changed out but I really doubt it will happen.
I have never found that 'top half' guy to be credible, he should have said a more reasonable figure like 1/4 of a tank. I generally keep my tank 1/4 or more filled in the winter due to the possibility of condensation yet I usually toss in a can of sea foam or another fuel drier product once every two months in the winter anyway even though I haven't had a single condensation related problem without adding anything to the tank or keeping it more than at the empty mark before filling in the past.
Do what suits you, I just don't agree with the 'top half of your tank' person and I don't like it how people can just do a multiple page write up that seems to make sense and it gets everywhere. It's obvious to not let your pump draw air, but I don't think that it needs multiple pages of wordy text to say that. This person compares the movement of ten gallons or more of fluid to a coffee cup. I can drive a car that is half full of gas for a mile down the street and open my gas cap and it will have the same 'whoosh' effect that he says if you don't "Keep the top half full" so it is nothing to do with heat. If someone takes a portable gas tank and shakes it and opens the cap it has pressure, it doesn't mean really mean much... I just don't like it when people use weasel words to try to hold an unreasonable implications such as 'getting better fuel economy with 1/2 tank or more versus 1/4 tank.'
Make your own decision but I have serious issue with people who try to act educated by adding excess words to a simple topic. He should have just said something like 'keep your tank full enough so that your vehicle won't run its fuel pump dry and cause misfires' as that seems to be his moral summarized in a sentence.
I digress.
The figure of keeping the gas filled beyond a certain level is going to be far less than 1/4 of a tank, even 1/8 of a tank. But of course many tanks usually have their gauges set with the Empty mark at a level that still contains a few gallons in the tank and with my 13.2 gallon tank, I have about 2.5 gallons left when the needle sweeps by the E. An 1/8th of a tank would be 1.65 gallons in the tank for me which comes after the needle sweeps past the E. I usually keep a one gallon tank of gas in my trunk that I keep fresh and I make sure it is in the trunk in case it is needed. I have never needed it even running for many miles beyond the E. I've taken a look and it is obvious that this was planned for through tank design. The tank is designed to be resistant to turning and braking as it has a 'deep trough' shaped thing on the bottom of the tank so there will be gas down there until it is very empty, less than 1/2 gallon it seems. The service manual even states that this was part of its design to prevent fuel slosh towards the bottom of the tank to prevent things from running dry. My pump is top mounted, accessible by removing the back seat and lifting the pump out. Looks to be an easy job if it needed to be changed out but I really doubt it will happen.
I have never found that 'top half' guy to be credible, he should have said a more reasonable figure like 1/4 of a tank. I generally keep my tank 1/4 or more filled in the winter due to the possibility of condensation yet I usually toss in a can of sea foam or another fuel drier product once every two months in the winter anyway even though I haven't had a single condensation related problem without adding anything to the tank or keeping it more than at the empty mark before filling in the past.
Do what suits you, I just don't agree with the 'top half of your tank' person and I don't like it how people can just do a multiple page write up that seems to make sense and it gets everywhere. It's obvious to not let your pump draw air, but I don't think that it needs multiple pages of wordy text to say that. This person compares the movement of ten gallons or more of fluid to a coffee cup. I can drive a car that is half full of gas for a mile down the street and open my gas cap and it will have the same 'whoosh' effect that he says if you don't "Keep the top half full" so it is nothing to do with heat. If someone takes a portable gas tank and shakes it and opens the cap it has pressure, it doesn't mean really mean much... I just don't like it when people use weasel words to try to hold an unreasonable implications such as 'getting better fuel economy with 1/2 tank or more versus 1/4 tank.'
Make your own decision but I have serious issue with people who try to act educated by adding excess words to a simple topic. He should have just said something like 'keep your tank full enough so that your vehicle won't run its fuel pump dry and cause misfires' as that seems to be his moral summarized in a sentence.
I digress.
I think that auto manufactures have to plan for drivers that are not too much into cars and may run the car low on fuel because they are forgetful or poor (college student) or careless.
So I drive quite often with less than 1/4 tank on a Camry and a Suburban. Camry seems fine, but I have put two fuel pumps in the Suburban (one at 150K and one at 151K). Replacing the fuel pump in the Suburban is a PIA job as you need to drop the 46 gallon tank, taking apart things above the tank you cannot see well as you cannot drop the tank more than a few inches with all hoses and wires connected.
So I drive quite often with less than 1/4 tank on a Camry and a Suburban. Camry seems fine, but I have put two fuel pumps in the Suburban (one at 150K and one at 151K). Replacing the fuel pump in the Suburban is a PIA job as you need to drop the 46 gallon tank, taking apart things above the tank you cannot see well as you cannot drop the tank more than a few inches with all hoses and wires connected.
The fuel pump in my Suburban was immersed in fuel and was at the very bottom of the tank.
The time it broken this past summer was when I tried to start the car when it was over 95 out one summer day. That pump was less than 12 months old and less than 1000 miles.
The time it broken this past summer was when I tried to start the car when it was over 95 out one summer day. That pump was less than 12 months old and less than 1000 miles.
From DTC table of my car's FSM:
-------
P0300
Random misfire detected
Misfire of such level as to cause damage to three way catalyst.
*1 driving cycle
P0301 - P0304
Cylinder 1-4 misfire detected
Misfire of such level as to deteriorate emission but not to cause damage to three way catalyst.
2 driving cycles
------
So my question to Mr. Hopkins and everyone that agrees with him, how come the ECU does not store the code and the CEL does not come up? With 50 misfires/min we're facing serious cat damage...
I'm sure nothing horrible happens when the pump gulps air because the fuel sloshes in the tank. The pump has so much more capacity then injectors that the fuel pressure does not drop one bit. The air bubble (if any) goes right back into the tank with the fuel excess...
Anyway, Mr. Hopkins’s theory sounds plausible, although there's no evidence to support it. I'm filling up after my fuel light comes up, or in 100-150mi, and I've never had any above-mentioned codes stored. It is easy to check with ScanGauge, and I check for codes regularly.
-------
P0300
Random misfire detected
Misfire of such level as to cause damage to three way catalyst.
*1 driving cycle
P0301 - P0304
Cylinder 1-4 misfire detected
Misfire of such level as to deteriorate emission but not to cause damage to three way catalyst.
2 driving cycles
------
So my question to Mr. Hopkins and everyone that agrees with him, how come the ECU does not store the code and the CEL does not come up? With 50 misfires/min we're facing serious cat damage...
I'm sure nothing horrible happens when the pump gulps air because the fuel sloshes in the tank. The pump has so much more capacity then injectors that the fuel pressure does not drop one bit. The air bubble (if any) goes right back into the tank with the fuel excess...
Anyway, Mr. Hopkins’s theory sounds plausible, although there's no evidence to support it. I'm filling up after my fuel light comes up, or in 100-150mi, and I've never had any above-mentioned codes stored. It is easy to check with ScanGauge, and I check for codes regularly.
The sloshing of gas in a tank and its effect on the fuel pump picking up air is somewhat minimized by the filter attached to it. The filter is made of a combo of paper and cloth (so it looks) and shaped like a babies sock (flattened). I would guess that fuel will not run out of this filter very fast if one sloshed the fuel around in a tank.
I run my '91 D150 with its 30gal tank low all the time. $20 only brings it to ~1/4 tank lol. I normaly can push it to a needle-with below the last "E" tick. It runs out of gas at 2 needles worth..almost touching the "E"...ive done that a few times. No light. 150K on the original pump.
My conclusion is that I definetly hate changing the fuel pump in my Suburban and there is circumstancial evidence that running it low (near empty) especially in the heat of the summer can cause a problem. Not sure if it applies to all cars. So given that it does not save one any money by running low on fuel, its probably best to run it with at least 1/4 tank.
Late model C4 Corvettes (mid 1990s) were well known for toasting their fuel pumps if you ran low on gas too often. I don't think it was just group hysteria.
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