71 Cutlass vapor lock?

Kestas

Staff member
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13,863
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The Motor City
I have a 1971 Cutlass 350 (stock) that I've had for 37 years. I used to drive it regularly until the past 5 years, where I was lucky to take it out once a year. The last time I took it out, I took it for a 15 mile drive. It ran well, but decided to stall out on my driveway and I couldn't put it in the garage. I came back next weekend and the car started up fine. Yesterday I had a conversation with another classic car owner. He told me that recently gasoline has been reformulated with a lower boiling point and I probably experienced vapor lock. He said the boiling point has been lowered from 175°F to 95°F. This isn't a problem with modern cars that have fuel injection. The fuel is under pressure, which precludes boiling. Is this a thing? The pattern of what i experienced matches this scenario. This last episode doesn't give me a good feeling about driving my car. I'd feel better if I at least understand what the problem is. I'd like to give my car some more exercise.
 
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1,629
Location
New England, USA
Starting with vapor lock. Automotive gasoline in most, maybe all states has a higher vapor pressure, (to be over simplified...boils easier) in winter than summer. Summer fuel is the opposite...there are specific times when the switch is made. There is a lot of info out there. Could you have had last year's winter mix fuel in there?? Ethanol is another subject and if you are only driving the car once a year, I'd be concerned about the how good the fuel is in your tank if you are running e10. The ethanol can cause many problems that could be contributing to your problem. For your car, I would try and run pure gas but if you can only get e10 (like us), I'd use a good treatment/stabilizer. If it were my vehicle I'd use Sta Bil marine and a lead substitute. Good luck.
 
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426
Location
Daytona Beach
Not sure what military vehicles have to do with your issue, and I'm certain that ethanol has absolutely nothing to do with it, but yes, you might be experiencing vapor lock. Anytime you have winter grade, eg. High RVP gas in your tank, and the temperature gets warmer, there is the possibility that the fuel can boil in the fuel lines near a heat source, such as your engine. Ambient temps of course would have an impact on this action, as well as routing of the fuel lines near any heat source. So this is an easy fix. Go exercise your car, and when you get home fill the tank with summer grade fuel. That would be fuel with an RVP (Reid Vapor Pressure) of 9.5 psi or lower. Winter grade gas is around 15 psi.
 
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10,592
Location
Cincinnati, OH, USA
Originally Posted by Kestas
I have a 1971 Cutlass 350 (stock) that I've had for 37 years. I used to drive it regularly until the past 5 years, where I was lucky to take it out once a year. The last time I took it out, I took it for a 15 mile drive. It ran well, but decided to stall out on my driveway and I couldn't put it in the garage. I came back next weekend and the car started up fine. Yesterday I had a conversation with another classic car owner. He told me that recently gasoline has been reformulated with a lower boiling point and I probably experienced vapor lock. He said the boiling point has been lowered from 175°F to 95°F. This isn't a problem with modern cars that have fuel injection. The fuel is under pressure, which precludes boiling. Is this a thing? The pattern of what i experienced matches this scenario. This last episode doesn't give me a good feeling about driving my car. I'd feel better if I at least understand what the problem is. I'd like to give my car some more exercise.
The Bakelite carburetor spacer/isolator is a very good way to lessen this issue by keeping the carb cooler, my brother had to put one on his Jeep's Chevy 283 to keep from boiling the fuel in summer, and the OEM GM fuel filter in the carb fuel inlet likely isn't helping much either. Guessing your Rocket 350 has a Quadrajet? I would make sure your heat riser is opening hot as well (if it has one) to stop the exhaust crossover in the manifold from getting too hot(?)
 
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21,846
Location
Apple Valley, California
Along with what everyone else said. Cars back then often came with an aluminum heat shield/Carb base gasket. They get thrown away by guys that do not understand why they are there. They helped keep the Carburetor cool.You may want to block off the heat crossover in the intake as well. Does the car still have a fuel return line that returns some of the gas from the pump back to the tank? You can also try the clothes pin trick too. Does the car still have this? this is a chevy version. Olds was simmiliar. https://www.camarocentral.com/1967_1981_Quadrajet_Carburetor_Pre_Heat_Shield_p/enc-859.htm
 
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4,177
Location
Texas
As far as I know , most / all electronic fuel injected gasoline engines have the fuel pump inside the gasoline tank . So , the gasoline is under pressure all the way from the tank to the FI system . Pretty much eliminating vapor lock . Your 1971 mode 350 engine likely has a n engine driven mechanical gas pump . Which creates a pressure lower than atmospheric pressure , at the gas line , going to the gas tank . This reduced pressure can cause the gas to boil ( vapor lock ) . Excessive heat ( gas line routed too close to exhaust system ) can make this situation worse . As has been suggested , using pure gas may help . Rerouting or insulating part of the gas line may help . A solution that was sometimes used back in the day was to install a generic electric fuel pump under the vehicle , near the gas tank . There by , the gas would be under pressure , the rest of the way to the engine . Unlikely to boil / vapor lock .
 
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2,790
Location
High Tax Illinois
Like other's have said make sure you got summer gas. If close to a pure gas station, use that kind of gas for the old critters we all have. If your car is stock then I'll bet it has that HUGE factory air cleaner that traps heat like all get out. Maybe think of putting on a smaller 10" open chrome ( +12 HP : ) that won't trap the engine heat at the carb. Check under the car and make sure the gas line is not around any exhaust pipes. If it is you can wrap the exhaust pipe or make a heat shield for the gas line. Another helpful option is. Is there any drag race or circle track race tracks around you? I'm sure you could buy some fuel from there. You could mix in 5-8 gals race gas with your gas. It helps. If it stalls spray some carb cleaner into the carb, that will tell you for sure if it's vapor lock. Good Luck..................Vapor lock can a pain in the rear!!!!
 
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36,108
Location
ME
Originally Posted by Gasbuggy
can anyone confirm if the boiling point change is factual?
Not as OP describes it. It will boil at a partial vacuum at warm-ish temps and it will be worse with winter-grade RVP fuel. There's also a chance that the evap emissions are primitive or non-existent, and that the year-old gas has already evaporated off the better parts. That should make it less likely to vapor lock, but more likely to run poorly. "They" generally try to have fuel that evaporates as slowly as possible while still starting an engine to help with ground level smog emissions. There's butane (?) in winter gas that isn't allowed in summer gas for this reason, and since they have to get rid of it, summer gas costs more. This would also help avoid vapor lock.
 
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1,389
Location
Indiana
Originally Posted by JohnG
Not sure what military vehicles have to do with your issue,
The military vehicle in the link has a Chevy 235 gasoline fueled six banger, typical of the old iron affected by this stuff. He was having the problem during parades in hot weather.
 
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12,467
Location
North Carolina
Could be vapor lock , but check for spark when it won't start. Just to make sure its not the coil. An ignition coil can work cold and not when hot.
 
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10,592
Location
Cincinnati, OH, USA
Originally Posted by WyrTwister
As far as I know , most / all electronic fuel injected gasoline engines have the fuel pump inside the gasoline tank . So , the gasoline is under pressure all the way from the tank to the FI system . Pretty much eliminating vapor lock . Your 1971 mode 350 engine likely has a n engine driven mechanical gas pump . Which creates a pressure lower than atmospheric pressure , at the gas line , going to the gas tank . This reduced pressure can cause the gas to boil ( vapor lock ) . Excessive heat ( gas line routed too close to exhaust system ) can make this situation worse . As has been suggested , using pure gas may help . Rerouting or insulating part of the gas line may help . A solution that was sometimes used back in the day was to install a generic electric fuel pump under the vehicle , near the gas tank . There by , the gas would be under pressure , the rest of the way to the engine . Unlikely to boil / vapor lock .
Yes, my '76 3.8 Buick engine powered Olds Starfire had one in the tank that fed the carburetor, that would eliminate the problem as well, just might need a regulator at the carb inlet to keep the pressure low onough (usually 7-9 PSI) for the float needle valve to control it.
 
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21,846
Location
Apple Valley, California
Originally Posted by spasm3
Could be vapor lock , but check for spark when it won't start. Just to make sure its not the coil. An ignition coil can work cold and not when hot.
Yep . good point. There is a cloth covered resister wire that powers the coil. Those can fail too.
 
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1,850
Location
British Columbia, Canada
Originally Posted by spasm3
Could be vapor lock , but check for spark when it won't start. Just to make sure its not the coil. An ignition coil can work cold and not when hot.
Absolutely true. I've had it happen twice on older vehicles (three times if you count the replacement coil being bad too). In my experience the engine starts normally when cold but starts to stall (probably missing like crazy) on any sort of slope or even on the level once it's warmed up. It may even stall while running, but starts up again quite readily once it's cooled off, to repeat the same performance a few miles down the road. Very irritating and very dangerous if it happens in traffic.
 
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Messages
10,592
Location
Cincinnati, OH, USA
Originally Posted by eljefino
Originally Posted by Gasbuggy
can anyone confirm if the boiling point change is factual?
Not as OP describes it. It will boil at a partial vacuum at warm-ish temps and it will be worse with winter-grade RVP fuel. There's also a chance that the evap emissions are primitive or non-existent, and that the year-old gas has already evaporated off the better parts. That should make it less likely to vapor lock, but more likely to run poorly. "They" generally try to have fuel that evaporates as slowly as possible while still starting an engine to help with ground level smog emissions. There's butane (?) in winter gas that isn't allowed in summer gas for this reason, and since they have to get rid of it, summer gas costs more. This would also help avoid vapor lock.
There is NO evap system on the OP's Cutlass, the gas cap or tank normally has a small hole or orifice that is open to the atmosphere to allow excessive fumes out & air in during operation, which makes the fuel go bad faster than it would in a modern sealed system.
 
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1,776
Location
Kingston
Lots of good suggestions already in here, I'll add that I just recently installed a return line on my Cutlass with a 76 350 Rocket, because every carbureted vehicle I had that came with one started instantly on a hot soak, without the return line on a hot day they would crank for a few seconds before starting up and clearing out. A fuel pressure regulator can also help as the mechanical fuel pumps often run slightly too high pressure. A combination of the spacer, a regulator and having a return line could make a huge difference.
 
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