Truck died and won't restart - the diagnosis begin

[The_Eric]

If it comes down to it being the fuel pump, make sure that it is getting an adequate ground. I believe these trucks had a braided ground wire that goes from the back of the cylinder head to the frame and up to the cab under the heater box. These wires like to rot away and can kill a fuel pump.

We had one customer that had replaced 4 or 5 pumps on his 98 Chevy before bringing it to us. After some checking, I found high resistance in the ground circuit (from the frame to the battery). A quick search found that his ground wire, while there was toast. As soon as I touched it, it disintegrated

 

[rationull]

That is a very good piece of advice that I would not have thought of. Thanks. This truck spent the first 9 or so years of its life (previous owner) near the coast, and so this would be a very good thing to check. You're literally just checking continuity between parts that should all be grounded?

 

[Trav]

The OEM injectors are a very poor design and fail often enough but usually at higher RPM or they stick open or closed.

If you need injectors PM me i have brand new custom Bosch units built for the 454 engine specifically.
These are a plug and play unit requiring no mods or reprogramming and are superior to the Multecs in every way.

 

[The_Eric]

Just check resistance from the fuel pump plug to the battery. This will check the pump's grounding point at the frame and the wires I mentioned. I think that there are two black wires in that harness, you'll want to check the larger gauge one.

Ideally you would see .5 ohms or less. On the truck I was talking about, I saw something around 7 ohms!

 

[rationull]

Originally Posted By: The_Eric
Just check resistance from the fuel pump plug to the battery. This will check the pump's grounding point at the frame and the wires I mentioned. I think that there are two black wires in that harness, you'll want to check the larger gauge one.

Ideally you would see .5 ohms or less. On the truck I was talking about, I saw something around 7 ohms!


Maybe I'm being dense but is there a better way to do that than to get a really long piece of wire to use as an "extension" for the multimeter probe?

 

[another Todd]

45 psi should be enough to start it. Pulse the fuel pump to obtain pressure then pull the fuel pump fuse out and crank it, if pressure drops the injectors are working. If the coil tested bad, I would replace it first, it may not be bad enough to cause this problem but it soon might so replace it now to eliminate it as a suspect in this problem and prevent a future problem. Next I'd check the cap and rotor. Those are a bear to get to behind the intake manifold and therefore tend to be neglected. I recently diagnosed a no start on a friends chevy truck, it was a bad (very high resistance) cap. After replacing just the cap and rotor, he mentioned the truck hadn't run that good in a long time. Like others have mentioned, check all the fuses and grounds too.

 

[rationull]

I guess the cap and rotor are just as likely to be causing a "weak spark" as the coil. I wish I had a good way to specifically diagnose "weak spark" considering I did actually see a spark at the plug. I think to do the cap and rotor the upper intake will have to come off again which I guess isn't too bad. I probably won't be able to get to that kind of thing for a few weeks though.

 

[mechanicx]

How has your diagnostics gone so far? Did you try starting with unplugging the MAF? Or tried starting fluid? It sounds like you are getting enough spark for the engine to at least attempted to start.

It seems your fuel pressure is low but it's hard to say if it's too low to start engine. Maybe pinch off the rubber section of the return line to eliminate the fpr. Also a noid light plugged into one or two accessible injectors to see if they're being piulsed.

 

[Doog]

replaced it a few years ago. The truck doesn't get a lot of miles but does a lot of sitting. I would do this just to rule it out but it seems like a waste since any problem the filter caused would show up as bad pressure at the rail, wouldn't it?


I had water in gas do a similar thing. Cut open the fuel filter and it was full of shaving cream like goo and completely clogged. It ran fine until it completely plugged.

 

[Doog]

Originally Posted By: rationull
I guess the cap and rotor are just as likely to be causing a "weak spark" as the coil. I wish I had a good way to specifically diagnose "weak spark" considering I did actually see a spark at the plug. I think to do the cap and rotor the upper intake will have to come off again which I guess isn't too bad. I probably won't be able to get to that kind of thing for a few weeks though.


The 7.4 cap/rotors are notorious for small cracks that cause arcing and ignition issues. what about the coil?

 

[rationull]

Originally Posted By: mechanicx
How has your diagnostics gone so far? Did you try starting with unplugging the MAF? Or tried starting fluid? It sounds like you are getting enough spark for the engine to at least attempted to start.

It seems your fuel pressure is low but it's hard to say if it's too low to start engine. Maybe pinch off the rubber section of the return line to eliminate the fpr. Also a noid light plugged into one or two accessible injectors to see if they're being piulsed.


I haven't had a chance to look at it the last few days (not a daily driver so it's just not as pressing as some other stuff). I'm hoping to be able to check it out this weekend or at least start one morning this week but I'm not sure how things are going to work out.

My current plan:

The coil is off right now and is kind of hard to access so I'm debating putting it back on vs buying a new one to put back on. However I should probably just hook it up again and do the quick MAF check.

Next step is to borrow a fuel pressure gauge again, hook it up to the rail and key-on a few times to see how high the pressure will go. Then, crank it and watch whether the pressure fluctuates (or remove the fuel pump fuse/relay and crank as has been suggested) to see if the injectors are actually firing. Depending on if/how that fails I will check/replace the fuel filter and check grounds to try to pinpoint the fuel problem.

If the above passes (50-60 PSI at the fuel rail and injectors firing), I will conclude that it could be the ignition hardware screwing up spark timing and will take off the upper intake and replace the cap, rotor, and coil.

 

[rationull]

Tried to start it tonight with the MAF unhooked, and then the MAP. No dice.

I also snapped a photo of the cable next to the belt tensioner that's worn down:

There is exposed wire inside (where the arrow is pointing, you can see the plastic "conduit" broken). This looks like it's been in this condition a while, is a single thick cable going from the positive battery terminal to the alternator, and is not close to any conductive material as far as I can tell (the tensioner pulley is plastic) so I don't see how this could be related.



Planning to pick up a fuel pressure gauge in the morning and probably try that line of diagnosis later in the week.

 

[another Todd]

Originally Posted By: rationull
I guess the cap and rotor are just as likely to be causing a "weak spark" as the coil. I wish I had a good way to specifically diagnose "weak spark" considering I did actually see a spark at the plug. I think to do the cap and rotor the upper intake will have to come off again which I guess isn't too bad. I probably won't be able to get to that kind of thing for a few weeks though.


You can use a spark gap tester to determine the strength of the spark, they are available at most auto parts stores and are innexpensive. If someone can tell me how to post a picture I can post one of it.

 

[rationull]

Haven't seen those before. Here's one: http://www.amazon.com/Thexton-404-Adjustable-Ignition-Tester/dp/B0002STSBM

Might be a decent thing to try depending on how the fuel diagnostics go, but I'd probably replace the cap and rotor first since it could be a spark timing problem rather than a strength problem. Would also have to find spec on how strong the spark is supposed to be.

 

[Vikas]

Have you not yet tried starter fluid equivalent test? I have never personally used it on a car but somebody should be able to give a full proof and least likely to cause catastrophe type instructions. I had used carb cleaner spary in the throat of the carbs decades ago but you will need to adapt that method for 21st century. The principle is still the same. You will know if your no-start condition is due to lack of fuel or lack of spark immediately after running the test.

 

[another Todd]

Originally Posted By: rationull
Haven't seen those before. Here's one: http://www.amazon.com/Thexton-404-Adjustable-Ignition-Tester/dp/B0002STSBM

Might be a decent thing to try depending on how the fuel diagnostics go, but I'd probably replace the cap and rotor first since it could be a spark timing problem rather than a strength problem. Would also have to find spec on how strong the spark is supposed to be.


That's the one. Very simple, and handy for diagnosing ignition issues on auto and small equipment engines. The scale on it shows the voltage required to jump the gap you set it at. Read the instructions, you do not set it at the spark plug gap, you set it at a much much larger gap because without cylinder pressure it is easier to jump a gap.

 

[rationull]

Originally Posted By: Vikas
Have you not yet tried starter fluid equivalent test? I have never personally used it on a car but somebody should be able to give a full proof and least likely to cause catastrophe type instructions. I had used carb cleaner spary in the throat of the carbs decades ago but you will need to adapt that method for 21st century. The principle is still the same. You will know if your no-start condition is due to lack of fuel or lack of spark immediately after running the test.


TBH I'm a little nervous about the starting fluid test, and it seems like diagnostics w/ a pressure gauge will give close to the same information (not quite as definitive of course). I probably have some carb cleaner and/or throttle body cleaner lying around that might work. Maybe I will consider doing this on Friday morning (or sooner if I can get to the fuel pressure test tonight) if it makes sense based on the fuel pressure check results, if I can find some really solid instructions as to how to avoid damage.


Originally Posted By: another Todd

That's the one. Very simple, and handy for diagnosing ignition issues on auto and small equipment engines. The scale on it shows the voltage required to jump the gap you set it at. Read the instructions, you do not set it at the spark plug gap, you set it at a much much larger gap because without cylinder pressure it is easier to jump a gap.


My concern is this still won't tell me if spark timing is wrong. That's why I'd probably do the cap/rotor first.

 

[Vikas]

By the way, if you go with the starting fluid/carb cleaner/throttle body cleaner, please make sure your MAF is not getting drenched with it! Try to spray a shot of the cleaner after the MAF right in the TB throat. You don't need lot to know if the engine now is at least thinking of starting without having to really start it. You will immediately notice if the behavior is different i.e. you will *know* if the cylinders are now firing. I have done this on my lawn mower but I absolutely understand your reluctance to try on the vehicle.

That is why I am hoping somebody else would chime in and provide a better and full proof instructions than mine.

And if the plug was not wet with the gasoline or if you don't smell the gas after cranking and removing the spark plug, then most likely you are not getting any gas in the cylinder.

 

[rationull]

The plug was not noticeably wet but I had only cranked a couple times before pulling it. I didn't smell gasoline after the cranking w/ the plug out but that was after the small pop + flame would've consumed the gas, which is another reason I'm thinking the injectors are firing to an extent, but maybe not enough.

If I do the starting fluid, I will definitely spray it into the TB after the the MAF. One reason I am reluctant about this is that since bad spark timing is one of the possible causes of the problem, if some cylinders are sparking at the wrong time, I wouldn't want to blow up the intake manifold or burn up anything inside it because a plug went off when the intake valve was open and there was a bunch of "fuel" sitting in the manifold. Maybe I'm being too paranoid though.

 

[Vikas]

I think you have every reason to be paranoid given that you have already seen a little flame! I had missed that portion in your original posting. If the timing has jumped and the car is refusing to start, can you use the timing gun to see if the marks are lining up during the cranking? I am not sure if that is valid test though because the cranking speed is generally less than idle speed.

There was a case of Lexus RX350 not starting which turned out to be case of too much gunk on the throttle plate and stuck IACV which was blocking air from entering the engine. But in that case, giving 1/4 gas pedal could have started the car. It turned out that most of us have now forgotten the old tricks of starting cars when we had chokes and carburetor but sometime they are useful in diagnosis!