Trying to Trouble Shoot 04 LT, engine cranks but wont turn over BUT...

[Jhart]

Ive had this problem the last 3 weeks where the car wont turn over. However, within the last three weeks the car has started three times, and each of those times the car ran no problem for hours at a time, before getting shut off and then not starting again for days at a time. I switched out the fuel pump relay, and alternator. Heres the weird thing, the last time I started it, I had the fuel pump relay backwards!

I am starting to think its a fuel pump, maybe its intermittent? I don't have a FPG....

 

[The_Roadie]

Welcome! How many miles, and is there any other maintenance history? Have you ever changed your ignition switch? They have a high rate of going flaky or failing, and one of the symptoms can be exactly this.

Any relay that can be installed backwards is perfectly happy both ways. They are rotationally symmetrical. GM was dumb in some ways, but not this. Relays that cannot work in other orientations can only plug in the one way they will work.

 

[Jhart]

Welcome! How many miles, and is there any other maintenance history? Have you ever changed your ignition switch? They have a high rate of going flaky or failing, and one of the symptoms can be exactly this.

Any relay that can be installed backwards is perfectly happy both ways. They are rotationally symmetrical. GM was dumb in some ways, but not this. Relays that cannot work in other orientations can only plug in the one way they will work..

Thanks for this. Its become much more clear that the culprit is either a fuel pump or this. I thought, however, that if ignition switch is bad it wont turn over hardly ever, and ive gotten it to turn over a few times in these few weeks..just gonna have to pony up and get a few tools. Oh, and 136k miles

 

[The_Roadie]

The ignition switch has a circuit that goes hot during CRANK mode, but there's another circuit that runs the PCM that needs to be on in RUN + CRANK mode that may be flaky. But then the PCM wouldn't be able to command the start relay to close to spin the starter, so you're right. It may be just in the fuel pump circuit. Assuming your coil and injector fuses are OK and all that rot.

 

[Jhart]

The ignition switch has a circuit that goes hot during CRANK mode, but there's another circuit that runs the PCM that needs to be on in RUN + CRANK mode that may be flaky. But then the PCM wouldn't be able to command the start relay to close to spin the starter, so you're right. It may be just in the fuel pump circuit. Assuming your coil and injector fuses are OK and all that rot.

Is there a best way to check to see if it's fuel pump circuit? Would unplugging fuel pump relay and tryin to jump it give any indication?

 

[triz]

Is the car cranking or is it just a dead start? No noise. If its cranking and just wont turn over I would say fuel pump. If your turning the key and are not getting anything I would look at your ignition system.

 

[Jhart]

Is the car cranking or is it just a dead start? No noise. If its cranking and just wont turn over I would say fuel pump. If your turning the key and are not getting anything I would look at your ignition system.

It's cranking but not turning over. is it possible that the fuel filter is clogged? I should check that first I guess seeing how that is much cheaper

 

[jimmyjam]

a fuel pressure gauge is $20 at harbor freight... i'd buy one of those instead of guessing...

 

[Jhart]

So there was hardly no fuel pressure. Checked the four relay circuits and they are all getting juice. So I changed the fuel pump... To no avail. It's still not turning over. Brand new pump. The truck is getting spark, so it can't be ignition switch correct?


Maybe I need a new ECM.. Makes sense the ECM that drives the fuel circuit is shot, I mean.. Chevy put that thing Right over the manifold.

Any else have input what it could be?

 

[budwich]

did you try temporarily wiring the pump directly to power (ie. by-passing the relay)? The pump should run "forever" (ie. as long as the "jumper" is in place). Further, you can try spraying some start fluid in the intake (carefully) to see if you get any "action". IF not, then you likely have an issue not related to fueling. IF so, then you can continue trying to understand why you aren't getting fuel especially IF the "direct wiring" doesn't work.... possibly a ground issue in that case.

 

[christo829]

When you turn the key to run (not to start), do you hear the two second fuel pump prime with the new pump?
Possible for new parts to be bad out of the box, or for there to be a wiring problem at the pump connector. When you
put the new pump in, did you check for voltage at the connector? Make sure grounds for the pump circuit as well as the
ignition switch are solid. Probably wouldn't hurt to check the ECM grounds as well.

Could still be the ignition switch, and that's a higher probability than ECM failure (cheaper to troubleshoot, too). Spark
and fuel are separately controlled, if I recall correctly.

Also, there could be a problem with the fuel pressure regulator, but if the pump isn't cycling, it won't have anything
to regulate.

Good Luck!

Chris