My L29 Laundry List

Tom P · Nov 16, 2017

No fuel pressure drop with key off. I have very few of the regular FPR failure symptoms.

Tom P · Nov 17, 2017

Well I popped my old Black Box back in the truck to try and eliminate my tuned VCM as the culprit of the issue. Of course it started and ran really well as soon as I made the swap, but I'd also disconnected the battery. It cleared a P1336 Code but I also have some rear wheel speed sensor codes as well, all of which could be caused by a faulty ground, so that is another thing I'm going to clear up tomorrow is my ground straps. And perhaps I'll throw on an extra one or two. Both of these issues (fuel pump and brake) started around the same time so its leading me to believe there could be a ****** connection somewhere.

Tom P · Nov 18, 2017

Ok, so im currently knee deep in my diagnosis and just stopped for a breather. I partially dropped the fuel tank enough so I could figure out how to disconnect the harness. It is the newer flat 4 style. I was able to manipulate the fuel pump via the test terminal and it runs. Going through the diagnostics I did discover that if your engine is running and you have oil pressure, and you remove the fuel pump relay, the engine supposedly should still run??? My truck dies within a second of removing the fuel pump relay...

John B · Nov 18, 2017

No, the truck will die when you remove the relay.

Tom P · Nov 18, 2017

Well I couldn't find anything wrong. I went through as many diagnostic checks as possible. I have good power and ground at the pump, the relay is working properly as it should, the I could hear the pump cycling on for 2 seconds as it's programmed to do so. The only thing I can think of it being is that possibly there was air trapped in the fuel rail. Twice when I jumped the fuel pump test circuit I ran it until I heard what sounded like air escaping the schrader valve. Once I had done that everything functioned normally. I dropped the tank to check for leaks or pinched hoses and found nothing. My next plan is to retest fuel pressure to see if I've made any positive ground.

Christian Steffen · Nov 18, 2017

Unfortunately, I'm betting the pump just isn't making the pressure it should. Keep us posted.

Tom P · Nov 18, 2017

Well I'm a ways off from dropping 150 on a delco fuel pump so hopefully it doesn't die all the way. At least now I could probably get the tank dropped in about half the time it took me today. Glad it was as empty as it was. I'm also thinking about taking the bed off. The frame is so rusty it could use a couple hours worth of wire wheeling and new leaf spring mounts. That would be the time to replace the fuel pump.

Christian Steffen · Nov 18, 2017

The more you do it the faster you'll get haha. Hopefully its not the pump.

Cleaning up under the bed would be time consuming, but probably worth the effort. That is on my to do list too.

Tom P · Nov 18, 2017

Here's the fuel pump relay diagnosis out of the GM Service manual.

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Tom P · Nov 18, 2017

If you go from step 10 on, it mentions that the fuel pump should still run at operating temperature and in nominal oil pressure even if the fuel pump relay is removed... It even states in another section of the book that the VCM controls the fuel pump and takes input from the key switch, relay and oil pressure switch to power the fuel pump, and apparently those systems are redundant to each other.

My L29 Laundry List

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