L29 Issues after FPR Repalcement

[Christian Steffen]

Alright I need some advice/insight.

So about two months ago I replaced a some stuff on my truck. I did new headers/ exhaust work, replaced FPR, and replaced old fuel pump with a walbro 255 fuel pump.

First Problem:

Since I completed this work I've had an issue where right off idle when I tap the throttle it stumbles for a second and then it goes, I've only noticed this happen while in gear but I cannot confirm right now that it wouldn't happen in park. The truck seems to run fine other than that slight tip-in hesitation, makes great power like it did before this work.

Second Problem:

It seems that while doing this work I created a starting issue similar to the very common hot start one, except this happens pretty much all the time. Before I replaced this stuff the truck would start almost instantly when I turned the key, without any sort of effort. Now I have to crank the engine for a few seconds and sometimes give it a little throttle to make it come to life (it sorta sputters while cranking then I'll give it throttle to make it rev up a bit) and then it runs and idles fine.

A bit if background here:
2000 K2500 with L29 that is now a 489 ci and Comp XE-270hr cam with wait4me tune
When I did the overhaul/stroker job I replaced the injectors with bosch 19 lb ones but not the fpr (stupid)
This was done 3 years ~ 35,000 miles ago


I figure I should probably grab a fuel pressure gauge just to start diagnosing, do you guys have any thoughts on what else I should check?

I appreciate any help offered.

 

[Christian Steffen]

So I have an updated.

I put a fuel pressure gauge on it, and it builds pressure right up to 58 psi when the key is cycled on, but then right when the pump quits cycling the pressure immediately falls off to about 6 psi. And then when I turn the key off it and the pump cycles again it goes right back to 58, then bleeds off again.

If I start the truck while the pump is still cycling, and the gauge reads 58 psi, it starts immediately like it used to. Then once its running the pressure settles out at about 50 psi, and occasionally does a strange very rapid oscillation between 45 and 55 psi.

I'm not sure what to think now, bad fuel pressure regulator? I haven't had time to check the vacuum line to it yet but that could't do it would it? If the line was disconnected I'd assume it'd allow full pressure since it'd be reading atmosphere.

Injectors? Seem unlikely since they weren't a problem before I replaced the fpr.

Something with the fuel pump?

Any thoughts guys?

 

[Jrgunn5150]

I would say you've effectively diagnosed a bad FPR. They build pressure by closing the return, cycling the key a few times artificially builds pressure and allows it to start.

 

[Christian Steffen]

I would say you've effectively diagnosed a bad FPR. They build pressure by closing the return, cycling the key a few times artificially builds pressure and allows it to start.

Well shoot, what are the odds that I got a bad fpr? Is it possible I damaged it during installation perhaps?

 

[Jrgunn5150]

Well shoot, what are the odds that I got a bad fpr? Is it possible I damaged it during installation perhaps?

I'm unsure, I suppose that depends on the brand. I'm unfamiliar with the Vortec 7.4's, so i don't know if you could have damaged it on install or not. But the pressure bleed down is symptomatic of a faulty FPR.

 

[Christian Steffen]

I'm unsure, I suppose that depends on the brand. I'm unfamiliar with the Vortec 7.4's, so i don't know if you could have damaged it on install or not. But the pressure bleed down is symptomatic of a faulty FPR.

Alright, I'll have to check it over and see what I can figure out. The one I bought was delphi so I'd hope it wasn't just bad from the get go. Thanks for the advice though.

 

[Christian Steffen]

So I have a bit of an update.

I couldn't remember if the oring on the old fpr came out with it so I pulled the new one back out and sure enough to old one was still in there. I thought for sure that would have been the issue but it's not apparently. Though it changed how it runs.

The truck seems to start better now. But when I hooked up the fuel pressure gauge it bleeds pressure off almost immediately still, and only sits at about 48 psi whIle running. It does still build right up to 58 psi when the pump primes as before. And oddly enough the fuel pressure gauge doesn't bounce around like it did before, just a very steady 48.

Any thoughts? Does this still seem like it's a bad fpr?

 

[Christian Steffen]

Can I rule out the fuel pump considering it initially builds pressure? Or should I dead head the pump with a gauge hooked up to verify that it does indeed build correct pressure?

I'm simply trying to avoid just throwing more parts at this problem.

 

[Biggershaft96]

Have you checked the fuel volume? you should be able to fill a pint in 15 seconds with a stock pump.

Edit: thats at the test port on the engine.
Heres a good writeup.
http://www.agcoauto.com/content/news/p2_articleid/360

 

[Christian Steffen]

Have you checked the fuel volume? you should be able to fill a pint in 15 seconds with a stock pump.

Edit: thats at the test port on the engine.
Heres a good writeup.
http://www.agcoauto.com/content/news/p2_articleid/360

Thanks for the advice, I have not check the flow rate yet. I think I may have figured out the problem though. I "followed" a write-up on gmtruckcentral about installing a walbro pump in the factory carrier, which advised to tap into the main feed line to keep the fuel bucket full for cooling of the pump. Below is a picture from the write-up.

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I think doing this is probably my issue. It would completely explain the almost immediate drop off of pressure that I am seeing. So I'm going to drop the tank and undo this, hopefully the pump will stay cool enough if I don't run the tank low.