Need help interpreting fuel pressure test

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mudpie

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I'm not a Chevy guy, as I've mentioned in other posts. I just happen to have this truck, so I"m not really up to date on all of Chevy's little quirks. But I'm learning....like fuel pumps that keep dying, that kind of thing.

My fuel pump died 2 years ago and I replaced it with a cheapie that had a 1 year warranty. Two months ago it died again. For about a month before that cheapie died, I had lots of driveability problems with it. No power, and sputtering at low rpm, but it seemed to get better when it warmed up, so I assumed it was a sensor of some sort. Then it died, and I figured it out.

So I replaced the pump again, 2 months ago, this time with a Delphi unit. The truck ran great. However, I noticed that it took longer to start. With the cheapie pump, it ran poorly but always started instantly. After putting the Delphi pump in, it ran awesome, but I had to crank it for 3 or 4 seconds before it would start. I didn't think anything of it.

A couple days ago the truck started to hesitate just a little bit, but seemed to run fine. I figured it was due for a tune up. Today it's dead. I checked the fuel pressure and it's a little odd. When I turn the key to the "ON" position, my pressure goes up to about 10 psi. When I crank the engine, the pressure goes slowly up to about 50 psi. It takes 3 or 4 seconds to get there. When I stop cranking, the pressure briefly goes up to 60 while the pump is still activated (1-2 seconds), then it drops almost immediately to about 40, and slowly drops back down to 10 psi.

I'm thinking fuel pressure regulator, but like I said, I'm not a Chevy guy. Am I on the right track? I know it's a hassle to get to, so I want to confirm my suspicions before I dig into the intake. Also, if I do have to pull the intake, is there anything else I should be looking at while I've got it apart? Any thoughts appreciated.

1996 C2500HD, 5.7
 

east302

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Here are the diagnostic tables for the fuel system, but, yes, it does sound like either a leaking regulator (leaking fuel back into the tank) or injector (how do the plugs look, is one in particular soaked with fuel?).

A revised spider injector assembly (which includes the regulator) is available and highly recommended. The lower intake stays on, you're just pulling the black upper intake off once you get all of the stuff off of it.

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mudpie

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It got dark and cold yesterday before I pulled the plugs. I put that off until today. I'll check and see, as I'm still of the opinion that it probably needs a tune up anyway.

When I replaced the pump originally (2 years ago) I had the upper intake off and noticed that the previous owner had installed an updated spider assembly. Don't know how old it is, or if it was new or used, but it's in there. Hopefully that isn't the issue. I do recall them being a bit pricey, as you mentioned.

If I replace the pressure regulator, is there preferred brand, like with the pump? My local Autozone has a Delphi for about $72, but it only had a 1 year warranty. My local O'Reilly's has a Borg-Warner regulator for $45, and it's got a lifetime warranty. Lifetime warranty means nothing to me though if I have to pull it apart every couple years. I happen to be off for Christmas now, but generally I use this truck for work, and downtime sucks.

Thanks for the info above. I'll go through it and see what I can figure out.
 

east302

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Hmm, so it already has the Multiport injectors on it? It's the one with the small electrical connector at the end of each injector.

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I'd go with a Delco or Delphi, though many report that the multiport Delco spider injector assembly is a re-boxed Delphi.




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magimerlin

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One thing I don't see... unless I read right over it. Was no mention of fuel filter being replaced at all. You'd be surprised how many times that was causing an drivability issue when they bring it to my shop... I'd pull it off and you can not even blow through it one bit....lol..

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mudpie

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Yep, that's the injector assembly it's got on it...a connector at each injector. The previous owner was a GM tech, and used this truck for road service calls. He maintained it fanatically. I assume the old injector assembly went bad and he put the updated one in.

I put a new fuel filter on the truck 2 years ago when I replaced the fuel pump the first time. Two months ago, when I put the Delphi pump in, I didn't put a new filter in. It's got maybe 10k on that filter, if that. However, I did put a new in-tank filter on the pump (again), and a new electrical harness.

http://www.gmt400.com/threads/bare-wires-in-the-fuel-tank.39287/
 

east302

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Well, it wouldn't be the first time for a new part to go bad I guess. If it's rapidly dropping pressure when the pump is turned off, it's either leaking through an injector, leaking through the regulator back to the tank or (I assume that it has this) through the check valve at the pump discharge back into the tank.

You might look into the throttle body, see if the intake is all clean on one side or wet with gas. Well, see whatever you can I guess. I suppose the regulator could be leaking externally if the diaphragm or seal is damaged. If you already have the upper intake off, jumper/energize the pump over at the driver side test port and see if you spot the leak.

You'd really need a valve and adaptor set to isolate the discharge and return lines at the fuel filter to otherwise confirm what is going on. I don't know if a parts store has a ready-made kit you could rent, but it'd probably be just as easy to fab something together.

It may be worthwhile to check the power and ground at the pump, but I see you already put in a new connection there.






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Coveman

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I'm on my 3rd replacement fuel pump now.. the third was a delphi, corrected the gage inaccuracy, and definitely takes longer to start now.

i change my fuel filters at the recommended interval, never seen one clogged up yet thou.

one other dianostic you can run is to check your fuel trim with a scanner. my leaky spider drives a -40 fuel trim, meaning its trying to compensate for all that extra gas leaking into the manifold

good luck
 

mudpie

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EDIT - Had issues with Photobucket. Tried posting pics several times, now trying to clean up the excess posting with this edit. I'm using Google photos now, and they don't have a way to embed a pic, just link to it. Best I can come up with for now. I'm curious what photo hosting site people are using.


Picture time.
This is the driver side plugs. From left to right they are # 1, 3, 5, 7. Gap on all 4 was around 0.68.
https://photos.app.goo.gl/00Ka2HzAoLjcv7Je2

This is the passenger side plugs. From left to right they are # 8, 6, 4, 2. Gap on all 4 was around 0.63
https://photos.app.goo.gl/OAcrDeO9uKQsWkO22

This is a closer shot of #2.
https://photos.app.goo.gl/s5K6GSoKYip1R4gd2


Obviously lots of oil fouling. I'm doing a compression test to see what I get. I got the truck a couple years ago and I did a compression test at that time. I don't recall the numbers, but remember them being very good, and uniform. It burns a little oil, but nothing abnormal given that it's got almost half a million miles.

I got new plugs but didn't install them yet. I checked the gap and they're all about 0.63 out of the box, if I can trust my cheapie little key ring gap tool. I've read several places that it's not a good idea to re-gap platinum plugs. They're obviously above spec. Do I go with them as is?

I cleaned up the old plugs and put them back just to see. They cleaned up pretty decent, but the truck still didn't start. Battery is pretty weak now though, so I'll charge it and see what happens.

For whatever it's worth, the distributor was new about a year and a half ago. I had issues with the cheap cap / rotor that came with it, so they were replaced with whatever the top quality Autozone brand is and I had no issues since. Plug wires looked fairly new when I got the truck 2 1/2 years ago, and they still pass the eye test...not worn or frayed or torn or any oily connections or anything.

Thanks for all the advice so far. I appreciate the help.
Ignore the rest of this stuff Photobucket nonsense. I've tried to edit it out, but it doesn't seem to be working.

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df2x4

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Photobucket strikes again unfortunately... We can't see any of your pictures.
 
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