96 5.7 vortec craps 22-2600rpm

[Caveman 81]

Newbie to forum and after perusing many threads and posts I "seem" to have a unique problem.
My 96 GMC K2500 only has 50K on it. (Don't hate me already. It really is a "little old lady story" and I am the second owner.) It starts and idles great. Engine misfires ("craps out") only in the 2200- 2600 rpm band and under light throttle. I can simulate the symptoms in any gear. Doesn't matter if engine is hot or cold, or if the day is dry or humid. One P-code prompted a new egr valve. Codes cleared & no more codes. New single platinum a/c plugs. I hate to hang parts to diagnose a problem so while I've done a visual check I have not changed out the cap/rotor or wires. Engine does perform normal below and above the troubled rpm band. Fuel pressure at the shrader is 55-58 at idle. I've been reading about the csfi to mpfi conversion. Any thoughts would be appreciated. I have enjoyed a lot of reading on this forum and already learned some helpful things.

[maxwedge]

Welcome to AF. Fuel pressure is supposed to be 65 psi, also look at the fuel filter for starters.

[Caveman 81]

Thanks Maxwedge, it was my mistake to not mention that I had already done the fuel filter before taking the psi reading. I have been frustrated trying to find a reliable source for OE fuel psi specs. If anybody knows of any online that would be great for future vehicles. So what you're saying is the crazy symptoms could be low psi? My deductive default setting is still old school I guess ("either it's a pumpin gas or it ain't") Would you just start with the pump?

[blazee]

Your reading was done at idle. GM lists no fuel specs with the engine running. Below is the GM test procedure and spec:

FUEL PRESSURE

1. Ignition "OFF" for 10 Seconds.
2. With ignition "ON," engine "OFF" and fuel pump running, pressure should be 415-455 kPa (60-66 psi).

[Caveman 81]

Thanks blazee for the procedure correction and psi info. Have you seen weird symptoms caused by low psi? By weird I mean that it really doesn't seem to make sense that the vehicle would be fine when throttle position calls for more fuel and the act up only at light throttle position? My thinking would be the opposite, but the wife has brought that up before;-)

[blazee]

Based on the symptoms, one of the things that I would check is the EGR valve. Just take it off and look to see if there is anything such as a chunk of carbon holding it open. If anything is found, clean it out and reinstall the EGR valve. The EGR valve opens to allow exhaust gases to enter the intake. If it were stuck open it would cause problems at low throttle.

[Caveman 81]

The earlier mentioned P-code for egr flow was the reason I went ahead and replaced it because it made sense that it would cause problems at low throttle (hwy / cruise). The code has not returned, but since it's so dawgone easy to get to, I'll R&R it and do a visual again. I need the practice.
*What sensor dictates to the egr? could that be the problem?

Also, I've been reading and getting an education on the old GM fuel injection experiment called csfi (hee-haw).
* What do you think about a stuck poppet?? It doesn't really fit the scenario.
*However at this point it looks like I am probably going to have to eradicate my low fuel pressure just to see what all is gained at that point.

[Caveman 81]

Well I changed out the pump last night and no change in pressure at the fuel rail. Does anybody want to advise? Still have 57 pounds of pressure. Truck still has exact same symptoms. I am thinking I'll have to pull the upper intake and change that dogone regulator??? thanks for your response in advance?
I also noticed that the truck was backfiring when gearing down a hill. hmmm

[Caveman 81]

OK boys, here's the deal,
New pump(delphi)& filter, new fuel regulator (a/c delco), new upper manifold gasket set (felpro),

I did notice (upon the removal of the upper intake manifold) the back half of the lower intake manifold was washed fairly clean (fuel) and yet the front half was a nice standard black carbon color and coating. I was satisfied to trace this "washing" to a leaking regulator. So I replaced it and put it back together.

No change in symptoms. Still starts great and keeps up with high demand under heavy throttle but under a light throttle it still craps and stumbles.

Does anybody know what a plugged poppet valve symptom would be?
Does it sound reasonable that one is plugged?

Should I be mpfi shopping?
Thanks for taking time to read and hope your detective skills are up.

[maxwedge]

Pressurize the system look for leaking poppets, a misfire would be the result with a bad poppet.