1996 Pontiac Grand Prix Performance Problem

Two GM dealers gave up on this problem, spent $500 just on diagnosis with them. 1996 Pontiac Grand Prix 3.1l automatic. Have ScanXL software to track sensor details. Performance falls off a cliff past 2100rpm. When it gets to 2100rpm on a road test, I can keep opening throttle but get flatlines for MAF at 6lb/min and MAP at 28in and revs don't want to increase. Sometimes will severely hesitate or stall at cruise speed, strong gas smell at idle. Replaced ALL sensors, some more than once.

Replaced ALL sensors, some more than once, even both crank sensors and cam. Replaced catalytic converter, fuel pump and filter, injectors, upper and lower intake gaskets, coils and ICM, plugs and wires, entire ECM. Chased entire underhood harness, checked grounds. No codes at all. Never had such a hard time diagnosing a problem, and I guess the dealer techs are lost too. Any ideas? Could ECM programming be a problem?

pontiac grand prix

 Answer: Have you checked the fuel pressure regulator for leaks?

I replaced it with the injectors and fuel pump, sometime in April this year. Could check it again.
 Yeah, that's an odd problem, especially with everything new. I doubt it has anything to do with ECM programming. Does it ever pop any codes when you get it to act up?

The only code that popped was O2 sensor, and both have since been replaced. I was told BWD sensors might be flaky for this engine, and that is what I used. I was thinking spark advance could be off, but no way to check timing except by watching computer output.
 I would definitely recommend using GM parts when you can. And yes, there is no way to even adjust timing, you can just watch it on scanner.

I am considering checking timing chain, (160K miles on car), but hard to imagine a chain slipping. Can you think of anything mechanical rather than electrical that could be suspect?
 If it was a mechanical issue, it would probably be more prevalent at all rpms, and not just at 2100. Something has to be glitching out at that rpm, whether it be electrical wiring or a sensor. Unfortunately, without setting any other codes or having another starting point for diagnosis, you're basically left at trying parts until you fix it. I'd definitely give the harness a good look over again, especially down the block on either side.

OK. Thank you.

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