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?

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.

Never get a CEL, no history.

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.
1996 Pontiac Grand Prix Performance Problem

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