1994 3.1 GP intermittent stalling

[marleythedog]

Hi,

I've been battling a stalling problem on-and-off for about the past year, and more regularly in the past 6 months.

1994 Pontiac Grand Prix, 3100 SFI, 115000 miles (I started having problems shortly before 100k)
We've replaced:
Fuel pump, twice -- I went about 6 months without a problem after the first replacement; second replacement was about 3 mos. ago under warranty, but the first replacement tested fine at that time
Fuel filter
Ignition control module and spark plugs
Coolant thermostat and coolant
CKP sensor
Battery
I've been driving a couple weeks on the other key, just to make sure it's not a VATS issue.

We've unplugged/checked some of the wiring harnesses, all clean and no corrosion.
We did not replace the TPS--we did the test from easyautodiagnostics.com and it was running within parameters.
I never get a check engine light, and a mechanic was unable to get any meaningful DTCs off of it (though I don't think his scanner could read the 94-95 OBD "1.5"-specific codes)

I drive about 75 miles a day round-trip, almost all highway. I haven't noticed any drop in performance or fuel economy since this began. Somewhere between 10-35 miles into a trip, it will just suddenly go into what kind of feels like a limp mode, except that pressing the gas causes it the engine to bog down and stall shortly afterward. We've gauged the fuel line, and it doesn't lose fuel pressure at all around stall time. It always starts right back up, but sometimes it takes 2 restarts or letting it sit 10-20 seconds before I can put it in drive again without stalling. Sometimes I will feel a single jerk (misfire?) on the re-acceleration after the stall. It will then go the rest of that trip without problem, and usually won't do it again that day (though occasionally it will do it twice, once per trip). It doesn't happen every day, but it happens more often than not.

Not sure if it's at all related, but some days I also feel a slight jerk or slippage in the powertrain. It doesn't seem to correlate with the engine stalls, but I'm not sure if maybe they're symptoms of a common problem.

My dad is thinking we try the ECM next. I've been trying to hold off on that until we've singled everything else out. Is there anything we've missed?

[TGP37]

Check upstream oxygen sensor? They tend to need replacement at 100k miles.

I would also check other maintenance needed at 100k miles, might clue into what's wrong.

Failing O2 sensor ground wire? Spark plug wires might have too high resistance.....check crank sensor wire, ...
VATS, the hammers in the key cylinder often fail. Changing keys won't make a difference there. That VATS, you can measure the resistance in your keys chip and make a series of resistors to add up to your keys resistance (+/- 2%) and wire it into the wires that lead to the hammers in the key cylinder.

Basically fooling the VATS system into seeing proper resistance (good key) and lets car start.

[robertisaar]

i see ICM and plugs, but not plug wires or coils... any reason those were skipped?

VATS won't be an issue... it will only prevent starting, it won't stop the engine once it's running for more than ~2 seconds.

[19Cutlass94]

what about testing the injectors?

[marleythedog]

Thanks for the responses, guys.

I've been holding off on responding so as not to jinx it, but we ended up replacing the ECT sensor two weeks ago, and I haven't had an issue since. We replaced the plug wires as well, as they were getting a bit worn.