Hi. My '93 Lumina, 3.1 L V6, MFI (T) started with this problem a few months back. When it acts up, the engine "races" at a high rpm (I have no tach, but when in drive it moves the car along at 40-45mph,or faster, with no pedal applied), then while braking to a stop, it helps to drop to neutral so to avoid fighting the engine 'pulling'. Once the car is stopped, the idle drops excessively low, sometimes killing. If it stays running, bad fumes can be smelled and a map sensor code (33) is thrown up because the low rpm messes with the pressure. If I keep the pedal depressed
enough to idle okay, the check engine light issue is avoided. Once the car starts moving, the high revs resume.
For curiosity, I took the car to the top of a slight incline, and in neutral, had a friend get it moving. Above 2 mph the racing commenced, until at the bottom when it fell below 2 mph again, the revs dropped to near "kill" state.
Now, this is an intermittent problem, acting up, or not, whether its raining or dry, 50 deg. F, or -8 F, engine cold or at operating temp. When the car is behaving, it runs like a charm, smooth operation, great idle, great acceleration,
and it may do this for one trip (however long), or 3 days.
When acting up, the same story as when good.
I checked the TPS off the car with a sensor tester and an ohmeter, more than once (and I never get a code for that anyhow), I checked, then replaced anyhow, the idle air control motor, after removing and cleaning the plenum and EGR assy., replacing all gaskets associated with both items.I disconnected the cruise control linkage from the throttle body (which I also had cleaned) and unplugged the electrical to the cruise control module. The gas pedal and cable are free and smooth, the wiring from the ECM to the idle motor rings out, even while flexing the wiring, all the ECM plug connections are dry, no corrosion, nice and shiny. I searched extensively for vacuum leaks, sucking on all lines, listening, etc. The fuel pressure at the rail checks out just as the Haynes manual specs indicate, (yes, even when the idle is bad). I blocked the line to the charcoal canister ( temporarily) to see if that was the culprit. Since my catylitic convertor was rattling inside, I knocked out the insides in case a defective covertor was at fault (and I needed to relpace it anyhow before next months test).
I checked the ECT on the car with the engine cold, then hot. I eventually replaced the MAP sensor just in case.
I'm starting to think the ECM is the culprit, especially after noting Rich Tazz's cautioning about GM service
#1227727. Is there anything else I'm overlooking? Has anybody else out there had a similar problem? The car is due for emissions testing before march 31, and I'd hate to buy an ECM and find I overlooked the real culprit!
Sorry this has been so long, I just wanted to cover what I've already done, so any 'detectives' out there had enough to work with. Thanks for any help; I'll let you know the eventual outcome. Rob