'93 Lumina intermitt. high revs. above 2mph

[59tech]

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

[richtazz]

Nice description of your issue, it helps to eliminate a lot if things. Just to clarify, does the idle only race when the car is in gear? You can try the "tap test" on the ECM to see if this changes the situation. When you replaced the IAC, did you also clean the bore in which it rides? An extremely high idle, especially an intermittent one, usually goes back to either the IAC, ECM or a vacuum leak. You are definitely approaching this issue with an intelligent attack plan, so keep us posted.

[59tech]

I hope this finally gets thru, this is my 5th attempt in 3 days. Thank for replying richtazz. I did clean the whole throttlebody(off car) because of the dirt I saw when I went to replace the IAC. Replaced all gaskets. The car revs high whether in drive or neutral(above 2mph) which I think means the VSS is okay and the tranny is not part of the problem. Do I perform the tap test during the acting-up period, when it's behaving, or both? Would idle circuit problems cause the car to idle at near 50 mph in drive, no pedal? Thanks again.

[maxwedge]

Do a full scan and you can see what the pcm is commanding the iac to do, this should narrow the problem down. Also make sure the TB throttle plate shaft is not worn in the housing, causing not it not to seat.

[59tech]

Wow. Those are both good ideas I hadn't thought of. I especially like the idea of a possible binding throttle plate. I'll check that out. As far as the full scan, how is that done on older cars such as mine? I just know how to pull up the codes. Does autozone have that capability? Thanks for the feedback.

[jeffcoslacker]

I'm a little baffled. But if it raised the idle itself when you let it roll in nuetral, I want to say the VSS may be involved.

I've seen a few older ones have a collapsed vacuum hose where it bends to meet the MAP , it flattens out and sucks itself shut and deprives the MAP of a vacuum reading under certain conditions/temps and causes bizzare behavior...

[richtazz]

IF you have the non-electronic trans, check the vacuum line to the transmission modulator. It may have a crack or break in it causing a massive vacuum leak when the car is put in gear. Also inspect the line for the presence of tranny fluid, as the modulator valve itself may have a split in the diaphragm causing the vacuum leak and a mysterious loss of trans fluid.

[59tech]

Guys, thanks for all the new suggestions. I'll check out what you're suggesting, but its going to be in about 10 days. My wife just called(I'm at work), and we're going to have to drive down to Texas from the Chicago area here to help with a family problem. I guess we're leaving early tomorrow morning. Here I am, hot to trot with your suggested lines of attack, and now they're on hold! Arragh!
(We're going in a much newer vechile, not the lumina!)