need help with a misfire

[3100 MPFI]

I took my 1990 Grand Prix STE with a 3.1 MPFI off the road to fix a MINOR surging problem. Occasionally, when I was coming to a stop, the car would surge until it was stopped, and also, SOMETIMES when I would take off from a stop, there would be a hesitation, then it would clear up.

I replaced my DIS (it was old, and one of the connectors was messed up anyways), my IAC, MAP, and the fuel rail and injectors. I had some issues with getting it to run, but finally today, I got it to start up, and idle like it should.

As I was cleaning up, I noticed what sounds like a SLIGHT misfire, so I decided to go for a test drive to make sure that the car was running right (and also to calibrate the compass since I had the battery disconnected)..

As I was leaving out the car was hesitating, misfiring, and surging REALLY bad. It would surge and hesitate all the way into second gear, then it would clear up, but if I got on the gas, sometimes it would hesitate, then clear up, but when I took my foot off the gas, the car would keep accelerating.

Any idea what I should check on this? I have been trying to fix a similar problem with my 1988, and seeing that BOTH CARS are doing the SAME THING, I feel that I am just overlooking something simple with both of them (the 1988 is the one with the 3100 MPFI).

[IsaacHayes]

Hmm. Bad TPS ?? Check it with a DMM

[3100 MPFI]

DMM?

[sil]

digital multi meter

with the ignition on but car not running you can probe two wires at the tps (not sure off hand what the colors are, sorry) and check the voltage response from approx .5 to like 4.95 or 5 volts at wot
check the smoothness, move the throttle slowly and check that the voltage doesnt stop or jump eraticly while moving the throttle slowly

you can check the manual for your car and see what the voltage the tps should be at closed

[3100 MPFI]

k, thanks. I got to thinking about it, and the only 2 things that I was coming up with was the TPS, and the ECM.

I will test the TPS tomorrow.

Thanks

[IsaacHayes]

Sorry, I assumed you knew the abbreviations and had a chiltons/haynes repair manual. Those usualy tell you how to check such things like that.

When you said it keeps accelerating after taking the foot off the accel, that's what made me think well maybe it's a whacko TPS. But that wouldn't really cause a bad missfire... Miss-fires can be from the ICM and also the 7x CPS unless you have having a fuel issue.

You don't have a vacuum leak do you? That's something too that will cause miss-fires and maybe even why it accels some after letting off the gas...

[sil]

i had a mega vacuum leak once, my plug for the egr popped off while driving lol
yea it made me not able to stop very well lol

check for vacuum leaks too

[3100 MPFI]

No vacuum leak. I have a GM service manual, but it is in storage right now, so I can't get too it.

I am not sure that it is misfireing. it sounds like a misfire, but when I am idleing, it idles fine. I have that weird in and out exhaust that was used on the 1989 SE, and on my STE as well, so it does not really sound like a normal Y-pipe exhuast. Once I get on the road, that is when it gets screwy.

I will test the TPS tomorrow, and see what it shows. If nothing else, I will just put a new one on.

[geoffinbc]

Before you go any further check your plugs. 99 percent of the time it is the root cause of a misfire.

Then check plug wires with an ohm meter while bending them around.

After these parts check out then go after the coil packs and sensors.

[3100 MPFI]

new plugs and wires (less than 3 months ago, and I checked them again after the car started acting weird to make sure I did not mess any up).

I friend said when her ECM went out, it was doing the same thing.

I have so much going on right now (trying to get that twin cam in my 1995 Cutlass) that I don't really have time to be messing with this. I picked up a TPS today. If that does not fix it, I will take the car to a shop, and let them deal with it.

[IsaacHayes]

Good luck. Shops can be expensive and rack up charges when they can't pinpoint stuff too.

Might want to clean the IAC valve or check it out too, maybe it's getting stuck open? Surging can be from this valve too... I totally forgot about it.

[3100 MPFI]

New IAC. That was one of the first things I replaced (I also cleaned the passage out to be sure that it was not blocked).