why are the simple fixes the hardest to diagnose?

[3100 MPFI]

I have been fighting this stalling/hesitation/misfireing problem with my 1990 Grand Prix STE (3.1 MPFI non-turbo) for almost 7 months now.

I have checked everything I can think of, and followed the book.

What my car was doing was stalling, didn't want to idle, and when I was taking off, it would hesitate and stutter from takeoff to about 15 MPH, then clear up, and be fine.

I finally found the problem. It was 3 small things that were easy to fix, and that I had the parts for (except for one).

First of all, I had a broken sparky plug wire. one of the front ones too.
Second, ALL of the plugs were kinda fouled. They were BLACK, but not crusty.
Third, Cylinder 1 (rear cylinder closest to the alternator), the heli-coil that was used to fix a stripped out spark plug had CRACKED!!!!!

I went to Napa, got a new heli-coil for it ($3.60) and put new plugs and wires in it, and it is like a new car again.

Now I just need to roadtest it too see if I still get the good fuel economy, but it was blocked in.

Why is it that an easy fix like this was so hard to diagnose?

[IsaacHayes]

Cause you take the simple things for granted? hehe

[3100 MPFI]

well..... The car is still not 100%, but it is the closest it has been in a LONG time. I still have a slight misfire when accelerating (I hear it more than feel it), and it still tries to die when I am coming to a stop.

I think it might be a coilpack going out, but any help will be appreciated.

The way the car is now, it was this way when it was fine. I would not hesitate to drive it like this, but I figure why not do it right now, BEFORE I put the car back on the road.

[IsaacHayes]

Wanting to die at stops could be the IAC, or a weak battery. Misfire while accelerating could be perhaps a bad injector or coil as you said. Also the 7x sensor could be a possibility, as mine was random before it went south.

[3100 MPFI]

7X, IAC, TPS, DIS, MAP and battery are all NEW.

Injectors are good (tested them first. It runs the same with either set of 3.1 injectors).

It is also doing something else that I failed to mention.

When I first start the car (cold start), it will idle REALLY HIGH for about 3 minutes. Like 4500-5000 high for 3 solid minutes. Then it will gradually go down to normal.

I tested the throttle plate to be sure it is not sticking open. It is fine.

I need to check for a vacuum leak (out of carb cleaner), but I don't think that is the problem.

This evening, before I went home, I disconnected the battery, and I am leaving it off overnight. Give it a good long time to reset the ECM.

Tomorrow, I am going to put my 3100 DIS and coil-packs on it, then test it again. Since i now have the 1992 ECM in there, I am going to take my lappy with me, and hook up to the ALDL to see if I can force data and get some sensor readings. I know the 1990 ECM will not read, but the 1992 one should. That will probably help me more than anything else.

I couldn't do that in the past seeing that I could not even get the car to idle. Now that it will, I can do that.

[IsaacHayes]

That's got to be a vacuum leak somewhere. When it warms it seals up..

[3100 MPFI]

Finally found out the problem. It was the new DIS. It was BAD.

I swapped in the DIS and coils from my 3100, and the car runs fine now.

I returned the DIS today and got my money back.

They watned to test it of course, and they said it was fine. I told them to run the test like 5 times and it would be bad. Sure enough, when it got hot, it started missing.

Internal short that gets progressivly worse the hotter it gets.

Here it is running!!!!!

[3100 MPFI]

Still not 100%, but pretty close. Surging now.

Okay. It is still not 100%, but it is pretty close. Lots of things have been changed, and it is at about 95% now.

The problem I have now is that the car has a slight surge when coming to a stop. As soon as you fully stop, or as soon as you start accelerating again, it goes away.

There is no stumble or misfire on acceleration. There is also no hesitation.

It now just has the slight surge when stopping. It does not surge when accelerating. Just stopping.

Here is a vid of it. Just watch the tach. I know the gauges are off (speedo and tach both read higher than they really are), but the surge is still shown.

http://s174.photobucket.com/albums/w86/riven2129/video/?action=view&current=STEsurging1.flv

[IanSzgatti]

I think Issacs' idea of a vacumme leak is still a good possibility to explore... hows the egr set-up on that beast?

by the way, I have never seen a car with more buttons on the steering wheel, it's like a fu%#ing space ship

[3100 MPFI]

I checked for Vacuum leaks. There is not any. Even checked all the way down at the tranny, and all the wy up at the brake booster. Even checked the lines that go into the interior, and the vacuum globe.

Has to be something else. I think it MIGHT be the gas (car has been sitting for a while). I am going to go monday and put fresh gas in it and see what that does.

And as for the Steering wheel, that is the standard steering wheel radio controls for the Grand Prix. Mine is an STE, but it is not a TURBO. I also have the weird front bucket seats with the power recliners.

Steeringwheel radio controls:


Armrest controls for seat position, lumbars, and recliners:

[IanSzgatti]

well you said you already checked the injectors... what did they ohm out at?

[3100 MPFI]

Not so much as checked them. I had replaced the stock injectors with 3300 injectors, and the car would NOT run. I put the stock injectors back in. The car was fine with the stock injectors and was not until I tried to use the 3300 ones that I had any problems at all. The 3300 injectors led to the fuel pressure regulator going out, and the car went downhill from there.

[IsaacHayes]

That bouncing idle I've had but it was the IAC. I had no codes but replaced it with a junkyard unit and it took care of it. I've also had that when the CPS was on its way out.

[3100 MPFI]

CPS is new. I replaced that when I got the car.

I might check the IAC. I did put a new one in there, but seeing that the new DIS was bad, I guess I am starting over will all of that.

I guess I will pull the throttle body, clean everything really good, and swap in the IAC from my 3100's 62mm throttle body. I know it worked before I pulled the engine out.