2004 Grand Prix PCM issues?

Hey guys this car has me stumped. Let me explain the problem. When you are accelerating anywhere from half to full throttle it may cut out at any given time. The RPM will drop down, it may bounce up and down for a bit and recover, or it will drop all the way down and stall. If you let off the gas it may or may not recover. If it stalls so far from the tests I have done if you never shut the key off it will never restart again. If you shut the key off and wait from 5 to 10 minutes it will all of a sudden restart normally like nothing ever happened. If you try to restart it immediately even if you shut the key off and back on it will not start.

When it does not start when I am attempting to start: Fuel pressure according to my gauge is between 55 and 60 psi. Inline spark tester shows that it is getting a spark. Engine sounds normal while cranking so in other words compression sounds normal. I have an LED test light rigged up showing when an injector fires, and it does show electrical activity going to the injector when it opens (flashes). Only clue is that it SEEMS dimmer when it wont start, but I have no clue if thats how it normally is on start up. Its only a slight difference from the brightness of a normal idle brightness. I have a snap on scanner hooked up, nothing that I know of looks abnormal in the data and it will not throw codes when this happens.

Parts I have changed or cleaned so far: Changed the fuel filter, crank sensor, cam sensor, ignition module, and MAF sensor. Cleaned the throttle body.

When the car is working normally it runs strong, good power, smooth. Starts great normally.

So what do you guys think? Throw some ideas out I'm ready.

Plugs and wires.

Plugs and wires.

The main counter arguement I have with that is that you can drive it for 15 minutes without a problem. And if you baby it you can drive it forever. Wouldn't it act up more consistently if it was that?

Ignition issues can be random. By you babying it you are not putting a heavy load on it so the ignition system can work properly. As soon as you get heavy on the throttle then the spark cannot keep up, hence the cut out.

One time I was driving my Stealth and started getting a random misfire. If I babied it I could get anywhere I needed to go, if I punched the throttle it spit and jerked and acted all sorts of crazy. When I finally got to look at it I found a spark plug had cracked. I did all 6 to be safe and was back to normal.

You've changed everything else, it almost has to be plugs and wires.

Ignition issues can be random. By you babying it you are not putting a heavy load on it so the ignition system can work properly. As soon as you get heavy on the throttle then the spark cannot keep up, hence the cut out.

One time I was driving my Stealth and started getting a random misfire. If I babied it I could get anywhere I needed to go, if I punched the throttle it spit and jerked and acted all sorts of crazy. When I finally got to look at it I found a spark plug had cracked. I did all 6 to be safe and was back to normal.

You've changed everything else, it almost has to be plugs and wires.

Thanks for your replies on this. Not trying to shoot you down on your idea but I don't see how it could be plugs and wires in this situation because of the fact that it won't even restart and run at an idle which has barely any load on the engine at all. I have seen what you are talking about though where gas engines with weak sparks will cut out under a load. I have especially seen this happen on older gas engines with a single coil. The coil was going bad and after the coil got hot the engine would cut out easier and eventually stop running altogether untill it cooled off. But this car is a different story of course because it has 3 coil packs, not very likely for all 3 to go bad at once and not likely that all 6 spark plugs and wires are bad at once. Scanner shows no misfires on this engine even when its acting up.

Hi curtis, I have a 2004 gt2 with same/similar issues. up to high idle runs fine, also I can intermittently "gun" the engine repeatedly and get it up to speed. revs up just fine, anything longer than half a sec above a high idle speed, she dies right out. I am thinking either throttle body actuator (built in TPS) or Map Sensor or PCV after some other research on this subject. A local mechanic friend also informed me of wiring harness issues and asked me to give a tug on each wire in the harness near plug...Have you come to a diagnosis on this as of yet? I really can't afford to go through a barrage of costly parts for a car with 270k. Any thoughts would be appreciated. Thanks in advance.:runaround::screwy:

Hi curtis, I have a 2004 gt2 with same/similar issues. up to high idle runs fine, also I can intermittently "gun" the engine repeatedly and get it up to speed. revs up just fine, anything longer than half a sec above a high idle speed, she dies right out. I am thinking either throttle body actuator (built in TPS) or Map Sensor or PCV after some other research on this subject. A local mechanic friend also informed me of wiring harness issues and asked me to give a tug on each wire in the harness near plug...Have you come to a diagnosis on this as of yet? I really can't afford to go through a barrage of costly parts for a car with 270k. Any thoughts would be appreciated. Thanks in advance.:runaround::screwy:

Well what have you done to the car so far? Have you changed the fuel filter? From what you describe it sounds like a fuel filter plugged. When you say "anything longer than half a sec above a high idle speed, she dies right out" do you mean when the car is in drive and moving? Or does this do it when sitting in neutral revving the car?

To follow up on the car I was working on... believe it or not it ended up being bad fuel. I never had the fuel tested so I dont have any scientific data to tell you about it, but whatever it was it had a slightly dark color to it when I collected a sample from the pressure test port at the fuel rail by the injectors. This dark "stuff" seemed like it would collect in the fuel rail over time but wouldn't readily go through the injectors until under heavy acceleration... when fuel flow was greatest. I ended up taking the fuel tank down, flushing it and putting new fuel back in. After running the car for about an hour the problem was completely gone and hasn't come back since. So it had to be the fuel that was the problem. That was the one of the weirdest problems that I have seen in my experiences and I have been wrenching for a solid 10 years.

Thanks for reply curtis.

Whether driving, in gear, neutral, park, whatever, this baby stalls out hard above a high idle unless quickly revving engine intermittently. idles all day long.

Work done thus far (to no avail):

I started by checking for a clogged catalytic converter and then went on to the fuel filter.
I have been thinking throttle may have issue in TPS.
This is a "drive-by-wire" vehicle, so TPS is non-serviceable, being part of throttle body assembly
This particular TB is year specific as well as model specific, which makes it a real bugger to find used.
Car has 270k, so $ is the object here. Dropping tank, etc. and trying to "run her through" sounds preferable to wasting cash.
If bad fuel is not my issue, then all I'm out is a bit of gas and some time.

Thanks for reply curtis.

Whether driving, in gear, neutral, park, whatever, this baby stalls out hard above a high idle unless quickly revving engine intermittently. idles all day long.

Work done thus far (to no avail):

I started by checking for a clogged catalytic converter and then went on to the fuel filter.
I have been thinking throttle may have issue in TPS.
This is a "drive-by-wire" vehicle, so TPS is non-serviceable, being part of throttle body assembly
This particular TB is year specific as well as model specific, which makes it a real bugger to find used.
Car has 270k, so $ is the object here. Dropping tank, etc. and trying to "run her through" sounds preferable to wasting cash.
If bad fuel is not my issue, then all I'm out is a bit of gas and some time.

Any codes? Not real sure what to say without being there. Gotta see what its not getting when it stalls. Watch for spark with an inline spark tester, have someone rev it and as it is stalling see if its still getting spark as the engine is decellerating. Fuel- check if you have decent fuel pressure (I think it should be around 50-60 psi on the gauge). Air- well that would be having to do with that fly by wire throttle control. With scanner you can see position of throttle body as a %. Also pay attention with the scanner with key on engine off slowly push accellerator to the floor and release. Make sure the percentage that it increases and decreases coincides with how far/fast you are pushing the pedal down. Some of these problems should throw codes tho. Hope this helps.