Missfiring.

[andyd00]

Ok this is whats going on.
Its a 99 GTP. 125k miles.

It all started with a missfiring at idle and only idle. A code would pop up if i let it idle for a long time but it would pop up p0300(random/multiple missfires). So i went to the hobby shop and had it scanned real time and sure enough it was missfiring all cylinders and the numbers kept climbing. I increase the rpms and it goes away. Went to the dealership and they told me alot of gtp's do this for some reason and i shouldn't worry about it. But it was annoying cause i also had a bad coupler. So i replaced the coupler and the noise went away. It almost seemed the misfiring went away along with it. But you can tell alittle bit if you look for it.


Ok now this week its missfiring at idle, a little under slight acceleration and real bad when i floor it.
So i put ngk spark plugs in it, nothing.
So i put new wires, nothing.
changed out crankchaft sensor, nothing.
Unpluged IAT sensor nothing.
And finally i unplugged the throttle position sensor, no change but tranny messing up.

I also noticed the car runs perfectly fine when the engines cold. But as soon as it warms up the car acts up again. I can't even pass people because of this. Are there any sensors the engine ignores while the engine is still cold? Besides the O2.
Has anyone had this problem. I just bought an O2 sensor and gonna install it tomorrow.

[Ripn12s]

I see you've rulled out the plugs and wires, but what about the coil pack?

Misfire is going to be a fuel or spark problem. Check the coil pack and if thats ok then check your injectors.

[andyd00]

crap i forgot.

I tested the coils and module. They all checked good.

And i run fuel injection cleaner on a monthly basis.

[andyd00]

But not all injectors would go bad would they?

[Ripn12s]

What u could do is determine what cylinder is the worse. Then take that injector and move it to a different cylinder and see if you get the same results.

Is it possible that you might have bad compression?? Maybe time to have that checked.

Any mods on the car??

[andyd00]

3.4 pulley and wizaird intake

Someone told me about the compression check but i would figure that would cause the prob to happen all the time and just in one cyclinder.

Anyway i can't scan realtime anymoreas the hobby shop changed the rules to pulling codes only.

[Ripn12s]

what heat range plugs are u running?

[andyd00]

NGK TR6

It was originally doing this with bosch platniums.

[Ripn12s]

I still think the best plugs are Autolite 104's. Just over a dollar a plug also.

I havent replaced plugs in almost a year, and still have 0 misfires.

What do u have the plugs gapped at? maybe the gap is too big and the boost is blowing out the spark.

Tim

[andyd00]

whatever the haynes manual said

i think 50 or 55 thousands

[Ripn12s]

mmmm....

All I can think of is try the compression check, and also pull the plugs again and make sure there are no cracks in the white part of the plug. You'll also be able to see if your running lean or rich and maybe if there is a leak in the head gasket.

Hope this helps.

Tim

[andyd00]

i"m stumped too. Guess this is the last modern car i'll ever buy. No complicated probs on my 68 camaro.

What kills me is there are no codes popping up besides p0300 and thats when i floor and hold it till it does pop up one.

[andyd00]

Ok popped in the new O2 sensor. The car runs a lot better but the missfiring is still there.

[andyd00]

UPDATE:

Bought an 8k ohm resister to fool the computer's AIT sensor and no fix.

Read the voltage level for the throttle position sensor and it read .4 at closed and 4.17 at full throttle. Think thats out of limits?

[spotts]

I am having the same PO300 code pop up on my 97 grand prix. I haven't figured out the exact problem yet but an Engine knock is what is causing the misfire. I don't know how familiar you are with GM and their knock sensor system but they retard the timing when the engine pings. The knock sensors are picking up an engine knock and retarding the timing too much, causing a horrible lack of power. The PO300 comes from the knock sensors telling the computer to retard timing and the other computer inputs tell the computer to advance, so, the code is tripped. I have the engine out of the car now to find the knock but haven't gotten to the problem yet. I'll let you know what I find if I could be of any help. I'm glad I found someone with a similar car problem because mine is the only one I've ever found like it. If your car has a slight knock, it is probably the same probem.