99 GTP missfires

Ok this thing has 120k miles on it. The plugs have 20k on them. They're performance plugs but i don't remember what i put in it(mabey plat-4's) This thing misfires like crazy at idle and anywhere below 1200 rpms. Any other range and its fine. I noticed when it missfires it feels a lot faster. When it rarely doesnt missfire the car runs kinda like a gt. So i had it scanned at the hobby shop and all the computer told me was random missfire all cylinders. So the coils can't the prob. The plugs are farely new and so as the wires. I just came from autozone today from testing the ignition module and it tested good. Tthis problem started when the car was stock. Now it has cold air intake and 3.4 pulley.

Has anyone seen this before? Whats the next step too go? If a sensor went bad the computer would tell me right? Are these things have magnetic trigger since its distributerless? Could a SC coupler cause this. Cause it is bad, it rattles when it missfires and i noticed some play when changing the pulley. I was planning on buying a performance pcm and hoping a case learn might fix it.

Thank you for advice in advance.

Try some new plugs first.

Go to autozone and get some autolite 104's

Are those the colder ones? To tell you the truth i don't think they're the plugs. I changed them on my 100k tune up. I remember paying 4 buck a pop on them. Any way i had the scanner look at it in real time and the missfires all cylinders evenly. Bu the dude wouldn't let me see any more stats like fuel mixture or timing. I did notice when i did each upgrade, it stopped missfiring for the first few miles then it would start again.
Computer?

What plugs are they again?

If they are those Bosch Plus 4 or whatever w/ the 4 electrod tips, you need to remove those.

Autolite 104's cost bout $1 a plug almost. There is no reason to spend $4 a plug.

I've been using 104's for going on 2yrs now w/ no problems. I've never had a 104 crack on me like I have seen the stock Delco plugs do.

I've heard that the Bosch Platinum Plus 4's weren't strong enough for the GTP's. You'd think a supposed high quality, high performance spark plug would work fine in a lil' ol' 3800.

I have heard that the Bosch do not work well at all on any car. If that is what you have installed you should either install the OEM, Autolites, Motorcrafts, Champion, whatever but not BOsch.
I am using Champions 408 without problems.

Ok I pulled a plug. They're Bosch Platinums. But no the 4's. Anyway they're completely rust color. What does that indicate. Burning rich? I have new NGK's on the way by the way.

the color sounds normal...did you see any cracks

They are normal if the color on the electrode are is light tan to moderate

from the one plug i pulled i didnt see cracks. Ill check the others when the NGK's get in the mail.

By the way its getting worse. It's starting to climb the rpm's. About midrange at part throttle. Full throttle it goes away.

Ok i put the new NGK's in there, replaced the coupler and replaced the overflow resevoir tube. No more rattling, but its still missifiring at idle, just can't hear it anymore. I think i pissed a sensor off or a wire. Now it missfires like crazy on acceleration, especially when i floor it. So i floored it and held it till the light came on so i can get the code. Well the service engine light began blinking. Took it to autozone and got scanned for a p300 code(multiple/random missfires). This sucks. Now i can't even have fun in it. So i put the old plugs back in the front. I'm not going back in th back unless i really have to. Well the missfiring didnt calm any. So its not the plugs.

It seems to run great and i can't feel a hp gain. But only until the car fully warms up, then missfiring.

What do you guys think? CPS? Mabey 02 sensor since i had to move it for the #6 plug?

I'm thinking of unplugging some sensors so the computer goes into loop and does pregrogrammed tables.

Has anyone changed an Crank position sensor? What to i need to remove to get to it.

Please help me guys this i have to have this fixed as i'm getting out of the military in a month and gonna have to make a lot of trips home.

man i have the same problem me personally i dont know but i was getting a reading of egr valve i fixed it and cat coverter reading now so i think mine is cat coverter but if you get it fixed plz email me at [email protected] if you would plz!

I had misffires and I installed NGK's with new set of wires and the car is running a lot better now

yeah i will have cat converter checked first it is running warmer than ever to so i really believe it cat converter it has a timing chain i think