2003 GP GT misfiring

[eshina17]

Hey all!
I'm posting about my brothers 03 GT with about 150k miles. Here's whats going on... At around 4500 rpms it seems to be misfiring, and the service engine light will blink a few times, then goes off after the engine run at lower rmps and is not misfiring. Seems to lose a lot of power while trying to accelerate and shift into the next gear. We replaced plugs, and coil packs, but it seemed the car ran even worse. So we picked up wires. Replaced those, and now it will start up, but not running right, and when it goes into gear, it feels like its going to die. Sounds like a couple cylinders are not firing. Any help is appreciated! Thanks

[Scottydoggs]

sounds like a clogged up cat. how many miles are on it.


blinking cel usually means of two things, tune up time, or clogged cat.

make sure the plug wires are on right. its easy to cross one up.

[eshina17]

Its got about 150k miles on it. We checked, double checked, and checked again to make sure we didnt just have the wires mixed up

[Scottydoggs]

remove the up stream o2 sensor, this will make a good exhaust leak pre cat, so if it runs better, feels stronger, the cats bad.

you can also drive it at night and then check to see if the cat is glowing red.

then theres the old bang on the cat and listen for it to rattle inside.

[eshina17]

Alright so we determined that it had to be the cat that was bad. It definitely rattled when we hit it with a mallet. So we ended up just cutting the old cat off and just straight piped the section. Engine runs loud, but better at idle. still having the same misfire and loss of power under load tho. I still think the cat was half of our issue, but what else is causing the poor performance? Bad spark plugs? Bad fuel filter? Bad injector? Were gonna take it to auto zone later and have them read the codes. Ill post those after i find out what they are.

[Scottydoggs]

rule one always start with the codes if the cel is lit.

im sure is got a code for the rear o2 now. and being a 03 you kinda need that to work or it will run poorly.

[eshina17]

Okay so here's what we've figured out
-Very hard/rough idle at start, once warmed up, misfiring at idle goes away (almost).
-Turning off the car and staring it minutes later it will misfires at idle again, until warmed up.
-I can give it no more than 1/3 pedal before it misfires.
-Two coil posts (from two different coils) look darker than the other four even though they are all new (can't remember which ones)
-Throwing a code for cat, and P0300, random misfire
-Already tried:
-Coils, wires, plugs
-Removing CAT
-Swapping (from my 97 GTP) ignition control module
-Removing heat shields around plug wire boots
It seems like there are quite a few things that could cause this issue. When we replaced the cat with a straight pipe, we didnt weld it, just used exhaust clamps. Would the gaps in piping be causing the engine to run like it is? Do we need to get an O2 simulator since there isnt a cat, or is that something that only throws a code and doesnt effect the engine? What about a bad spark plug gap size? Im stumped on what exactly it is

[Scottydoggs]

some good info on the rear o2 here. http://www.grandprixforums.net/threa...uld-know/page9

id start to trace all the icm wires and look for damage to them, down to the cam and crank sensors too. crank sensor tells the icm when to fire, so a bad wire could cause the issue.