Misfiring

[Kazmoe]

I have a 2000 GTP w/150k on it. Here's what's going on.

If I walk out and start the car right now, I have about a fifty percent chance that it will be misfiring constantly, and flashing SES if you drive on it. Sometimes this resolves itself after just a minute, yesterday it came on as I started it from work and remained on all the way until I stopped at a gas station to refill.

This is where the other fifty percent comes in. As I went to leave the station, start it up... and no misfiring. The same thing occurs in my drive cold ... hot ... whatever. Half the time, there is no misfiring or at worst it's a single misfire that is just noticed and gone.

If it is running smooth, it never starts misfiring without key-off/resting. However it can go from rough/misfire to smooth/no-misfire in the middle of a drive.

I have it coded and we have a 128, 303, and 442 recorded.
I'm going to try a new cap for the 128, looking for help on the 303, and I am not sure where to start on the 442.


Any help appreciated.

[nault31]

It's a good time to change your spark plugs and maybe wires and do it today because a misfire will dump fuel into your exhaust and fry your catalytic converter making this a more expensive repair. Just stick with platinum ones if you plan to continue running a stock motor. P0303 is cylinder 3 misfire, it's the center cylinder towards the front of the car and at the very least change that plug now.

If new plugs and wires properly installed don't solve your problem then the misfire is caused by a bad fuel injector, I doubt a bad coil pack because cylinder 6 is not misfiring.

[nault31]

P0128 means you need a new thermostat, a cheap and easy fix. Make sure your coolant level is good.

P0442 is a leak detected in the Evap system, this is the code that could actually be the gas cap. Check for fuel leaks under the gas tank or any fuel lines. If it's not that then I'm assuming a vacuum leak. After you fix the misfire run the motor and spray a can of carb cleaner on the rubber vacuum hoses to determine which one is leaking. The engine will change it's idle as it sucks in the carb cleaner.

[Kazmoe]

Should have mentioned, front 3 plugs/wires were changed before I posted.
Swapped the coil with another I had from before and no change.

I'll check out the injector right away.