Misfire woes P0303 P0300 [Archive] - GrandAmGT.com Forum


View Full Version : Misfire woes P0303 P0300

10-25-2011, 12:07 AM
Hi all, need help figuring what to do next. Have a 2000 GT 3.4 125k miles auto trans that runs very good and smooth for less than 1 minute then starts missing and progressively gets markedly worse and more jerky. It keeps throwing a P0303 code after a few restarts. Here is what I have done so far:
1) Put an ignition module with all the wires from a donor car (99GT3.4) on the 2000 and no change in the symptoms on the 2000. And the ICM and wires off the 2000 worked fine on the donor.
2) Put new spark plugs in the 2000 no change in symtoms.
3) Checked the fuel rail pressure on the 2000 which stays good at 50psi at idle when it starts and when it misfires.
4) Never has any P02XX codes to suggest fuel injector circuit malfunction.
5) Swapped out the MAF sensor off the donor car to the 2000 and no change in the problem/symptoms on the 2000 and the donor car ran normal with the other MAF sensor.
6) Pressures in the 2000 cylinders #2=155psi; #4=160psi; and the #3 cylinder giving me misfires is 150psi.
7) Took the 2000 engine apart and put new UIM gaskets on it, and externally decarbonized and cleaned fuel injectors and swapped the #2 and #3 fuel injectors on the wire loom/postiton then reassembled. Still the # 3 cylinder misfires and throws a p0303 with the #2 injector on it.
8) Once in a while a P0300 no other codes.
9) After all this pulled the new plugs on #2, 3, and 4 cylinders and the # 3 looks slightly blackened and wet, the others tan.
10) No white smoke out of either pipe on startup or run.
11) Put a new EGR valve gasket on it.

1) Where to go next diagnostically, please do not say the dealer...this is a challenge I am taking personally now?
2) Can I assume that there are no opens in the Fuel injector circuit if there are no P02XX codes?
3) Why are there no P0305 and P0306 codes in the 1999 factory manual?
4) Is my biggest clue that it runs so nice for half a minute to a minute then dramatically deteriorates, ie what in the control system changes from default to sensor closed loop?
5) Two good cylinders have 155 and 160psi while the misfire cylinder has 150 psi, is that really much of a difference...does not bleed down.

Cheers, JPS

10-25-2011, 10:21 AM
ya know when my valve seals weren't holding, i had engine misfire like a mofo... 3 of them. Car didnt smoke, but i was getting same codes... could be a possibility. Also check for vaccum leaks; anything... even faulty injectors.

10-25-2011, 12:24 PM
Today I took the Front O2 sensor out of the working donor car and put it in the 2000GAGT and no improvement in symptoms, still misfires. Also the donor car runs great with either O2 sensor in it.
I guess I am impressed with how nice the 2000GAGT (problem car) runs when it is cold, for example if it sits overnight it runs good for a few minutes but when it gets hot it starts missing bad. Its like once it is hot and using all its sensors it is acting like it is getting bad info from one.
What else could act like this?

10-25-2011, 01:47 PM
What plugs are you running?

10-25-2011, 01:57 PM
Bosch Super Plus 7980 @ 0.060"

10-25-2011, 07:21 PM
Get rid of the Bosch plugs and put some AC Delcos or NGKs in there.

That's likely your problem.

10-26-2011, 08:24 PM
Well today did a few more things. Swapped out spark plugs again to champion, still threw a P0303. Took out the front O2 sensor and ran it cause I might be building up too much back pressure but that did not improve anything either
Took a stethoscope apart and put a 3/16 aluminum tube in it with a bevel cut on the end. Listened to each fuel injector up close and personal. Each injector is clicking like a metronome with no misses, great way to tell they are getting a signal and trying to pump. Anyway with all I have done I figured it is time to look for a bad LIM gasket, bad head gasket, lifter problem, or a cracked head port so I am doing a top end teardown, I will post what I find in a day or two.

10-26-2011, 09:20 PM
Have you switched plug wires and coils yet?

10-26-2011, 09:40 PM
I did switch out the plug wires and the coils in the ICM switch, both sets of coils wires and ICM worked fine on the donor car, and the 2000 still threw a p0303 code with either set of coils wires and ICM.

10-27-2011, 09:54 AM
Found it. It has a blown LIM gasket around #3 intake port....even was one of those heavy duty LIMs with the metal frame.