2000 Pontiac GTP Miss Fires

[jdttaylor]

Please Help. I have changed the plugs to AC Replaced all 3 coils and all Plugs and Wires. I have a PO300 code multiple miss fires on multiple cylynders. It only miss fires under load like going up a hill and the super charger barely kicks in. Yet I can get on it Full Throttle and it won't skip a beat. I have had it into my local GM repair shop 5 times and they can not find the problem. Any ideas would be great.
Thanks

Josh

[BNaylor]

Welcome to AF.

Well, it looks like you have the secondary ignition suspects changed. DTC P0300 can be caused by many things. Basically crank variation caused by a misfire condition is being sensed by the PCM module.

Other possibilities are:

MAF sensor/air induction system
fuel pressure
fuel injectors
EGR valve
CAT convertor
02 sensor if missing is at idle, cruising speeds or transition from closed to open loop mode.

[DJ JEFF]

Originally Posted by bnaylor

Welcome to AF.

Well, it looks like you have the secondary ignition suspects changed. DTC P0300 can be caused by many things. Basically crank variation caused by a misfire condition is being sensed by the PCM module.

Other possibilities are:

MAF sensor/air induction system
fuel pressure
fuel injectors
EGR valve
CAT convertor
02 sensor if missing is at idle, cruising speeds or transition from closed to open loop mode.

Probably another dumb question from me but doesn't the MAF and EGR and the 02 sensors have their own fault codes? I'm just wondering, can you elaborate what you mean about the 02 sensor?
Thanks

[BNaylor]

Originally Posted by 01TahoeJunk

Probably another dumb question from me but doesn't the MAF and EGR and the 02 sensors have their own fault codes? I'm just wondering, can you elaborate what you mean about the 02 sensor?
Thanks

Yes but at times they will not generate a DTC or be the sole cause for triggering a SES/CEL Mil light.

O2 sensors degrade and as a result the recovery times leaves alot to be desired and may not generate a DTC. The fuel injection system relies on feedback from the 02 sensor (front) in PCM closed loop mode to keep fuel/air mixture ratio at the ideal 14.7:1 figure. As a result in the transition period from open loop mode to closed loop mode and vice versa an 02 sensor can be responsible for the P0300 random misfire DTC, therefore it is one of the items to suspect. Basically the fuel injection system looks at 02 for idle and cruising after engine warmup to help control emissions. When accelerating to include under load like going up a hill the 02 sensor signal is typically inhibited and other engine sensors are used like the MAF, MAP, etc.

Basically my reply above (post #2) is a recital from the FSM but in my own words so I do not violate GM copyright.

[DJ JEFF]

So in your opinion then, would you start with changing out the front 02 sensor? Isn't there 2 of them up front? I have this gentlmen's exact same issue and have gone through the same items that he has plus my fuel system and have not been able to correct the issue. (sorry don't mean to thread-jack) I have not changed the 02 sensors. Also you brought up a point about after warm up.... I've notice my 97 GTP doesn't really start doing this until after its warmed up. For the first 5 min or so of driving everything feels normal. I have been told by 2 different shops that my converter is fine. Thanks again.....

[jdttaylor]

I have had it to a reputable garage a few times. As I said before I have replaced plugs, Wires and all 3 coils. The car does not do it all the time. Only some times. Mostly going up hills when the supere charger kicks in. Are you saying that this could really just be a O2 Sensor problem? What about the PCM or the Ignignition control module. Any help is greatly appreciated. I am about ready to just give up and get rid of the car. Oh they have already checked the fuel pressure on the rails and did and injector test by meassuring the amount of fuel being used??

Thanks

[BNaylor]

Originally Posted by jdttaylor

I have had it to a reputable garage a few times. As I said before I have replaced plugs, Wires and all 3 coils. The car does not do it all the time. Only some times. Mostly going up hills when the supere charger kicks in. Are you saying that this could really just be a O2 Sensor problem? What about the PCM or the Ignignition control module. Any help is greatly appreciated. I am about ready to just give up and get rid of the car. Oh they have already checked the fuel pressure on the rails and did and injector test by meassuring the amount of fuel being used??

Thanks

The 02 sensor is one item that can cause that specific DTC. Your GM dealer or mechanic didn't have much luck 5 times with all that fancy diganostics equipment so the next step if you want to keep your GTP is to undergo a systematic troubleshooting procedure starting with the easy low cost parts or possibilities. You don't want to be throwing money away. Keep it simple and look at areas possibly overlooked.

I would get the ICM module tested but bad ICMs do not give you those symptoms. Normally you will get an ICM related DTC or no spark at all. On the PCM possible but it looks like it is working. The PCM is detecting some sort of crank variation and generating the P0300 DTC.

On the MAF thats easy disconnect it and then test driveability. Or try cleaning the wire element with MAF spray cleaner.

On 02 sensor, remove it and clean with throttle body or carb cleaner. Depending on your mileage they will get carboned up and the receptor vanes get clogged. Test output with a scanner or DMM (purple wire). Look for a varying .1 volts to .9 volts.

On EGR, remove it and clean it with carb cleaner. Make sure pintle moves freely. Clean the EGR manifold exhaust passages bewteen the exhaust and lower intake manifold.

On the CAT, maybe actual misfires that were uncorrected damaged the convertor. You can get the exhaust system and CAT tested for excessive backpressure. Sometimes a vacuum test may show a clogged CAT convertor.

On fuel injectors a balance test and/or cleaning but it looks like this was tested and passed. The injector coils can be ohmed out for proper resistance. A noid light can be rented and used at each injector to make sure the pulse and timing is correct from the PCM module.

Another suggestion is to disconnect the battery negative cable overnight and see what it does the next day.

The intermittent problems are the worse but the object is not to give up. It most likely won't fix itself. Good Luck!

[DJ JEFF]

Your awesome Bob... Thanks for the all that typing you did. We got a little more out of you then usual this time. I'm sorry for prodding but I have been battling this issue for over a month. As you probably remember. Anyway THANKS AGAIN for your advice.