Chasing a misfire...

[hectikart]

So, the jeep started idling rough and misfiring when I started it up yesterday morning. Ran the codes and it's just giving me a generic P0300 random/multiple misfire. Coil packs 1-5 are almost brand new. (they only had 5 in stock when I did the coil packs) I've just been waiting for #6 to go, so I saved the old, but still functioning coil packs when I replaced them.

So, the first thing I did, was swap #6 with one of the old coil packs...no change, same code. The spark plugs only have 20k on them, so I doubt that's the problem; although they were changed by a shop while I was having a valve job/upper engine rebuild. I may pull a coil to make sure they used the right type...but it was my regular shop, and they have a good reputation, so I would be very surprised if they screwed up the coils.

I did just fill it up with gas the night before this started....I'm wondering if it is as simple as bad gas....some dry gas will probably be my nest step.

Also thinking possible ASD relay.

Any other suggestions to help diagnose the problem? I'd like to try to avoid just throwing parts at it until I find the issue.

'06 KJ sport 3.7L, automatic, 107,000 miles
also has a P0038 o2 sensor code....I doubt it's related as that has been there for a while, I just haven't gotten around to changing it out yet.

Thanks

 

[streetglideok]

Don't assume because you haven't replaced #6 coil, your issue is there. You have a P0300, which means more than one cylinder has been misfiring. 20k miles on plugs, the plug life is 2/3 gone. You also have a p0038, B1S2 oxygen sensor code, most likely a faulty heater in the sensor. You might add some HEET to the tank and see if that helps. Also, replace the plugs and inspect them while at it. NGK copper plugs are the OE plugs for it. Always verify the gap of each plug before installing it. Spec is on the emissions sticker. You may also want to attach a vacuum gauge to the hose going to the brake booster and look for flutter when you start the engine up. Anything past that, you are likely going to need a capable scan tool to help pin down what cylinders are randomly misfiring.

 

[Snail Farmer]

I would bet money the O2 sensor is related. Replace them with NTK only. I had this problem and I did throw parts at it. O2 sensors fixed it.

 

[hectikart]

Thanks for the ideas. I did the o2 sensor first since it needed to be changed anyway. That didn't solve the misfire, but it resolve the P0038 code and gave me a new misfire code...P0301. Once I had cylinder 1 pinpointed, I swapped that coil pack with an old one and that fixed the misfire.

That coil pack had less than 2000 miles on it so I brought it back to the store and they exchanged it.

 

[streetglideok]

Parts store electrical items tend to be subpar.

 

[Porkchop]

Replace with part of well known quality and new parts are not of well known quality. Book 101. This is why I have many used parts of well know quality sitting on a shelf. Glad to hear you have it resolved.