Jeep TJ throwing P0303 code and idling rough on cold start

mudcrewsr

mudcrewsr

Member
Joined
Sep 18, 2012
Location
Wilmington NC
I've got an 2001 TJ 4.0 and within the past few months it has started idling rough on a cold start for about 10-20 seconds. Last month I drove the the Gulches in SC and it was one of the hottest days on record. Before hitting the park I stopped to fuel up and when I restarted the jeep was running rough like there was a fouled plug. I've got the coil packs not a distributor and after looking up the code I saw where it might me #3 plug fouled, bad O2 sensor or bad coil pack. I just replaced the plugs about 3 months ago but I went ahead and replaced the 3# plug. This seemed to correct the issue and I wheeled all weekend with no issues. I still have the rough idle though on a cold start (cranking it up in the morning or after sitting for a few hours. Today on the way home from work I stopped fueled up and when I restarted the engine light came on and the dash (after cycling the key) threw the same code. I saw online where there was a TSB for the TJ where the #3 injector could be getting hot and this causing the problem. The only difference I see is mine is doing it cold and today when the code was thrown it was mid 70's temp.

Any ideas or suggestions on how to troubleshoot this? My thought was to swap the plugs again (I'm running NGKs right now with a Champion on #3) to all all champion plugs and then if that doesn't work, replace the coil pack and the fix from the TSB. Last would be the O2 sensor. The only difference from the TSB, is that I don't think heat is causing the code or rough idle since it is happening on a cold start but I don't know how else to test this theory without just starting to replacing stuff.

I just crested 115k on the motor and it's running fine aside from this code (and the rough starts). Once running restarts are smooth. Fuel mileage has been less than great (10-11mpgs) but I attribute that to the 37's and I'm okay with that. However I thought a bad O2 sensor could be causing it to dump more fuel than needed.

The TSB I was referring to can be seen here: http://www.wjjeeps.com/tsb/tsb_wj_1803103.pdf

Thanks for any ideas or suggestions.
 
Regular 87 ocatane. I've read where some fuels with higher levels of ethanol can cause this but I have been using the same gas/station for well over a year. I did just add some gunk-out injector cleaner on the last tank... I thought from the idle I may of had a clogged injector or the sending unit was pushing too much fuel but otherwise I haven't switched up much over the past year.
 
Have had several come through the shop with the cyl 3 code, seems when tje coil strip fails or starts to go it often sets that code. With that milage may be due for one.
 
Thanks guys, cbjeeper.. I replaced the coil rail this past weekend, I reset the code with an OBD reader today. Got 40+ miles on it so far and no code that may have been the issue... we'll see. If not we go through the routine of testing everything else.
 
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