Join Date: Dec 2013
Location: South Fulton, TN
JeepForum newb, 1st thread: P0300, P0301, P0303, P0304 on cold 4.0L
I apologize for the length of this post - I'm trying to provide you with as much information as possible. This one has me stumped for now.
My wife and I bought a 2003 Jeep Grand Cherokee 4x4 QT-II with 4.0L I6 new, and it now has 235K miles on it.
Around a month ago, the crankshaft position sensor went out (P0320), so I replaced it with a BWD, and this issue was resolved.
A few weeks ago, it threw P0300, P0301, P0303, P0304 codes and was severely misfiring after starting it in the cold (20 degree) weather and immediately operating it. I limped back to the house, and by then, it was warm. I killed it, then started warm, and it ran great. Looked at the plugs, and they were worn out (nearly 0.070 gap), so I pulled the cable off the battery to clear the codes and replaced the plugs with plain Autolites gapped at 0.035- ran great for two weeks, although it threw a P0456 code (fixed this with new gas cap).
Two days later, again when the weather was cold, this time in the lower 20's, it was started and operated immediately - same thing, missed severely and threw P0300, P0301, P0303, P0304 codes. Limped back home, by then it was warm. Killed it, then started it, and it's been fine ever since.
After researching, many say that I picked the wrong plugs - I should've put NGK's back in it, but I had always run Autolites in old technology engines. Even with Autolites rather than NGKs, I wouldn't think that Autolites at 0.35 would be no better than old NGKs burned out to nearly 0.070 and cause the exact same problem as the worn out plugs. I don't mind putting new NGK's back in it, but its just hard for me to believe that new Autolites at 0.035 are no better in a 4.0L Jeep that old NGK's worn to nearly 0.070. If that fixes it, though, I'll gladly admit that I'm wrong - it won't be the first time that I've been wrong, not even the first time today.
I know that some say to warm one up, some say not to - I've always liked warm a cold engine up before operating, but sometimes I just can't - I have to go, and when doing so, it should not start misfiring severely and throw misfire codes if everything is working properly.
The problem seems to be cold weather related, but I'm having a hard time pinpointing the problem. It has performed flawlessly when allowed to warm up in cold temperature.
I've read that it could be engine coolant temperature sensor (?), oxygen sensor, bad or dirty injectors, bad coils, carbon deposits on the valve stems, or a wire in the harness for the coil packs being rubbed on a rear driver side head bolt to the point of shorting.
Both times that it's acted up, it's been problems with the same cylinders. This is my wife's daily driver, and she still gets 18-22 mpg in it. I just need for it to be absolutely reliable for her and our kids. Any suggestions? Thank you.