2000 XJ, 4.0 with 156xxx miles.

Ive had a constant, recurring P0301 code, Cylinder 1 misfire. As well as an idle at 500 rpm. Besides the rough idle it runs perfectly fine. Ive done the screwdriver test and can feel all 6 injectors are working. I already changed the spark plugs, to Champion Coppers which didnt change anything. Ive traced all the vacuum lines i can see and theyre all plugged in. I cant seem to hear any leaks either. Ive had the code reset a couple times and it continues to come back.

From cold startup it will idle fine and smooth at ~800 rpm but once it reaches about 140* the rpms drop to 500 and thats where they stay.

Not sure what to do next or what to suspect. I was gonna do a seafoam treatment through the throttle body and see if that makes it any better. Would that be a good idea or a waste of time?

 

Probably have some carbon between the exhaust valve and it's seat.
Seafoam might work.

The only thing that fixed by buddy's was the old water in the throttle body trick.

 

What is that?

 

Here's a video on it.

 

I'd be way to scared to do that. lol

 

Im gonna do a seafoam treatment today and see how that works. Id live it if it was something simple that needed to be done lol.

Should i do it from a cold start or hot?

 

[QUOTE=Keaton;2122848]Im gonna do a seafoam treatment today and see how that works. Id live it if it was something simple that needed to be done lol.

Should i do it from a cold start or hot?[/QUOTE well at least ya got 50/50 chance that way.

 

No guts. No glory. It's an old tried and true fix. I dare ya to do it.

 

Went ahead and put some fuel system cleaner and did a can of the Seafoam Spray down the TB. After i filled up and drove with the fuel cleaner the CEL went off but the idle was at 500 rpm still. Got home, did the Seafoam and now its back on LOL. Idle is still at 500 rpms and acts exactly the same. Oh well, what to try next?

 

WATER!!!!

 

Havent had a chance to do this yet.

Yesterday when i was under the hood me and my friend were both sure we could hear a vacuum leak from the back of the engine. Whats a good way to test for vacuum leaks? Spraying water on them?

 

First off, look. There are a coupla vacuum fitting to the rear of the manifold. One on top and one on the side. Make sure they're plugged in securely.
Just for fun, see if your intake manifold bolts have loosened up.

Spraying suspected areas with throttle body cleaner with the engine idling works well for finding the vacuum leaks.

 

I did my best to look and all lines i can see and follow are connected. Ill check the bolts, i hadnt even thought of that.

 

I have had the same problem you do for about 9 months now. I have tried water, seafoam, new plugs, looked for vacuum leaks. nothing worked. Only when my check engine light comes on its always different codes, except for P0300, always have that one no matter what. Im gonna try the o2 sensors, the cps, and a new coil pack next. is your jeep running too lean?

 

EDIT: I should mention that ive owned the jeep for a year, and that whole time i would get the CEL on, but then it would turn itself off before i could get it checked. I assumed it was the heatsoak issue that occurs. Then about a month and a half ago when i left for work early one morning when it was quite cold out, i didnt let it warm up at all and i got a flashing CEL and limp mode. I drove to work with it like that and I got the CEL read and thats when i got the P0301. I know i shouldnt have driven it like it was but i didnt know what else to do. Ever since then the CEL has been with me and its had the rouogh 500 rpm idle. Before then it idled around 800 and felt way smoother.

Everytime i check my codes its always the same P0301. The only things ive done are plugs, seafoaming it and running fuel system cleaner. Ive heard 02 sensors mentioned a few times, same with cps. Maybe ill have to go that next. I know i hear a vacuum leak though so i need to track that down first. I can hear a misfire in the exhaust though when i walk behind the jeep.

If anything its running rich. I seem to be burning more gas than before i was consistantly getting the CEL.

It actually went off today when i went out to my jeep after letting it sit for a couple hours after i drove to work. Im assuming this is the heat soak issue.