Quick rundown:
'01 Cherokee; 176,000+ miles, engine all stock

Been running fine until Saturday morning. Started it to go get some gas and noticed it idling, running a little rough. Drove 2 minutes to gas station, got gas (had about 1/8 tank, filled to 1/2), on the way back home noticeable misfire and CEL flashing (I know that means cat damage potential). Got home, shut it off for 10 minutes. Restarted, ran fine. Hooked up trailer to haul some stuff to the dump, and about 10 minutes down the road at a stop light, idling became rough and CEL flashing again. Shut down, restart, all OK. Dumped trash, on the way home, again at red light, idling became rough, CEL flashing, only 5 minutes from home so parked it in driveway.

Let engine cool, started again and at idle I noticed it was rough but no flashing CEL (steady). When I give it some gas, get to about 2000 rpm or so, then it starts flashing. Shut it off until this morning. Took it to Advanced Auto 2 minutes from home to have codes scanned. Again, idled rough with steady CEL, when rpm got above 2000, flashing began on CEL. Noticed at red light at idle, went steady, then flashed again as I began getting to speed. Misfire was noticeable.

Codes pulled:

P0203 (#3 injector)
P0300
P0303
P0304
P1494

I know the P03's are misfires (multiple and #3, #4). No P0204 to go with the P0304. The P1494 I looked up on this forum and found info- went and looked and found 90 degree boot detached on intake manifold. Reattached that.

Checked wiring on #3 fuel injector. No visible shorts or loose wiring. Cleaned connector, still misfire with flashing CEL as rpm increase. Have the heat shield, etc for the heat soak issue. Next step to replace injector? Is it worth running injector cleaner first to see if that helps? Any ideas on the P0304 with no corresponding fuel injector code?

Thanks in advance, folks.

 

When was the last time you changed your spark plugs? This just happened to me. I ended up also putting on a new coil rail but it's been on for a week now with no more misfire or rough idling.

 

Spark plugs were changed a little over a year ago. Used Champion plugs. I also just checked my plugs a month ago looking for a coolant leak and they all looked OK. BTW, I found the coolant leak and it was a hose connection.

I went ahead and got a new injector- just getting ready to put it in. I figured it's worth a shot since I'm getting the P02 code for the #3 and the wiring looks to be OK.

 

Did you verify the gap on the plugs? Mine also "looked" ok but when I brought out the feeler gauge they were way off.

 

Yep. Checked the plug gap.

I just swapped out the #3 injector for a new one and changed all the o-rings on the others to be safe. Started it in the driveway a couple of times, revved the engine- no CEL and no rough idle or vibration when revving the engine. Took it out for a 15 minute test drive from back roads to interstate and it did fine. Actually idled and had better throttle response than it has in a long time.

I also noticed something else gone that I didn't think would happen. I had been having a problem with some weird vibrations as speeds got higher (65+ especially). That even matches the pattern I had of a steady CEL at idle and a flashing CEL as rpm climber above 2000 (mentioned in my original post today). I had attributed the vibration to rotated tires after wearing too long on their original corners, because it started about that time. But just now at highway speeds 90% of the vibration was gone. Makes me wonder if the thing has been slowly going bad- not enough to throw a code, but enough to cause some poor performance in that cylinder until the injector final went kaputz.

Anyhow, seems to be OK now. I'll hold my breath for a couple more days I'm sure. I'm by no means a master mechanic, but this may be the first time I've ever over-diagnosed an XJ problem. Seems like any other time I go down a long check-list before finding the real problem (ah, the joys of owning an XJ). Any chance I fixed the symptom but there's still a problem lurking there somewhere?