Greetings all,

2001 Cherokee Sport / 114k miles / 2WD / clean and well maintained

My XJ typically runs as smooth as silk. A very reliable daily driver. But recently it's developed a rough idle when the A/C is on (and here in Central Texas – that's pretty much all the time!).

It's thrown a P0304 misfire a few times – but only because the A/C was on. One of the times, I got the flashing engine light, which immediately stopped (and went to the non-flashing light) when I turned the A/C off. After clearing the code, I can drive without using the A/C and all is completely normal. Turning on the A/C gives me the rough idle, and at some point during that day, the check engine light is sure to come on.

I've looked for obvious things like an exposed wire shorting out on something, but everything appears to be where it should be. The throttle body is very clean. The plugs and coil pack have only about 15k miles on them.

I didn't find anything (about this specific A/C-related issue) in a search. Have any of you experienced a problem like this? Any advice would be greatly appreciated!

 

Remove the AC Compressor clutch connector and see if this still happens.

 

Would this be the wiring harness that clips into the compressor? Would you perhaps have a photo to point it out?

And just to clarify – the A/C is working. I think it needs a charge, but it does blow cold – even with this rough idle issue.

 

Here's a pic of a '99 (similar to the '00/'01) AC compressor walk around. You can see the Grey AC Compressor clutch connector.


Let's just see if the AC compressor has anything to do with the running rough. Simple enough task to unplug the connector. If there's no difference, plug it back in.





Simple stuff first.


There may also be an issue with the Idle Air Control valve. The PCM is supposed to kick the RPM up a bit when AC is selected. If the IAC pintle is dirty/sticky, it may not be responding to the PCM signal.






.

 

Many thanks, CCKen. I'll give this a go today after work.

 

Update: I unplugged the compressor clutch power connector and drove around the block with the air on. Perfect idle. Plugged it back in and the rough idle was back.

Let me add a bit more detail to what I mentioned earlier. A few days ago I was sitting still at a light with the A/C blowing. The idle was rough, but not about to die, and no engine light was on. I accelerated away from the light. When I reached about 40 mph I noticed the check engine light was flashing. I turned the A/C off and the flashing immediately stopped. The code was P0304. After clearing the code, I was able to drive and use the A/C for a couple more days before the code came back.

There's something about having the A/C on that's pulling too much of a load and causing the misfire. I'm starting to think it might be triggered more easily when I accelerate hard.

What's next?

 

It could be border line lean condition in one or more of the cylinders in bank 2 (4,5,6) caused by a vacuum leak or bad fuel injector and the PCM is responding to the 2/1 upstream O2S by adding more fuel to compensate for the lean condition, and the additional fuel remains unburned and is dumped into the CAT, causing the flashing CEL.


You'll need a scanner that shows live data so you can look at the Short Term Fuel Trim from O2S 2/1. A negative (-) fuel trim number is lean and a positive fuel trim (no - ) is rich. See what happens to the STFT with AC and without AC. Check at idle and at around 2,500 RPM.


#3 cylinder may be the only cylinder operating normally in that bank and when additional fuel is added it overwhelms #3 causing a misfire code in that cylinder only.


Why turning on the AC would cause an additional load on the engine to cause this is unknown to me because that additional load is minor (if the IAC is bumping the RPM up a bit), unless there's an issue with the compressor clutch wiring causing an upset in the engine management sensor(s) control circuits - think MAP here.

 

Thanks, CCKen, for investing so much time answering my questions!

I spent 30 years driving and restoring 65/66 Mustangs, so carburetors and manual chokes make sense to me. Scanners and O2 sensors are from another planet! I may need to pass this off to a pro.

 

Let us know what it turned out to be the problem.