Folks I'm new to the forum but have read through a bunch of posts regarding rough idle. None of the posts fit my particular issue and I put it out there to you all for your thoughts. This is a 1997 XJ 4.0 automatic. It is idling rough and will stall once in a while if you goose it in neutral or when you get off the gas like exiting the freeway in drive. This has never happened except at idle, so I don't think its the PCM. I have taken off and cleaned the IAC valve and TB, changed the spark plugs, looked for vacuum leaks. I ran a compression test today while I had the plugs out, all Cylinders were between 149-155. After I put everything back together I let it warm up then put on a vac gauge, rock steady 13 inches at idle. I would like to adjust the timing but I know that isn't an option. Any suggestions?

 

What's the idle rpm's at operating temp?

 

You should have closer to 18" of vacuum at idle. Look for a leak.

 

Yep. Since I got my IAC issue resolved my gauge now sits right at 18mmHg.

 

13 in/hg is minimum for a 4.0.

Take a look at senario 10.

http://www.secondchancegarage.com/public/186.cfm

 

I was looking for leaks by spraying carb/TB cleaner around the intake because it said highly flammable, but when I sprayed some into the air intake it stumbled instead of raced, I'll try again with starting fluid. As far as the rpm's at temp I'll have to check today. Listening at the tail pipe last night it may have an intermittant miss in one of the cylinders, I never noticed it before but under the hood the 4.0 is noisy and clanky and just never noticed it before. I'll get back into the garage this evening. Thanks for all the suggestions up to now, I will post anything new I find.