Weird idling problem

When starting my XJ will idle at 900 rpm like it should.
But lately when I shift it to drive, it will go up to 1100 rpm and it will stay there.

What kind of problem does my XJ have and how can I fix this ?

Year?

I'd likely check my exhaust/intake mounting bolts, could be a vacuum leak. The vacuum leak may be anywhere but this is all I can think of that might cause it when shifting to drive (the motor rocks).

It may be an electrical issue that is messing with your IAC, but I doubt it. Does your volt meter drop when you shift to drive?

Could be bad injector o-rings....

Since the idle bumps up once your engage it into gear (I had the same problem), the ECM may not recognize that the engine is warmed up. I had a few problems. one was that my XJ was running rich ('87) and the MAP sensor was bad, it was stalling. Replaced it, the rich problem went away, but still idled high. The IAC valve had been sticking for awhile periodically, so I changed that, no change on the problem with it in gear but it no longer sticked. Then I changed the Engine Coolant Temp sensor and the high idle in gear problem disappeared. The resistance was off on the coolant temp sensor, you can check it with a ohm meter. Mine was off a decent amount.

8Mud said:

Year?

I'd likely check my exhaust/intake mounting bolts, could be a vacuum leak. The vacuum leak may be anywhere but this is all I can think of that might cause it when shifting to drive (the motor rocks).

It may be an electrical issue that is messing with your IAC, but I doubt it. Does your volt meter drop when you shift to drive?

Click to expand...

It's a '98
My engine does make a rocking sound but only when in park.
I've diagnosed this as being my flex plate but I still have to check to be sure.

Ol' Ironsides said:

Since the idle bumps up once your engage it into gear (I had the same problem), the ECM may not recognize that the engine is warmed up. I had a few problems. one was that my XJ was running rich ('87) and the MAP sensor was bad, it was stalling. Replaced it, the rich problem went away, but still idled high. The IAC valve had been sticking for awhile periodically, so I changed that, no change on the problem with it in gear but it no longer sticked. Then I changed the Engine Coolant Temp sensor and the high idle in gear problem disappeared. The resistance was off on the coolant temp sensor, you can check it with a ohm meter. Mine was off a decent amount.

Click to expand...

This is interesting.
IMO my Jeep has been running rich also since the day I got her.
I get 8mpg while running with aircon and I'll be lucky to get 10 without aircon.
Do you still have your cat on your exhaust ?
I'm also having serious O2 sensor problems.

How do you know that your MAP sensor is stalling ? I might have the same problem.

What is the IAC valve ? And what do you mean that the gear no longer sticked ?

I will check the temp sensor.
BTW, was it before you swapped your sensor that the resistance was off ?

I know some of these questions may sound dumb but it is only my first XJ
I still have a lot to learn.

The Idle Air Control valve should be your main suspect. Clean it, if that doesn't help, replace it.

Tim_MN said:

The Idle Air Control valve should be your main suspect. Clean it, if that doesn't help, replace it.

Click to expand...

Will do, thanks.
Do you know of any high performance air valves ?