I have a 99 xj with a 4.0 was told motor and tranny have 175k miles. Ive posted about this issue before but my jeep started having a fluacting idle when cold then once it warmed up itd go away. Now its doing it constantly and going down the road sometimes it feels like the tranny is slipping which it isnt. Ive cleaned tb and iac used contact cleaner on the sensors and plugs today which has made things worse it has new champion copper plugs in it and isnt throwing any codes. I dont want to buy sensors if i dont have to. Itll almost stall in drive idling but when put it in park itll idle at 1200 rpm any ideas what it is

 

Whats the fuel pressure in psi?

 

Ill have to find someone with a pressure gauge as i dont have one

 

Drove home from work tonight and it didnt sputter like did on my way to work but in drive idle was normal put it in park and it went up to 1200 rpm. Put it back in drive and they went back down

 

Drove jeep to work today and ran fine although when first started it idle started to fluctuate so i hit throttle then it settled down. Got to work idle was fine in drive, no fluctuation put it in park and it idled normal and not at 1200 rpm. Im wondering if theres a loose connection somewhere.

 

Check for vacuum leaks. You can do this by spraying brake cleaner around the engine at likely places for leaks. Do it while it's idling. If the engine RPM changes, you found a leak. Plug it with bubble gum, and continue looking for leaks.

(I'm kidding about the bubble gum.)

 

Checked for vaccum leaks and idle never changed while spraying. I did notice that if i tapped on iac or tps with jeep running idle would change.

 

Here's how to test the TPS. It's an easy test with a voltmeter.

The IAC is likely gunked up. Get some throttle body cleaner, some Q-tips and pipe cleaners, and clean it up. Can't hurt, might help. Probably needs it.

 

Ive already cleaned iac and throttle body with an old tooth brush and cleaner but ill check on the tps. Would either of them make rpm chsnge from tapping on them though.