HIGH idle. (im stumped)

when my XJ is at a cool temp the idle will be around 800. somewhat normal correct? then as i start driving it does not want to stay there. it will start to idle aroud 2,000 rpm. If i have it in gear it will go back down to normal. it is only high when i am in park or neutral. If i happen to shut it off for any anount of time, then start it back up it will always be idleing around 2,000. i just want to get it fixed so i can have some fun again at the rubicon. also if it is warm with a high idle and i try to put it in 4LO it will act like it wants to start going then it will start dieing like it is starving of fuel. Does anyone know what the heck this might be. thanks for your time.

Smittty9785 said:

when my XJ is at a cool temp the idle will be around 800. somewhat normal correct? then as i start driving it does not want to stay there. it will start to idle aroud 2,000 rpm.

Click to expand...

Dunno what year your XJ is, but things to check (this is based on going through a similar problem with a friend's '89 Limited a few weeks back) in a quasi-order of difficulty:

- Condition of PCV hoses and grommets
- IAC and IAS
- Valve cover bolts (make sure they're torqued down, this was a large part of the problem on that XJ)
- EGR valve
- Main vacuum line to vacuum bottle

His Jeep still idles high (usually around 900RPM or so), but this brought it back down from gradually creeping up to 2250. Current suspects on what's causing the remaining issue are the manifold gaskets - everything else is OK at this point, and there is some noise from them suggesting replacements may be needed.

FWIW, it should normally idle between 650 and 750RPM.

I would add the following:

- Clean the electrical contacts of your throttle position sensor (TPS)
- Clean the inside of your throttle body.

Assuming 4.0 engine, most likely cause is faulty TPS. I'm on my third since new on the 1990, still original on the 2001. You can test with a multimeter- the search function should pull up the procedure. Up to 1990 they're adjustable and must be calibrated, 1991 and later no calibration needed. Next likely should be vacuum leak- check the lines, especially around the battery, and the accumulator (blimp) in the front bumper passenger side for cracks. Third on the list, IAC faulty, not so likley based on my experience (only repalced one over 3 XJs). The EGR is only found on 4.0 engines up to 1990, and when it fails (sticks opemn) you get low idle or stalling, not high idle (but the engine runs normally at higher rpms, where the EGR would normally open).