I have a 2001 jeep Cherokee sport with a 4.0 straight 6 that I got off a family member. It had been sitting for 5 years after over heating and wrecking the motor. Finally got a new motor in it and new radiator, water pump and all that. And tried starting it and when it starts it runs smooth but it idles at a really high RPM almost like you had the foot on the gas.
I've changed the starter, fuel injectors, and the fuel pump as well. And ideas on what the problem could be?

 

How about the Idle Air Control valve on the throttle body? Clean that off with some carb/throttle cleaner

 

I ended up replacing that too because it seemed to be stuck and I guy I was working with thought he accidentally broke it so he bought me a new one to be sure.

 

How high are the RPMs? I kind of have the same thing going on and am now replacing a dead O2 sensor. Loads of info on this site for driveway mechanic like myself.

 

I honestly have no idea. I usually shut it off pretty fast because I don't really feel like paying for a new motor. It sounds like you're flooring it.

 

I've goofed up plenty of times by over looking the obvious. How's the throttle area? Butterfly closed and nothing in the way of the linkages?

 

A mechanic friend who took a look at it keeps trying to tell me the timing is 180 off but that doesn't seem right to me for the problem we're having. Does that sound right?

 

When I replaced the exhaust manifold on my '99, I had t also take off the intake. I had the same problem until I realized, the intake manifold was not lined up correctly. If felt like it was on correctly and on the little locating dowels, but it was sitting on them instead of directly over the holes. Eyeballing it from the top, it appeared to look correct. If you removed/replaced the intake manifold, it would be worth a check. The large air leak causes it to rev really high.

 

If it idles really high, it MUST get that air from somewhere. Regardless of timing. So that air is likely coming from 1 of 3 possible places.

1. The IAC
2. An opened butterfly in the throttle body
3. A vacuum leak at the intake manifold or throttle body

More air (mechanical) = more fuel (computer) = more RPM.

Try ruling out each of these as a possible source of unwanted air.

 

That fixed the problem. I took the in track manifold off and the doles weren't in right. After some trail and error we finally got it in right. And after that it started running fine.
Thanks.