'00 Cherokee Sport 4.0 high idle
 

I have a 2000 Cherokee Sport, I6. It was drinking coolant like hell, found out I had to change out the head gasket. Did all of that, replaced every other gasket I could find (head, valve cover, thermostat, and intake/exhaust). Changed the oil, new spark plugs, everything.

After getting it all back together, I'm having an issue: a rough idle in the 5k rpm range. I start it and see the rpm gauge shoot up to 5k, then it wobbles between about 4.5k and 5k. I tried disconnecting the TPS and IAC from the throttle body separately and doing a test start, but saw no change. After that, I replaced the AIC (it was stuck in the full closed position) with no noticeable change.

I know I'm missing something, but none of my research is leading my anywhere. I made sure that the intake manifold was properly torqued, every wire and cable and line is properly placed. How do I fix this? I need to finish my school registration, can't get that done until my vehicle is up and running again.

There is a vacuum leak somewhere. I would bet the intake manifold is not mated properly. If it is not then torquing the bolts wouldnt fix that, it would still be crooked. It might be very hard or impossible to actually see

Could be the throttle body gasket(assuming you replaced it)

could be the line to the brake booster

I highly advise you to not keep starting it and letting it run at high rpms like that.

That was my first thought, so I have pulled off the intake manifold and checked that it is seated properly before bolting it in place.

I did replace that gasket, not seeing any leakage there.

​​​​​​​I'll check that shortly. I'm worried that the booster itself is the issue. Some of the research I've done suggests that is a possible issue. No idea how it could, since I never touched it when changing out the head gasket.

​​​​​​​I don't let it run for more than about five seconds. Long enough to hear it rev up and then I shut it down.

How?

Engine was practically drinking coolant. 2 gallons a month. No other evidence of a coolant leak.

Did you perform a pre-tear down compression/cylinder leakage test? Was the cylinder head inspected to ensure it wasn't FUBAR?

I’m betting on a FUBAR’d 0331.