I've got a 1999 4.0 in Calif with about 216000 miles on it and it's starting to show a few problems. The one I want to mention here is
a high idle problem that will go away as soon as I shut off the engine and start it up again.
When hot, sometimes,the idle hang up at around 2000 rpms. Put in in park, shut it off and restart and its fine.
Recently...new.throttle position indicator, IAC, and MAP sensor, TB cleaned, new coolant sensor. 195 degree thermostat. Very clean, well maintained jeep but this
has got me stumped, mainly because it will restart with a good idle.
I have a smog check coming up next week and i don't need this to happen while being tested.

 

Sounds like it could be the TPS sensor, or maybe the wire leading to it? The throttle could be seen as 12% off or some other value to the computer if this sensor is not operating correctly, among many other possible s/s.

You could also check the throttle plate to make sure it is fully closing.

 

It might be a little rare to happen, but not too unusual if the throttle body throttle plate was getting hung up.. Just above idle and near 2,000 RPMs, especially when warmed up as the metals begin to expand.

The throttle plate could be rubbing on one edge, especially when it gets hot. I know a throttle plate can be removed and reinstalled, carefully centering it if this were the reason. I've seen a throttle body plate being installed on a build.. and I know this was one place we looked at checking over carefully for having correctness.

 

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Did it pass?