Q: Erratic RPM when car is at operating temp 210-220

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98 Jeep Cherokee. 4.0 liter. Automatic. When car gets to operating temp 210-220 RPM's start erratically jumping- mostly at idle, but will do it when taking off from a stop. Only does this after reaching operating temp. Replaced air idle control valve but problem continues. Any suggestions?

My car has 200000 miles.
My car has an automatic transmission.

A: When the vehicle is cold the computer will ...

When the vehicle is cold the computer will run the vehicle richer than when warmed up. The engine may be running too lean once it is warmed up and causing the misfire. The fuel injection system or ignition system may need to be serviced. The injectors may be dirty, the spark plugs may need to be replaced along with plug wires. The car may also be running to rich when it's warmed up due to a failed coolant temperature sensor. Have these items checked and the computer scanned to see if the O2 sensor is working correctly once the vehicle is warmed up, and if the coolant temperature sensor is working properly. The oxygen sensor should go into closed loop fuel control, and the coolant temperature sensor should be indicating that it is at a warm operating temperature. If you don't have access to a capable scan tool, you can have a qualified technician, such as one from YourMechanic, come to your home or work and diagnose your rough idle.

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