1997 jeep grand cherokee with 4.0L engine idles fast (1500-2000rpm)

It likely has EFI (electronic fuel injection) and an idle air control valve that regulate idle speed. They can get gummed and fail to have a fast idle when cold, or get stuck the other way, and have a too fast idle all the or regularly take its time slowing down after you take your foot off the gas. An engine temp sensor can also cause a lingering high idle if it reports the engine as still being cold after warm up.

its most likely a bad sensor. there are Many! have the dealer or autoparts store run a diagnostic.

Code 12 = Recently disconnected battery

Code 14 = Coolant sensor high resistance or shorted

Code 43 = Knock sensor or Electronic spark control circuit fault

Code 55 End read code.

Hope helps (remember rated this).

Change fuel filter and air filter. If that doesn;t help you may have a fuel transfer pump pressure problem.

Most of the time an engine is supposed to idle at 5-600 RPM in Driveif it's not you can up the idle. But you also may have a vaccume leak.

check for air leak in the air intake.

If you have the V8 engine, the filter is actually part of the fuel pump. WHICH IS IN THE TANK

I don't think the PCM is your problem, is the check engine light on? It could be you idle air control motor this is located on the throttle body, but because this happens while driving I am thinking you will need to replace your crank shaft position sensor it is located on the transmissinon bell housing it is very hard to replace without the correct tools. If the check engine light is on take it to a local auto parts store and ask if they can check the check engine codes for you they usally will do this for free.

check for vacuum leaks make sure throttle bodygasket is not broke and inplace and check fuel pump pressure.

Replace the throttle postioning sensor, and mass air flow sensor