I have a 2001 Oldsmobile Aurora that has a high idle; had diagnostic done, says to replace PCM. Replaced PCM after having programmed by Chevy dealership in town, and it still has a high idle of 1100 RPMs. This PCM operates just like the one that was taken off. When the IAC was replaced, RPM jumped to 2500 RPMs and wouldn't come down. Put back on the IAC that was on the car originally. Idle was set to 500 RPMs.

No clue, either, so to start I would check the validity of the wiring from the computer to the diagnostic connector. Find a wiring diagram and get out the old trusty ohmmeter. You know where the computer and the diagnostic connector are.
Take off the negative battery cable. Pull the PCM connector off. From the wiring diagram you know what pins on the PCM connector go to the diagnostic connector. Now, one ohmmeter lead on the diag. conn. and the other meter lead on the PCM connector-turn the meter to ohms and see if very little resistance is in that wire. Resistance to current flow means very high ohms. A short is a break in the wire, and the ohmmeter will show infinite resistance. A wire still connected by two little strands with the others broken, will show very high resistance, just not infinite.
Check all the wires going to the diagnostic connector in the same way. If all wires are good, check the computer is getting fully powered. Is check engine light on with key on? Is your diagnostic tool working okay on other cars?

For engine misfiring problem check the help links mentioned below. Click the link below to go through the troubleshooting procedure :---- Engine misses and truck jumps? Hard to down shift? Issue with high gear only? http://technoanswers.blogspot.in/2012/02/engine-miss-and-truck-jumps-hard-to.html

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Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Engine Misfire codes for car?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html
Engine Misfire codes?

http://schematicsdiagram.blogspot.com/2011/12/engine-misfire-codes-on-chevrolet.html

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Misfire on cylinder 4?

http://schematicsdiagram.blogspot.com/2011/12/misfire-on-cylinder-4.html

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Oil on spark plugs and car misfires?

http://technoanswers.blogspot.in/2012/02/oil-on-spark-plugs-and-car-misfires.html

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This will help.

Thanks.

Radiator is restricted, flush or replace.

Disconnect Battery for 30 Minutes to Be Sure all Residual oltage is Gone as ECM/PCM/PTCM Operate off of only a 5 Volt Sygnal, and There Should Be NO ADJUSTMENTS on IAC Valve, Be SURE OF Proper SEALING of it Also.

problem with throttle position sensor maybe..... engine light on? Leaking manifold gasket??

if you dont get an engine light on the iddle control valve(its on the intake manifold)is to blame for this kind of problem, do a resistance check using a Fluke Meter(you can get a cheap digital muntimeter at Harbor Freight set it to the horse shoe then do the reading) if you get an "OUT OF LIMIT" reading it needs to be replaced.

You didn't mention replacing your water pump....that is what it is if you have NOT replaced it.

Sounds like your oil pump pressure relief valve is stuck ?

first, make sure the bulbs are in good and snug, 2nd, make sure the wire harnase is plugged in snug.

Call autozone and ask them to define the codes for ya then get with your mechanic and get em fixed.