Just sounds like the throttle body valve is not closing all the way, possibly stuck a bit from some type of build-up, it may need a clean
if this doesn't work you should be able to adjust the idle manually by taking a screwdriver and backing down the idler screw which is located on the throttle body that closes the butterfly valve inside
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Try cleaning the throttle valve and the idle speed control air bypass valve. Only use throttle valve spray cleaner and harsh abrasives, soak them and clean with the throttle valve with a soft cloth. Soak the idle speed control valve for an hour and then blow it out with compressed air.
It could be a defective sensor or just a sludged up throttle valve and automatic idle speed control valve, you will need to test the electronics for fault codes and clean the intake system, the web link below will show you how to draw out codes using the check engine light, after that is a procedure to clean the throttle valve and idle speed control valve.
Go here to find out how to get the fault codes: http://www.extreme-check-engine-light-codes.com/GM%20OBD1%20Decoder.htm
Here is the cleaning procedure: Get a can of intake cleaner (ABOUT $5.00) from any local auto parts store like NAPA or Carquest, do not use carburetor spray, intake cleaner only or u will damage the Teflon coating inside the throttle valve and air bypass valve, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.
on the s series it was very common for carbon to build up inside the idle air control valve-IAC valve.there are too expensive and are easy to fix-buy the part so you know what is looks like the look at your throttle body(follow your air filter box tube to the top of the engine-the throttle body)the iac is right there.
The Idle Air Control valve is located on the throttle body. If you follow the intake tube from your air filter all the way back to the intake manifold, where the intake tube connects to is the throttle body. Remove the intake tubing and you can see the throttle body.
There are two sensors on the throttle body and a vacuum line. You'll see two harnesses going to the throttle body, one is the IAC.
However, a quick diagnostic check can be done without touching the throttle body to confirm your problem. If the car has a normal idle when initially started but then rises to a higher idle and stays there BUT then if you were to shut the car off and restart it and it comes back to normal that is the Throttle Position Sensor (TPS). This is also located on the throttle body (the other sensor to the IAC). If it is the IAC it will be high and maybe fluctuate a bit.
Sounds like a dirty IAC (Idle Air Control) valve. It is located on the throttle body. Remove it and clean it throughly as well as the throttle body underside as well where a lot of carbon build up is.
However, since it is such a high idle, check for any vacuum leaks. Check your hoses and lines running inside the engine bay, specifically near the intake. Spray some carb cleaner around while the engine is running to see if you can hear or see a leak. Check the intake gasket as well.