The TBI gasket was a good place to start, but not the only thing that could
give you a high idle. The next thing is the IAC (Idle Air Control) valve.
The IAC allow air to bypass around the main air flow path when at a zero
throttle condition - it's position is controlled by the ECM depending on a
number of factors, which in turns adjusts your idle up or down. On a motor
that old, it probably has some crud on the pintle or valve seat not allowing
it to seal completely, giving you a high idle. Either that or it's worn and
won't seat, another age related condition. A new IAC valve will run about
$60.00 and take around 20 minutes to install. Remember to clean the seat in
the TBI before installing the new IAC valve.
After the IAC valve, check your coolant temperature sensor. A faulty sensor
giving an abnormally low reading will make the ECM think the motor isn't
warmed up yet causing it to idle high.
Good luck - Jonathan
Jonathan A. Race
Lieutenant, EMS Supervisor
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