Replaced IAC valve, but now a different problem

I recently replaced the Idle Air Control valve in my 2005 Buick Century because it was shutting off after starting. Now, it starts OK but after driving for about an hour the idle speed increases and also starts to race. The “check engine” light never came on while running. After arriving at work one morning I shut it off and let it rest for about 5 minutes and it re-started at regular idle speed. Did I buy a defective IAC valve? Or, could there be a problem with the Electronic Control Module?

Which one would be cheaper to replace?


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I’m wondering if a complete cleaning of throttle body, both sides of butterfly might help.

At 16 years of age it might be a good idea to check for vacuum leaks also.

When the engine idle speed is too high, wiggle the wiring near the IAC motor, if this affects the engine speed there is a break in the wiring or a poor connection in the connector.

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Could the Throttle Position Sensor be involved? It’s easy to test it with an ohmmeter.