What should I be looking at to diagnose my Ford Ranger which surges when stopped? This typically only happens while the vehicle is warming up. After that, it's fine.
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My first thought is to clean the throttle body and Idle Air Control (IAC) motor passages with carburetor or throttle body cleaner. They get a black buildup on the edge of the throttle plate and IAC passages that interferes with the idle speed regulation.
As Larry said, check the throttle body and the IAC, but I'd also check for vacuum leaks.