An engine that surges or revs without you touching the pedal is almost always a vacuum leak, dirty idle air control, or a sticking throttle. Here are the most likely causes ranked by how often they turn out to be the problem.
A cracked intake hose or PCV grommet lets unmetered air in. The ECM adds fuel to compensate, then drops it, causing surging.
Carbon buildup at the throttle plate makes the ECM hunt for correct idle. $10 throttle body cleaner and a rag often fixes it.
A dirty mass air flow sensor reports wrong air volume, so the ECM cycles between rich and lean. Clean with MAF cleaner first.
On cable-throttle cars, a sticky cable holds revs up. On drive-by-wire, a failing pedal sensor can cause similar erratic revs.
Older cars use an IAC stepper motor to set idle. When it fails, idle hunts or stalls.
The throttle is stuck high and the car wants to surge forward, the check engine light is flashing, or the engine is revving so high it sounds like it will damage itself. Shift to neutral, brake firmly, and pull over. A stuck throttle is dangerous - turn the engine off if you cannot slow it.
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Idle hunting almost always means a vacuum leak or a dirty throttle body / MAF sensor. The ECM is correcting back and forth trying to find correct idle.
Yes. Carbon buildup at the throttle plate confuses the ECM about the actual airflow at closed throttle, so it overshoots.
Throttle body clean: $0-$30 DIY. Vacuum hose: $20-$80. MAF or IAC replacement: $150-$500.
Mild surging at idle is annoying but safe. A throttle stuck open is dangerous - shift to neutral, brake, and stop the engine.
Yes, on most cars. Disconnect for 10 minutes, then reconnect. Drive 10-15 minutes to let the ECM relearn idle.