Engine idle surging up and down
A rhythmic rise and fall of idle speed is the ECU chasing a target it cannot hold: it sees the mixture or airflow drift, corrects, overshoots and corrects again. On most modern engines the root cause is unmetered air from a vacuum leak, a dirty throttle body or a coolant temperature reading that is wrong. A surge only when warm and in gear often points to the idle-air control or a load input such as the air-con compressor.
Urgency
Visit a workshop soon
Possible causes, most likely first
- Vacuum or intake leak after the throttle
Idle hunts and long-term fuel trim goes strongly positive (above +10%). Spray brake cleaner around the intake manifold gasket, PCV hoses and brake servo hose with the engine idling; an rpm change marks the leak. - Dirty throttle body or idle control valve
Carbon on the throttle plate edges restricts the small idle opening, so the ECU over-corrects. Clean the throttle bore and plate, then perform the throttle adaptation with a scan tool. - Faulty coolant temperature sensor
The ECU thinks the engine is colder or warmer than it is and sets the wrong idle fuel. Compare the live ECT reading with an infrared thermometer on the housing; more than 5-8 degrees difference means replace it. - PCV valve stuck open
Drawing excess crankcase air gives a lean, hunting idle and sometimes a whistle. Pinch the PCV hose briefly: if the idle steadies, the valve is the problem. - Weak or lazy lambda sensor
A slow upstream sensor makes the ECU swing rich and lean. Watch the sensor voltage: it should cross 0.45 V several times per second at idle; a sluggish trace confirms it. - Low fuel pressure or dirty injectors
Surge worsens with electrical load and when hot. Check pressure at idle and confirm it holds after the engine is switched off; a fast drop means a leaking injector or regulator. - Alternator or electrical load cycling
Idle dips and recovers in time with the cooling fan or compressor engaging. Switch off all loads; if the surge stops, check charging voltage and the ECU idle-up strategy.
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What you can check yourself
- Listen for a hiss around the intake and vacuum hoses at idle
- Switch off the air-con and all electrical loads and see whether the surge stops
- Check the air filter and that the intake duct is clipped on properly
- Note whether the surge is only when cold, only when warm, or both