BMW E34 hunting or surging idle
E34 idle hunts or surges - what usually causes it?
The most common causes are a dirty or stuck idle control valve, a faulty throttle position sensor, a vacuum leak or a dirty mass air flow sensor (MAF). Check these first before replacing more expensive parts.
How do I check the idle control valve?
It is under the intake manifold. Check the electrical connector and the two air hoses, clean the valve with, for example, brake cleaner spray, and make sure it moves. Oxidised connectors and too much grease can jam the idle at high revs.
Can the throttle sensor or mass air flow sensor cause idle hunting?
Yes. If the throttle position sensor does not detect the throttle in the closed position, the engine may race at idle or the revs fall slowly. A dirty mass air flow sensor can give erratic readings; cleaning with a suitable spray helps. Unplug the connector while the engine is running: if the running does not change, the sensor is probably worn out.
What else should I check if the idle is uneven?
Look for vacuum leaks: the intake boot, vacuum hoses and intake manifold gaskets. Also check that oil is not leaking from the valve cover gasket into the spark plug wells, because that causes a misfire. Replace the spark plugs, distributor cap and rotor if needed.