BMW E30 idle hunting and vacuum leaks
E30 idle hunts or surges when warm – what causes it?
The most common cause is a vacuum leak: the engine gets air that the air flow meter does not measure, and the mixture goes too lean. Other causes are a sticking idle control valve, a worn air flow meter and a leaking brake booster diaphragm. Fix the leaks; do not adjust the air flow meter.
How do I find a vacuum leak on an E30?
With the engine running, spray starting spray or pilot fluid next to the intake-side hoses and joints and watch the revs. When the spray hits the leak, the revs change. The most common spots are the bellows rubber between the air cleaner and the throttle body, vacuum hoses, the brake booster connection, the breather and the intake manifold gaskets.
Where is the E30 idle motor or idle control valve and how does it work?
On the M20 Motronic, the idle motor is in the rubber pipe between the air flow meter and the throttle, next to the intake manifold. On the M20 L-Jetronic, the idle control valve is under the intake manifold: it is open when cold, and a wax element closes it when warm. A stuck valve can be removed and cleaned.