BMW E28 idle hunts, stays high or stalls
The E28 idle hunts or the engine stalls – what is the fault?
The most common cause is an intake leak: a cracked vacuum hose, breather hose, intake manifold gasket, or the rubber boot between the air flow meter and the throttle body. A small leak is enough to cause trouble, and you will not find it by squeezing the hoses. Old vacuum hoses are worth replacing.
How do you find an intake leak?
With the engine idling, spray starting fluid on the suspected spots. If the revs change, the spray is being sucked in at the leak. Also check the large rubber piece on the intake manifold and the base of the air flow meter.
Why is the E28 idle high or rising by itself?
The cause can be an idle control valve that does not close fully when hot. The valve is worth removing and cleaning or replacing; replacement is recommended if it is at least 15 years old. Idle is adjusted with the idle screw on the throttle body, not with the throttle cable.
What else can be behind a hunting idle?
A faulty air flow meter, a sticking centrifugal governor in the distributor, or a broken diaphragm in the distributor's vacuum regulator. Valve adjustment is also worth doing if the idle hunts especially when cold.