1992 BMW 525i


I am wondering what would cause the idle on a 1992 bmw 525i to drastically rise and fall continuously. Any and all comments would be helpful. Thank you very much.

Air leak. Most likely a split in the tube between the mass airflow sensor and the throttle body, but could also be multiple leaks at the intake manifold gasket or a leak in some central vacuum hose.

Less likely but also possible is a leak in the exhaust manifold that is letting in air and confusing the oxygen sensor.

I’m going to go with a vacuum leak as the most likely suspect. After 20 years, the hoses are probably brittle, so your odds of it being a vacuum leak are good. Finding it could be done by spraying ether (starting fluid) in suspect areas until you find the spot that affects the idle, but to be honest vacuum hose is dirt-cheap by the foot and it may be easier to just replace all teh vacuum lines… ONE BY ONE, so’s you don;t cross them up. You’ll either solve the problem or eliminate the vacuum hoses as potential candidates.

Last car I saw with this (surging idle) had a stuck open thermostat.
That was a 1985 fuel injected Accord.

I agree that a vacuum leak is the most likely cause.

However, if this Beemer is carbureted, rather than fuel-injected, there is also a potential for this to be a carburetor problem. The high-tech feedback carburetors that were used by some manufacturers before they adopted fuel injection were…problematic.

Just about positive it’s fuel injected.