'86 533i surge question

[Dash01]

Brother has '86 533i which surges after warmup.

It starts and warms up normally, but then after warmup it idles at ~1,000 rpm, then momentarily surges to ~1,200 rpm, then back to ~1,000 rpm, continuous pattern. Slight clicking sound from the electric valve between the intake manifold and vapor canister.

Got suggestions as to diagnosis and cure?

[Bellicose Right Winger]

Is this a UK car? A 533i wasn't offered in US in '86, so this may not help.

The clicking from the solenoid valve is likely a consequence of the hunting idle. This valve opens/closes at an RPM just above idle.

Is the AC compressor cycling?

Solve the fast idle problem and hunting might go away. Should idle at ~850 rpm. Check throttle position switch adjustment. Engine won't idle correctly if TPS doesn't see an idle postion. Check vacuum hoses to fuel pressure regulator, idle control valve, evap fuel cannister, vacuum hose to interior temp sensor. Check idle control valve. Does it have a bypass screw adjustment? Count the turns it take to close it completely and record. Close completely and open 1/2 turn only. Remove idle control valve and clean w/ carb cleaner.

My US spec '83 533i had a similar problem. My idle speed varied intermittently a hundred rpm or so, but didn't hunt. One stop light idle would be normal, at the next it would be 100 rpm high. There is a temp switch in the thermostat housing that turns off the fast idle as soon as coolant gets to ~40C. As I recall it's the sensor with the (2) separate spade connectors. The (2) terminals on the switch had rivetted connections. The rivets had worked loose creating an intermittent connection. I was able to repair the connections by soldering.

Paul




Brother has '86 533i which surges after warmup.

It starts and warms up normally, but then after warmup it idles at ~1,000 rpm, then momentarily surges to ~1,200 rpm, then back to ~1,000 rpm, continuous pattern. Slight clicking sound from the electric valve between the intake manifold and vapor canister.

Got suggestions as to diagnosis and cure?

[Dash01]

Thanks, Paul, my bro appreciates the insight and advice. Turns out, his 533i is an '84.