Idle Fluctuations on 928S

Idle Fluctuations on 928S

The following started yesterday morning in a traffic jam on the M25 (where else!) This is an intermittent fault which doesn't seem to be affected by engine temperature, after the initial warm up.

Engine was at normal working temperature and there were no other 'side-effects' The engine will not hold a steady tick over, varying between 300? and 750? - difficult to tell with the standard tacho. The revs rise to the higher level, then fall back to the lower, then rise again.

The engine was tuned about 2000 miles ago and has run perfectly up until now.

Blipping the throttle sometimes clears to a steady idle again. Apart from this, the engine starts, warms up, and runs fine.

Does anybody else have any experience of this kind of behaviour?


Jon Jeffryes
85 928 Manual

Your '85 model has an "idle speed charging control", or idle stabilizer. It may have failed, or be failing, or there may be a bad electrical connection.

Look at the top front of the engine, just under the cross-brace, and find a 50 mm round electrical connector under a cover plate, There are three connectors, at 11:00 o'clock, 1:00 o'clock and 6:00 o'clock. Temporarily connect the points at 1:00 and 6:00 o'clock with an electrical jumper, and try the idle. The engine should idle at 660 - 700 RPM. If the idle speed is far off, the idle control screw (bypass) is on the throttle housing. Go in with a long screwdriver between the second and third intake tubes, just on the right side of center. If you can't tell any difference with the connectors jumped, your idle unit is bad, or there is a bad connection in the system.

If you can tell a difference with the connectors jumped, but you still have idle problems, start looking at vacuum leaks - perhaps a failed hose.

Wally Plumley
928 Specialists

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