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high idle 700 1988

posted by  someone claiming to be pete on  Sat Oct 6 07:51 CST 2001 [RELATED]

Here's another high idle question. My '88 740 turbo suddenly started idling high. It starts at 1500 rpm when cold and increases to 2000+ when warm. When the throttle is revved and released it drops below 1000 rpm then quickly pulls up as if some control is activating. Engine vaccuum at 2000 rpm is about 18 psi and very steady. I checked for vaccuum leaks and replaced one bad hose that went to a control mounted on the turbo (you can tell I'm not that familiar with the "newfangled" engine -- I've been a 122 guy since 1963). The engine speeds up when vaccuum hoses are removed. It slows down when the hose from the crankcase vent to the intake manifold is removed and blocked. I wiggled the injectors and pulled on the intake manifold to test for a leaking gasket. None of these things changed idle speed.

So here are the key facts as far as I can tell:

(1) It happened suddenly, as if something broke.

(2) The vaccuum system seems to be functioning properly.

(3) Idle speed increases as the engine warms up.

On another note, what book should I get to help with diagnosis and troubleshooting, tuneup procedures, stuff like that? I've got the Haines manual, but it just tells how to take the thing apart.

Thanks for the help.





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