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Variability in '87 740 idle speed[ALL/1988] posted by R. Abel on
Friday, 30 January 1998, at 11:29 a.m.
For a "constant idle system" the idle speed of my '87 740 B230F with
113k miles seems to vary more than it should. When at it's low point the
engine (door panels, etc.) has a pretty decent vibration, and when at it's
high point it's smooth. This happens only when in gear (auto trans.) at idle.
It almost seems to cycle in about 5 second intervals. Runs great otherwise (with
the only exception of stalling when in gear when cold).
Plugs are fine, plug wires, cap, rotor, air filter, fuel
filter are new. I bought the vehicle with 112k; reportedly it had a new
timing belt at 100k. Setting the timing seemed to help initially, but now the
symptoms are back. Suggestions appreciated.
Re: Variability in '87 740 idle speed[ALL/1988] posted by Don Willson on
Saturday, 31 January 1998, at 12:09 a.m.
Clean the throttle body. Check that the constant idle valve is working properly. If is stumbles when the engine is cold, not necessarly the weather, you might need a cold start module. It effectively changes the resistance of the temperature sensor so the computer thinks it is 10 to 20 degrees colder than it actually is. This causes the computer ti give it more gas and helps relieve the cold start problem.
Re: Variability in '87 740 idle speed[ALL/1988] posted by Randy Starkie on
Saturday, 31 January 1998, at 6:00 a.m.
It wouldn't hurt to clean the throttle body. I've cured similiar problems several times by removing and cleaning the throttle body. It can get pretty builtup with "crud". Disconnect the electrical connector, the throttle linkage, and hoses. Remove the three nuts holding it to the intake. Bentley's warns against spraying carb cleaner into the body for fear of damaging the switch, but if you are careful how you apply it the cleaner will dry the accumulated sometimes sticky deposits and make them earier to scrape off. A small brass brush or toothbrush (make sure the toothbrush is compatible with the cleaner !) will help in scrubbing it clean. Make sure the pathways from the hoses are clean and open- one is extremely small, I use a parts tag wire to get it open. Remove the small cap from the body and clean that out as well. The more miles your car has it the more often it will be necessary to clean the throttle body.
Randy Starkie
Hamilton, Ohio
Re: Variability in '87 740 idle speed[ALL/1988] posted by Lee Lowrey on
Sunday, 1 February 1998, at 7:39 p.m.
Identical symptoms in the wife's '89 744GL (130k miles)were cured with a new idle air control valve -- the old one was shot and could not be cleaned enough to salvage. When you clean out the intake manifold, as suggested, also douse the inside of the IAC valve with carb cleaner. It gets "gummed up" like the intake and flame trap. The IAC is a cylinder with two hoses and a two-wire connector under the intake manifold. Her car also needed a new intake manifold gasket to make it idle perfectly (in or out of gear, a/c on or off), so I would also check for leaks around the intake manifold. Replacing old, stiff and cracked vacuum hoses is also a good idea.
Good luck.