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Slightly Erratic Idle

Posted by alanrw 
March 25, 2010 07:53PM
Well, after 26 years, she hiccuped a little yesterday and today. When sitting at idle, I get a burble that causes the rpms to drop about 300 rpm and then it goes right back up. The car is a 1984 5 speed 325E. It is not all the time which tends to make me think it is not a vacuum leak as I would expect that to be constant. At this point do I take out the Idle Control Valve and hose it down with carbl cleaner?

And for general interest, I have read you are never supposed to touch the adjustment screw in the ICV, true? If so, how do they adjust the ICV/

thanks guys

alan
March 25, 2010 09:52PM
Quote
alanrw
Well, after 26 years, she hiccuped a little yesterday and today. When sitting at idle, I get a burble that causes the rpms to drop about 300 rpm and then it goes right back up. The car is a 1984 5 speed 325E. It is not all the time which tends to make me think it is not a vacuum leak as I would expect that to be constant. At this point do I take out the Idle Control Valve and hose it down with carbl cleaner?

And for general interest, I have read you are never supposed to touch the adjustment screw in the ICV, true? If so, how do they adjust the ICV/

thanks guys

alan

True...don't touch the ICV screw. The engine management computer takes care of adjusting the idle so if you change the setting the screw is at, you will upset the system and throw out the balances built in to it.
March 25, 2010 10:37PM
Quote
Archeo-peteriX
Quote
alanrw
Well, after 26 years, she hiccuped a little yesterday and today. When sitting at idle, I get a burble that causes the rpms to drop about 300 rpm and then it goes right back up. The car is a 1984 5 speed 325E. It is not all the time which tends to make me think it is not a vacuum leak as I would expect that to be constant. At this point do I take out the Idle Control Valve and hose it down with carbl cleaner?

And for general interest, I have read you are never supposed to touch the adjustment screw in the ICV, true? If so, how do they adjust the ICV/

thanks guys

alan

True...don't touch the ICV screw. The engine management computer takes care of adjusting the idle so if you change the setting the screw is at, you will upset the system and throw out the balances built in to it.

What Peter said. The system assumes that all parts are in proper working condition, and the computer is able to adjust the system to make up for slight differences or slightly used parts. If the idle is erratic, the cure is to find the part that is upsetting the system, not adjust the screw.

Sporadic idle problems are a pain to diagnose. Is there any correlation to the engine temperature?
A problem that comes and goes makes me think an electronic part (like a sensor) and not a broken mechanical problem. You may try letting the car idle and going through the engine wiggling all the wires. This is how I found a problematic relay in my car.

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1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

April 01, 2010 06:33PM
Yeah, could be any number of things unfortunately (check table 3f in the Bentley handbook). Also check fuel pressure (pump may be dieing), distributor cap wear, leaking/sticking injectors.

Cheers,
Simon



Edited 1 time(s). Last edit at 04/01/2010 06:35PM by nomis3613.
April 04, 2010 11:01PM
One of the most successful fixes on mine was to clean the injectors. I also use injector cleaner in my fuel every month or two.

Bob in Everett
April 05, 2010 09:55AM
Well, last week, I pulled the connector off the ICV as she was running. Re-connected it and the idle stabilized. Shut her down, cleaned the connection to the ICV and she has been stable ever since.

We will see......

thanks to all who responded.

alan
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