Idle Adjustments

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528i-1981
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Idle Adjustments

Post by 528i-1981 » Wed May 29, 2019 8:27 pm

How many M30 threads worldwide have begun with this subject line?

Based on experience, what's the best way to attack the throttle body idle screw and the AFM idle air bypass for the smoothest idle? After confirming the idle switch was closed, my first pass was to adjust the TB screw first to get the idle to around 750 and then tune the AFM screw for smoothness, but doing so brought the (warm engine) idle up to around 950 or 1000.

The AC additional air valve seems to be doing its job (idle steady with AC on), but the aux air valve was dirty, so once I get the warm idle to spec I'll need to monkey with the AAV a little.

Thanks for your hints. I could just adjust them back-and-forth until I get to spec, but if there's a more scientific way I'd like to know it.

Eric
(oo=00=oo) Eric
1981 528i Manual

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Mike W.
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Re: Idle Adjustments

Post by Mike W. » Thu May 30, 2019 1:10 pm

They were never known for a smooth idle, really they weren't even supposed to idle smooth. The O2 sensor always dithering, cycling up and down, up and down was a large part of it. It's best described as hunting. Not quite a miss, but hunting, never quite settling down. There were other cars that also did that at that time, BMW wasn't the only one, but it was also a high performance engine in an era when there were also 120HP V8's.

Technically it's supposed to be adjusted by CO at the sampling tube off the exhaust manifold, but I think you can get fairly close if you disconnect the O2 sensor lead and dial in the bypass to about .45V. Actually it will idle smoother with the O2 disconnected, but at the expense of some MPG and excessive emissions with no increase in performance. Personally I just make sure the O2 sensor is dithering and doing it's thing running the mixture, then dial in idle speed as best I can. Extra ignition advance seems to make consistent idle speeds more difficult too.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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528i-1981
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Re: Idle Adjustments

Post by 528i-1981 » Thu May 30, 2019 2:36 pm

Thanks Mike. This is helpful.

So, is the little air bypass on the AFM just a fudge-factor device? I understand that while the AFM won't measure the small amount of air that the bypass admits, the O2 sensor sees the result at the exhaust end. Can I consider the TB screw as the coarse idle adjust and the AFM bypass as the fine adjust, or something like that?
(oo=00=oo) Eric
1981 528i Manual

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Re: Idle Adjustments

Post by Mike W. » Thu May 30, 2019 3:05 pm

No, they're two different things. The mixture adjustment on the bottom is a AFM bypass, that air goes around the AFM flap, the volume adjustment on the side is metered air that does go thru the AFM and is measured. But the O2 sensor has a wide range of compensation and will correct what you do with the mixture adjustment. I've found it useful a couple of different times. Once when it was cold blooded before it went into closed loop off the O2 sensor and another time when I had a fuel pressure regulator problem, it enabled me to lean it out enough at idle to get thru smog. But normally I don't mess with the mixture adjustment.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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528i-1981
Posts: 175
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Re: Idle Adjustments

Post by 528i-1981 » Thu May 30, 2019 5:31 pm

This is great stuff, thanks. Last question - is it best that the AFM mixture screw be closed, open, half-open, or does it not really matter if I leave it alone (i.e., constant)?
(oo=00=oo) Eric
1981 528i Manual

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Mike W.
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Re: Idle Adjustments

Post by Mike W. » Thu May 30, 2019 8:18 pm

I'd set it at 2 or 3 turns open.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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528i-1981
Posts: 175
Joined: Wed Feb 07, 2018 5:50 am
Location: Boston North Shore, MA

Re: Idle Adjustments

Post by 528i-1981 » Sat Jun 01, 2019 12:22 pm

OK, so I set the idle using the TB screw to around 800 RPM and the AFM mixture screw 2 1/2 turns open. It wasn't happy at all. I connected a small wire to the O2 sensor plug to I could pick a measurement from the sensor using a multimeter. The voltage was consistently falling below 0.5V, indicated a lean condition, and clawing it's way back up to 0.5V (0.47 being stoichiometric). The needle on the multimeter sweeps around, so this is a generalization, but it was in lean territory a majority of the time. Closing the AFM mixture screw using a short screwdriver from beneath with the engine running, I turned the screw until the sweep from the O2 sensor was approximately centered around 0.5V. This coincided with a reasonably happy-sounding engine, with the "hunt" described by Mike alternating between richer and leaner.
(oo=00=oo) Eric
1981 528i Manual

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