Idle speed on an E28 520

My brother has a much loved '87 E28 520. Done nearly 250,000 miles. He recently had to change the head gasket (and have the head planed) after a hose blew and it overheated, but it's now running better than ever. Except
that the idle speed is far too high. He has the Haynes manual - but that's as much help as usual. I think it has the L-Jetronic system - the warm up fast idle is by a vacuum valve. The AFM has an adjustment screw on it - but on other cars with this system that's for the idle mixture? My first thoughts was an air leak somewhere - but it has just about the most tortuous plumbing I've ever seen. We tried wiggling all the hoses we could get to, and removed the AFM and trunking to check that, but all fine. Any guesses?
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*Why do we say something is out of whack? What is a whack? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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By AFM do you mean the idle air control valve?
I would get out a propane torch and make a serious search for vacuum leaks before doing anything else.
Then I would clean the IAC.
Then I would disconnect the O2 sensor and see if the problem got better or worse. Likewise I would disconnect the throttle position sensor (which is really just an idle switch) and see if it gets better or worse. --scott
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'Flapper' air flow meter. There is no electrical idle control valve that I can find - despite Haynes reckoning one that age should have one. The fast idle is provided by a vacuum servo acting on the throttle mechanism. Which I've no idea how it works on the control side, as it just appears to be connected to the inlet manifold. If you disconnect the vacuum (and blank it off) the idle speed shoots up. So that appears to work as it should. As it does slow down somewhat as it warms up.

No o2 sensor on this UK car. No cat. either.
What does the TPS do on this version? I'm more used to a potentiometer type.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Hmm... my (US model) E28 535i has a full IEC and an O2 sensor, and the O2 sensor feedback is definitely used for mixture control which is a good thing to my mind.
If the flapper valve isn't sticking or leaking, then you are good to go and there isn't much else to look for other than vacuum leaks.

I don't really know, because if I disconnect it, the RPMs don't change at idle. If you disconnect it and they do, something is wrong. --scott
--
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Not really. 14.7 AFR is only required where there's a cat. A much wider range is beneficial in all sorts of ways. Like leaning off to perhaps 16:1 when cruising which aids economy. But you need a wideband o2 sensor to check on that.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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London SW

The idle switch is to reduce emissions created on the over-run, which is when the most noxious emissions are generated. What it does, IIRC, is stop the fuel from being injected whilst slowing down (off throttle) with engine rpms above a set value. This value differs from manufacturer to manufacturer.
regards
David
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wrote:

AFM = Air Fuel Mixture.
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Air flow meter.
Air fuel ratio.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Hi, I have a 1997 328i BWM. Recently my car has been making this loud clicking noise while I drive at low speeds (under 15mph). It doesn't do it every day and when it does it will do it for about 10 minutes before it goes away, so i'm having trouble pin pointing the problem. The sound sounds like it's coming from the rear right side and goes along with the movement of the car (not while stopped), after a certain speed it goes away. My brake pads light had recently gone on and had the pads and sensor changed and the light went away. So I don't know if its a brake problem or not. I inspected my rotors and they look perfectly fine. The best way I can describe the sound is it sounds as if I had a trash bag or soda can stuck on the tire but it's obviously not that. Any ideas as to what this might be?
--
elywaldoya

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