Violently cyclic idle - MC engine

John Sherrow sherrow at
Mon Sep 13 19:54:19 EDT 2004

I'd be surprised if the problem was fuel related. I'd expect the engine too
be very rich or lean. Not idle merrily away.

The ECU is responding to "metered air" moving across the MAF and supplying
fuel accordingly. There's 5 scenarios I can think of:
1) ISV is being pulled open by a strong engine vacuum (what happens with the
ISV unplugged?)
2) ISV is being driven too far open by the ISV contoller (possible bad
3) Thottle bypass screw is cracked too far open
4) Throttle plate is stuck part way open even though the switch is closed

If  items 3 or 4 are true the ISV you may be too far beyond the ISV's range
of operation.

5) There is a signal to boost idle speed when the AC is on. Without the
compressor load (was the4 AC removed?) you can get some strange "high"
idles, in any setting but econ.

----- Original Message ----- 
From: "Phil Payne" <quattro at>
To: <quattro at>
Sent: Sunday, September 12, 2004 11:16 PM
Subject: Re: Violently cyclic idle - MC engine

> > When you say 'occasionally', is there any particular running condition
> > that seems to make it appear more often than not?  With my car it was
> > only after the engine warmed up and the cycle was more like 1k->1.5k
> > every second, but it would hold at 1.5k for around a second as well,
> > almost like a square wave.
> Make that 1/2 second in my case.
> Someone else posted a clue that checks out.  What is really happening is
that idle is surging
> up to 3000 rpm, but the ECU gets pissed when it sees 1500 rpm with the
idle switch closed and
> chops the fuel frequency valve.  As idle drops, it restores it and off we
go again.  This is
> demonstrable if you get it doing this at rest - pull the plug off the
throttle body switch and
> up you go to 3000 rpm.
> What confuses me is that it usually idles at 1050 rpm - almost at random.
I've done the full
> proper setup (ISV at 430mA, fuel pressures correctish, 1.25%, etc.) but I
can't get a stable
> idle below 1050.  If I set it where it should be, it won't start from cold
and often stalls.
> There are no air leaks - pulling the dipstick stalls the car INSTANTLY.
The only thing I've
> ever seen produce this sort of effect is a defective metering head - the
infamous upper
> O-ring.
> I've got both a spare metering head and a spare warm-up regulator.  II
have some engineering
> to do today on the ur-quattro, but once I've done that I'll gut the CIS on
the bus and take a
> look around.
> One further clue - I had a major problem with the car about three weeks
ago - stalling, no
> power, stumbling, etc.  I thought about the fuel pump screen (there's a
TSB about it blocking)
> so I pulled the pump and found several fluid ounces of water in the tank.
Some of it
> obviously got into the system - I ran the fuel pump (via the diagnostics)
to flush it round -
> maybe it's done some damage?
> -- 
>   Phil Payne
>   +44 7785 302 803
> _______________________________________________
> quattro mailing list
> quattro at

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