Violently cyclic idle - MC engine

[Phil Payne]

> 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
  http://www.isham-research.com
  +44 7785 302 803