re. Violently cyclic idle - MC engine

[Ben Swann]

Phil,

Most likely it is the throttle body screw - that is the O-ring is not sealing.  Sometimes it is compounded by a marginal/failing ISV.  Most times when I have diagnosed the problem it has been the result of two combined problems at once and the ISV may still be good and just need cleaning, but once the screw starts leaking, seems all H#!! breaks loose.  Sometimes a there is another air leak somewhere.

Another culprit to check/verify - wiring at the throttle switch.  Verify operation of the idle switch. I once found the wire beneath the rubber connector boot to have failed.  This resulted in a continuously high idle and not the bounce, but is something else to check.

Basically the idle problems occur due to ISV, Throttle Body screw(o-ring), WOT/Idle Switch wiring or switch adjustment, air leaks and lastly and leastly the ISV Relay itself as I have not seen one go bad.

Ben
With too many Type 44's to stick a shake at.

[From: "Phil Payne" <quattro at isham-research.com>
Subject: Violently cyclic idle - MC engine
To: <quattro at audifans.com>
Message-ID: <00c301c495ad$1d0404e0$0700a8c0 at KMSITLTD>
Content-Type: text/plain; charset="iso-8859-1"

Type 44 - 100 TW MC engine.

Occasionally the idle goes into a _VIOLENT_ cycle - quite agressively cycling between ca. 1050
rpm and 1500 rpm.  Cycle is around 1Hz.

Only code is 2312 (temperature sender) - I've tried three now and still get the code, but the
harness has been butchered at some time so I need to restore it.  I plan on diving in
tomorrow.

I've checked all the air mass sensor valves - decel shutoff, fuel frequency, carbon canister,
ISV, etc., and they all seem nominal.  I've got a boxfull of spares and even a complete spare
car to cannibalise.

I can sort this out from first principles just by taking it all to bits and putting it
together properly.  But has anyone seen this exact symptom?

-- 
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803]