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Engine cutting out.

In message <3402DD15@smtpgate.mavericks.bt.co.uk> "Lowe, Jonathon, LOWEJD" writes:

> My name is Joe Lowe and I bought an '89 UR quattro 10 valve (MB engine) 
> three months ago.  The car has covered a total of 51500 miles (1500 since I 
> bought it) and is completely standard.  I had an unusual fault  with the car 
> the other day and I was wondering if anyone had any ideas what might have 
> caused the problem.
> The circumstances of the fault are as follows:  It was at night around 10pm, 
>  I had driven approximately 3 miles from cold. I was following another car 
> down a country road at between 35-40 mph.  The oil and water temperatures 
> indicated that the car was now warm so I decided to overtake.  I changed 
> down from 4th into 3rd, pulled out and accelerated.  When the turbo came in 
> the car took off as though it was in first gear and accelerated viciously 
> much faster than it should have done.  After approximately 2-3 seconds of 
>  this vicious acceleration the engine cut out just as viciously with a real 
> jolt on the transmission, the rev counter read out went out (as if the red 
> line had been reached) and the speedometer was reading 60 mph.  My initial 
> reaction was to go for fourth gear as I thought I had hit the rev limiter. 
>  The engine then came back to life just as normal and continued to 
> accelerate as normal from 60 mph in fourth gear.  Luckily I had just passed 
> the other car when this happened so this did not cause any problems on the 
> road.  The car performed as normal for the rest of the journey, another 5 
> miles and still continues to function normally a week later.  I can find 
> nothing visually wrong with the car under the bonnet.

Welcome to the ur-quattro (MB) owner's club.  Don't forget to clean the seat 
before leaving.

> My questions are, has any one experienced this type of fault or knows of 
> this problem ?

Yup.  We've even got a name for it - "MB warm-up overboost".  It's 
particularly prone to happen on cool days when the water temperature display 
is on the segment below the middle/normal one.  You'll find my initial 
descriptions of the phenomenon in the archives for November/December 1994.

> Is this a bug in the management system ?

Very possibly.  I've just bought a second MAC12 ECU for diagnostic purposes.   
The problem has manifested itself on my car and on John Robinson's car, and we 
have been investigating it with the help of Martin at BR Motorsport. 

 Phil Payne
 Committee Member, UK Audi [ur-]quattro Owners Club