MB engine overboost.

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <34A79555@smtpgate.mavericks.bt.co.uk> "Lowe, Jonathon, LOWEJD" writes:

> As you can imagine it is embarrasing for me, uncomfortable 
> for my passangers and dangerous for other road users. So please if there is 
> anybody that can help I would really appreciate it.

Sure.  Bring it round - John Robinson and I would _LOVE_ to find a car with 
repeatable symptoms and finally lay the ghost of this thing.

As a note to other readers - the MAC11/MAC12s that control the I5s with 
hydraulic tappets dating from 1988/9 _do_ permit a degree of overboost during 
initial acceleration.  Scott Mockry has some logging equipment and has produced 
traces of this phenomenon, and there is clear provision for it in the ECU code.

This is _NOT_ what we're talking about in 'MB warmup overboost'.  In this, the 
car takes off like a tweaked Sport or an S1 - only to hit a brick wall seconds 
later when the fuel pump cuts.  It's _very_ frightening if you're passing a 
long truck on a short road.  My guess is that the engine goes to over 300bhp 
for one or two seconds.

I've had two workshop sessions at BR Motorsport and spent a small fortune on 
improved capabilities of my own.  Somewhere along the road, among all the 
changes I made to get everything back to nominal specification, I fixed the 
problem.  Similarly with John Robinson's car - while we were fiddling around 
swapping metering heads back and forth to check out his performance problem, 
his overboost problem went away.  I don't know whether Steve Weeks or John 
Scott have it on their MBs - I seem to remember discussing it with Steve 
earlier in the year.

So, at present, no one knows what causes it.  The problem seems to be absent 
from the very similar I5 implementations used in the 200TQ and 5000TQ.  

We have better instruments now, partly thanks to Scott Mockry, so we could have 
a go at exorcising the swine.

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