And it continues

[quk@sievers.com (Phil Payne)]

In message <Pine.SUN.3.91.970109110509.14637A-100000@marjoram.compnews.co.uk> Mike writes:

> If you have any ideas, let me know, but the car is going back into the 
> garage next Thursday night, and Im using one of theirs until they fix it.

Guess #1:

It has a _massive_ air leak between the mass sensor and the turbo, or in the 
crankcase/camshaft breather system (which bypasses the mass sensor, but 
should normally contribute little or no air). The mixture has been set _very_ 
rich to compensate for the unmeasured air while the engine was warm, and the 
cold start enrichment is simply swamping the engine with fuel.
 
Once the warmup regulator leans out the extra cold start richness (after about 
two minutes, according to the specs - it's electrically heated) the over-rich 
mixture setting compensates for the air leak and you achieve the stoichiometric 
ideal. It runs "better than ever" because the air leak was previously making it 
run lean.

The air leak can come from a number of sources - most frequently it's complete 
rotting of the large rubber hose (034 103 221L on an MB - about GBP25) that 
comes up the side of the engine between cylinders #4 and #5 from the crankcase 
breather to the breather assembly that goes across to the air mass sensor.  It 
rots on the inside because of the heat of the engine.  Also, the wire 
reinforced hose from the mass sensor to the turbo has a nasty habit of 
developing splits that are _very_ hard to see while the hose is still on the 
car.
 
Make _absolutely_ _sure_ that they are following the mixture adjustment 
procedure _to_ _the_ _letter_; it's different between WR and MB engines.  This 
includes detaching and clamping breather hoses, which many of the b******* are 
too idle to do, or claim is "unnecessary".  If they resist, the Audi notice 
about the MB engine is on D000.5147.00.00 page 25-33.  A little knowledge is a 
dangerous thing, especially for mechanics.  If the setting obtained with the 
breathers off and clamped goes out to lunch when the breathers are reattached - 
you have an air leak in the engine, or several smaller leaks are getting 
together to party.  Check that the camshaft cover is tight (unbelievable but 
true) and that the oil filler cap and dipstick are making good seals. 

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club