Phil, you're on to something-urq positive crank ventilation

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

In message <b5d578db.35abf852@aol.com> GaidosIII@aol.com writes:

>  Last week Phil had mentioned pulling the dip stick or cracking open the oil
> filler cap when I asked about a possible TOB failing. I tried that and it did
> not work. However, my trusty mechanic took that idea a step further and tried
> something else and discovered that he was able to eliminate the sound or
> reproduce it another way.
>  As it turns out, the noise is not the throw out bearing as Phil had
> suspected. Ron did a little more investigation and found that there is crank
> case vacuum. This is great because I don't need an expensive clutch job but
> ....
> ...He was unable to figure out what is causing the problem. So I need to know
> where he should look. Alldata was no help to him. He suspects there is a
> missing restrictor in the hose from the air sensor housing to the valve cover
> but as I said Alldata could not confirm this.

?

I think the ur-quattro is _supposed_ to have crankcase vacuum.  There
are certainly no restrictors in the breathers.  On the MB engine, those
breathers are even wider - mostly 1" or more internal diameter, and
that turbo can _SUCK_.

The "stumble trick" works because the engine is trying to suck air
through the air filter and up past the metering head paddle - the
so-called 'air mass sensor'.  The breathers come into the system _above_
this - so if you suddenly supply airflow into the breather system, less
air passes the paddle and the fuel is partly cut.  On a properly set
up MB, the engine stumbles and almost cuts when the dipstick is pulled
out, and dies instantly if the oil cap is removed.

HOWEVER - this is only true for engines that have good cam and crank
seals, and have been set up as Audi lays down - with all the breathers
_DISCONNECTED_ and plugged.  Do it this way, then reattach the
breathers.  If the engine performs differently, you have a leak in the
engine.  The VAST majority of people who set up an ur-quattro engine
don't disconnect and plug the breathers - over time, they gradually
'correct' for the increased air leaks in the engine every time they set
the CO.  It isn't a true correction, though - it actually moves the
enrichment curve slightly and leads to a top-end performance fall-off.

There's good news in all this.  Positive crankcase pressure, when
there's too much of it, foams the oil in the breathers and blocks them.
Plus (IMO) it's usually a sign of worn rings.  I had a car here a few
months back where I plugged the air mass sensor breather port and
held my thumb over the breather pipe from the engine - there was a
very marked gas pulse once per revolution.  #3 was also at 1/2 rated
compression - go figure.  A sustained crankcase vacuum at least
implies good rings.

I think you still have a problem - the crankshaft rear seal - that is
just as expensive to fix as a clutch bearing.

And, as an aside - since I ran the Passat for 2 x 1/4 hour oil flushes,
compression has improved.  I've forgotten who it was that said "black
sludge" in oil could gum up the piston rings - but you seem to be
right.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
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