Some unexpected vacuum leak observations

[four.rings@MCIONE.com]

Hello, club!

Yesterday I and Mike Zamikhovsky have been chasing an evasive oil leak
in my 200 in his 5-car Audidrom.
We sprayed the messy engine compartment with some foamy grease eating
stuff after which
gave the 200 a pressurised bath with Mike's new two-stroke Honda thing.
Being true perfectionists we also treated the remaining few oil spots
with brake cleaner, after which one could eat off the valve cover.
However we decided not to and went out for some sushi, warming up the
engine to dry it out and to get some fresh oil leaks at the same time.
The car ran well. No oil leaks either.
At about 20:00 I headed home. Since it was getting late and the 1 hour
drive home via empty 202 promised to be rather boring, I stepped on it.
The car did not want to go. Actually it did, but very sluggishly,
slightly better than a stock 200. The boost was barely reaching 12psi. I
immediately lifted off and checked the vacuum. It was -4" Hg vs normal
-14" Hg on my analogue boost gauge and 0.7 bar vs normal 0.4 bar on the
trip computer. The engine did not develop both the full boost AND the
full vacuum. And it did not sound right at all. It sounded as if you
filled 0.5 of a bottle with water and then blew over it's neck.
A classic syndrome of a vacuum leak. Damn, half-past-eight, in the
middle of nowhere!
Pulled into a gas station, checked all hoses. The bullet-proof Samco is
still tight, the dump valve is in place. Most importantly, the idle is
rock steady at 800, no fluctuations.
Pulled the dipstick, the car stumbled. Pulled the valve cover cap, the
car stalled.
Vacuum OK. 
Disconnected Idle Stabiliser, the car stalled.
Disconnected Idle Switch, the car stalled.
A friggin' mystery!
Proceeded to limping home at whopping 40mph, trying to stay off-boost at
all time.
Called Mike. Mike in his infinite generosity told me that he'd come and
get me with his executive vehicle (an Econoline 350 with a trailer,
which he uses for hauling his AllTrac to rallies) should I break down
completely.
Made it home uneventfully.
At first I suspected a leak in the dump valve. Disconnected it's inlet
and outlet and plugged the ports in the IC and the L-pipe. No
difference, still no full vacuum. Hmm, the dump valve is OK...
Decided to measure vacuum directly off IM, coz my analogue boost gauge
is T-ed off the MAC-11 port, so if the main line got clogged or
something, both the OEM and the analogue gauges would be running low.
That hypothesis explained the low boost too.
When I built my vacuum gauge, I made a needle pointed conical
attachement and drilled a 0.2mm hole in it's point, so that it would fit
almost any hose. Looked for a narrow vacuum line port that would fit my
vacuum gauge snugly. Here it is - the damp valve release line, unused
for the time being.
Fit the gauge in and it showed -15"Hg at the IM! Looked inside the car.
Yep, -14"Hg on the analogue gauge and 0.4bar on the computer. Bingo! 
Disconnected the line from the vacuum gauge and reconnected it to the
dump valve. Back to -4"Hg!

Moral of this story:

1. Don't wash your engines, they mysteriously tend to lose boost.
Kidding of course :)
2. If you do, DON'T spray brake cleaner onto those plastic Bosch dump
valves.
Evidently the ether eats through the plastic/glue/rubber seals, which
hold together the valves halves and they start to leak.
3. The dipstick-pull vacuum leak diagnostic, which we all love and
trust, ain't worth $hit, coz it can cheerfully mask a small leak.

Oh, the oil leak. It was the low pressure oil switch (again!). $8 and 15
min later it was fixed.

-- 
Igor Kessel
'89 200TQ -- 18psi (TAP)
'98 A4TQ
Philadelphia, PA
USA