Re: Vacuum Test - 88 5KSQ

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

In message <199812280107.TAA06394@smtp2.mailsrvcs.net> cjr1@gte.net writes:

> If the idling engine does not stall or miss when the dipstick or oil filler
> cap are removed, does this indicate a vacuum leak?

As a generality, this test is only applicable to turbos.

What _actually_ matters is where the breathers are attached to the air
inlet - before or after the air mass sensor.

On my Passat the breathers are attached to the air box immediately
above the air filter and below the air mass sensor.  Any change in gas
flow through the breathers is measured correctly and compensated for -
this car will never show any symptoms when the breather system is messed
with.

On my ur-quattro (MB = MC engine) the breathers are attached _above_
(after) the air mass sensor.  Any extra gas introduced will not be
compensated for, and the engine will react strongly to any messing
with the breathers.

Removing the oil filler cap simply opens the breathers to atmosphere.
On the Passat, you just get oily fingers.  The ur-quattro stops dead.

Removing the dipstick opens a smaller channel to atmosphere.  Again,
the Passat doesn't react at all.  The ur-quattro almost stops until the
ISV controller reacts and closes the ISV.  Then it returns to stable
idle.

The potential problem is when a car like my ur-quattro - which SHOULD
react - simply doesn't.  What this means is that the level of air
leakage into the breathers (cam oil seal, crank fore and aft seals,
hoses) is already very high and has been manually compensated at the
mixture adjustment.  Leaks of this magnitude affect performance,
because they distort the mechanical enrichment graph.  If the car runs
closed-circuit most of the time, you might not notice it.

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