20 Valve Loss of HP and MPG

[Phil Payne <quk@sievers.com>]

In message <9610111527.AA29120@hunch.zk3.dec.com> Andrew Duane USG/PE writes:

> Other than that, my 20V is having some of the same problems. The car
> failed the vacuum test (opened cap and dipstick, car didn't even hiccup).

I think we should go into this "vacuum test" thing in a bit more detail.

The fuel injection on these cars is adjusted at idle.  Point one, of course, is 
to have the idle right before you adjust.

Nominally, there should only be a small amount of air entering the system 
through the breathers.  Thus, when the system is set up, the vast majority of 
the air entering the engine is displacing the mass sensor.  The mixture will be 
set as appropriate.  Opening a massive airway (the filler cap) has the effect 
of drastically leaning-off the mixture, and the engine stops.
 
If there is a large vacuum leak _before_ the engine is set up, the mixture will 
be set up nominally rich to counteract the leaning-off effect of the air 
entering via the breathers.  In such an engine, opening the oil filler has 
little effect.
 
The true problem is the mis-set mixture adjustment - but this is a consequence 
of the setup being performed with the leaky vacuum.
 
On most engines, Audi recommend that the breathers be disconnected and plugged 
during initial setup.  There is then _zero_ air entering the engine via the 
breathers.  Replacing the breathers will show if there is a vacuum leak - if 
there's a big one, the engine simply won't run.  Diagnose as appropriate.

The propane trick, BTW, is harmless.  In more temperate climes, some might 
prefer butane.  Although propane is not particularly dangerous in this context, 
it's a nasty gas to have around in large quantities because of its high 
specific heat and low latent heat of evaporation - there's enough thermal 
energy in liquid propane under pressure at room temperature to evaporate over 
half the volume instantly.
 
A catastrophic pressure release caused the Spanish camp site fire that killed 
over 70 people.  In the sort of quantities needed here and used this way, it's 
a pussycat.

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