Audi 100 Won't Start (Vacuum Leak?)

[Marc Boucher]

I performed the vacuum test as described.  Getting a good seal on the air
bonnet was a challenge but once I tried latex gloves for sealing it, the
problem was solved.

The system was 'essentially' sealed.  By this I mean that if I created
either a positive or negative pressure situation (small pressure, using lung
power) and sealed with my finger, it would take at least a five seconds for
that pressure difference to dissipate.

I investigated and found at least one source of this small leak:  The hose
that leads from the ISV to the air bonnet actually has a small branch at the
back that goes to a small opening in the engine.  From what I can tell from
various diagrams, this opening leads to the injectors.  The hose itself has
no leaks.

Could such a leak be the cause of my non-starting condition (I'm doubtful).
Remember that this car went from 'never failed to start', to 'impossible to
start and drive'  overnight.

I just remembered that many things get energized once the ignition key is
turned (I think including a vacuum reservoir to open close vents/climate
controls?) So I'd best repeat this test tomorrow with the ignition on.



Thanks,

Marc


----- Original Message ----- 
From: "SJ" <syljay at optonline.net>
To: "Marc Boucher" <mboucher70 at hotmail.com>
Sent: Monday, January 26, 2004 9:32 PM
Subject: Re: Audi 100 Won't Start
**** I havent done this yet, because its too damn cold . .. but to find a
vacuum leak I would take off the air bonnet at the fuel distributor. Make a
wooden or metal or plastic plate to position on top of the fuel distributor.
Cover that with a plastic sheet. Install the air bonnet over the plastic
sheet.
Now you have the intake sealed. The other end is sealed by the engine
valves. You now have a completely sealed system.
Use one of the unused ports at the bonnet to supply air to this sealed
system . . .and look for leaks. You can just blow into it using lung power.
If it doesnt hold air, you have a leak. A steady supply of air at a low
pressure . . .say 3 lbs would be ideal for locating the leak. Just use a
hose pressed to ear to look around for the sound hissing air.

Some hoses you can block off . . .like PVC hose. But, the crackcase system
is also sealed.

SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q



> Today I pulled both control units.  Their plugs were pristine as were the
> units themselves (wrapped in plastic).  No sign of moisture or rust.  The
> wiring I'm not so sure of.
**** Good idea to use electrical contact cleaner and plug/unplug several
times to clean contacts.
>
> Thanks again,
>
> Marc