Audi 100 Won't Start (Vacuum Leak?)

Tue Jan 27 04:30:11 EST 2004

Marc,

I am joining this problem late so I apologize in advance if you have looked
at the suggested items.

>From the sound of it your car went from running to non-running without any
sign. Didn't sputter or anything of the sort... Small leaks can account for
rough running and poor running conditions but will not cause a car to die
just like that. Take a broader look at the problem. In order for an engine
to run (I'm not saying "run well"), just plain run, you need only the basic
components of the internal combustion system. Once its cranking over on it's
own, go in further detail to check the little things.

#1 Valve timing. Ensure that the valve timing is happening on your car.
Valves need to open and close at the correct time in order the engine to
run. You don't need to check in great detail if the timing is right on the
nuts, just make sure that your timing belt is on and that the engine is not
making knocking or clinging sounds when cranked. If it is off by a few
degrees, you should still be able to start the car. I had a running DOHC car
with 30 BTDC skip in timing. It still ran but made a hell of a lot of noise.

#2 Compression. When cranking you should hear the familiar cranking sound of
a motor - engine turning over at variable speeds making a repetitive winding
sound, "woo-wooo-wooo-wooh". If the engine sounds like an eclectic motor,
turning over very quickly like a straight "wooooooooooh" you have a severe
compression leak. Check if your spark plugs are tight. If they are, common
problem is severely bent valves usually resulting from problem #1.

#3 Air. When turning over make sure the car is sucking in air. Leak or no
leak, I have seen EFI cars run with golf-ball sized holes in the intake
piping, cracked intake manifolds and I myself have left the brake booster
hose off by accident more times than I can count. Your car will start and
run with a HUGE leak that you will be able to hear from a mile away. Make
sure the engine is sucking air and if it is, call it good for now.

#4 Fuel. I bet this is where your problem is. Pull out an injector, crank
the motor over and see if anything comes out. (Idea: put the injector in a
cup so you don't spray fuel everywhere) CAUTION gasoline is MUCH more
flammable and explosive than most people think! Be careful! If you have no
fuel pressure, check out that Satan's child - fuel distributor (good luck!)
and more likely the fuel pump.

#5 Spark. Take out a spark plug, put the tip of it on the block and crank
the motor. See if you have a spark jumping across the terminal. CAUTION:
Remember the fuel test in step 4? Give make sure there is no fuel near the
spark and let the engine bay air out for an hour with the hood open before
doing this test!

#5.1 Spark Timing. The spark timing would have to be REALLY off-whack for
the motor not to start, but if your distributor came lose or something that
could be your issue. Get a timing light, hook it up to your #1 spark plug
wire and shine the light on the crank pulley while someone cranks over the
car. Others here may have better instructions on how to do this on your
particular model.

If #1-5 are all in good order, your car WILL start or at least sputter. Once
you get it going, hunt down the little things like those pesky vacuum hoses
:)

Just my 2 cents.

Tyson Varosyan
Technical Manager, Uptime Technical Solutions LLC.
tyson at up-times.com
www.up-times.com
206-715-TECH (8324)

UpTime/OnTime/AnyTime

-----Original Message-----
From: quattro-bounces at audifans.com
[mailto:quattro-bounces at audifans.com]On Behalf Of Marc Boucher
Sent: Monday, January 26, 2004 11:26 PM
To: SJ; quattro at audifans.com
Subject: Re: Audi 100 Won't Start (Vacuum Leak?)

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

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