[200q20v] Re: Subject: Re: '95 90qs: Cut Out, Hard Starts, Stall

Brett Dikeman quattro at pdikeman.ne.mediaone.net
Thu Aug 31 23:03:03 EDT 2000

Open the hood; look at the passenger side headlight, grill side, and 
look directly behind it.  You'll see a small valve with a vacuum line 
that goes up along the metal intake tube and then connects to the 
back of the intake manifold, after the throttle.

The valve has two other large diameter connections; one is right near 
the intake for the turbo, the other is right near the exit.  I 
-strongly- recommend the use of a socket, not a screwdriver, to 
loosen the clamps; I believe it's a 6mm.  Use a flex shaft and those 
clamps will be off in a second.

When you accelerate, boost builds.  You then shift; the throttle 
plate snaps shut; the air compresses at the plate because it had 
momentum; there's a pressure spike.  Pressure behind the throttle 
plate(the intake manifold) drops radically.  The turbo, still 
spinning very fast(several tens of thousands of RPM's) is still 
pushing air into the intake.  The vacuum generated in the intake 
manifold goes back down the small vacuum line and pulls open the 
valve, providing a loop immediately at the intake+output of the 
turbo.  The turbo can now come close to freewheeling because it has 
no resistance(this is good.)

Now, you hit the throttle again; manifold pressure rises, the valve 
snaps shut, and now the spinning turbo resumes boosting air into the 

You'll notice that the vacuum line runs behind that metal shield. 
Unfortunately, that shield doesn't do much to protect the hose; buy 
some hose, and replace the line.  You'll probably find the hose a 
charred mess at worst, or just really stiff at best.

The valve is available from most parts suppliers; check the vendors 
list on the homepage(www.audifans.com.)  Remember, a few of them 
offer a discount for qlist members.

By the way, I think the bently has a diagram somewhere with the 
engine compartment and all the various valves/sensors/etc marked, not 

Chris's advice is almost certainly right, this is a very common 
problem due to the poor routing of that hose.

BTW, if you want to do a permanent solution, I've seen copper tubing 
used from near the valve all the way back of the engine compartment, 
with the normal hose used to connect it all.  The copper of couse 
doesn't rot.  Others route the hose around the engine compartment, 
but I think the copper is more elegant.

I would also thoroughly clean the hose pieces the valve mates with.


At 9:15 PM -0400 8/31/00, Thomas J. Donohue, Jr. wrote:
>Chris...got my trusty Bentley out after your post and a few others 
>who confirmed your dx and found the technique for testing the bypass 
>valve at 21.29.  However, after searching the 3B diagrams, I can't 
>seem to locate where the bypass valve is to check
>it!  Am I missing something obvious, or is it just something that 
>you're supposed to know before you open the Bentley?  Any guidance 
>would be appreciated!  Regards, Tom Donohue
Brett Dikeman				Systems Engineer
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