1990 10vt avant - falls on its face at 4k rpm

[Ben Swann]

Vittorio,

The recirc/bypass valve should open abruptly when throttle shuts and is controlld by
manifold vacuum.  Orientation of the valve can be important as the opening and closing
of the valve is parly dependant on pressure differential.

The valve may be open when engine not loaded - low or no boost conditions.

You definitely don't want to see the valve opening under positive pressure.  

If the valve is plumbed incorrectly, it is possible that it can remain open under boost
and the turbo spins in a runaway fashion without full boost being attained.  Make sure
the control line is true manifold pressure and not one of the lines that works on
partial pressure or venturi effect, such as the ones on the turbo or under the throttle
body. 

See: http://www.gtquattro.com/files/Vacuum5kt.jpg for a picture.

Ben

Date: Sun, 22 Feb 2009 11:45:28 -0500
From: "Bares, Vittorio" <Vittorio.Bares at nuance.com>
Subject: RE: 1990 10vt avant - falls on its face at 4k rpm
To: "LL - NY" <larrycleung at gmail.com>
Cc: quattro at audifans.com
Message-ID:
	<624BC75DA4460545849070E141EBCD6B041E3571 at BN-EXCH01.nuance.com>
Content-Type: text/plain;	charset="US-ASCII"

Thanks, and also to the listers that responded off list.

 I checked the WG solenoid hose - it was indeed occluded - which would explain the over
boost. The old rubber hose had collapsed on its self and was blocking the passage of
most of the air.

 
That is now taken care of :-) Gauge shows 1.9, and never gets to fuel cut.


However, the 4k rpm problem persists.

 
I looked for boost leaks, basically looking for oil as I would imagine that oil vapors
would accumulate and exit in any spot where there is a leak. Nothing of any
significance.
 

Then I thought, ok, what if the recirc valve is toast? I have one of the plastic ones
(bosch I believe), plumbed into the IC just below the exit tube to the TB.

*       I believe boost is measured there, at the exit of the IC

*       I'm theorizing that if the recirc is always open, that boost
would actually still be ok, since the recirc dumps into the hose that goes to the turbo
inlet (not sure about this though).

*       What I did as a test is pull the hose that is used to dump
pressure, blocked the hose its self and the inlet on the pipe that goes to the turbo.
Started the car, then unblocked the hose going to the recirc valve - motor died.

 

If my understanding of how the recirc valve works, it should be sealed at idle and only
open up to dump pressure when you come off the throttle (activated by a vacuum line that
goes from the IM to the recirc valve).

 

Do I have this all sideways? Or does this make sense?

 
Vittorio -