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RE: Wastegate Diaphram??

Hi Dan!

Almost exclusively, problems with the wastegate diaphragm cause overboost
events.  The wastegate is mounted to the back of the exhaust manifold on the
WX and MC engines, and it usually pokes up behind the valve cover and
slightly to the right of it (as viewed for the driver's seat).  The
wastegate has a cylindrical cap that is held in place by four nuts on studs
that come up out of the wastegate casting, and the cap also has a brass
fitting on it to which the WGFV is connected on the MC (on the WX this is
simply a vent).  A spring underneath the cap pushes down on the wastegate
diaphragm trying to keep the wastegate closed ... thereby forcing all of the
engine exhaust to pass through the turbo's turbine.  Below the wastegate
diaphragm on the MC an WX a line attaches to the intake manifold, making it
so that the manifold pressure will work against the wastegate spring (when
the intake manifold is above atmospheric pressure), and eventually once the
boost gets high enough the boost pressure will force the wastegate valve
open, allowing some of the exhaust gasses to bypass the turbo, which
regulates the boost pressure in the intake manifold.  Over time the WG
diaphragm either stiffens, which in effect makes it more difficult for the
intake pressure to open the valve, or develops a crack and leaks, which also
causes the wastegate to stay closed ... and can mean a catastrophic failure
of the engine due to overboost unless there is some sort of safety
mechanism.  This is why the MC and WX have an overboost shutdown mechanism
built in which is supposed to shut off the fuel pump if the intake pressure
exceeds some predetermined maximum value.  

For the wastegate to fail in a way that would cause low boost it would
require the diaphragm to get weak and respond more quickly to the intake
manifold pressure ... which I would consider to be an unlikely occurrence.
I suppose another mechanism where the WG could fail to cause low boost would
be if it did not close completely ... but this would not be the fault of the
diaphragm.  I've never actually looked at the WG's valve closely enough to
speculate how likely this is to happen.  I would be more suspicious of a
leak in the intake tract or a problem with the turbo in situations of low
boost ... IME vacuum leaks are the most common sources.  If you wanted to
test to see if your WG was leaking I suppose you could try blocking the
intake manifold line that goes to the bottom of the wastegate ... this
should cause you to experience overboost, so be very careful not to damage
anything ... just run the engine up slowly and see if the max boost now
increases.  Another common problem on the MC engine that could be your
problem is a break in the line from the WGFV to the top of the WG.  It tuns
out that the MC engine uses the WGFV to augment the wastegate spring with
boost pressure ... if this line becomes brittle from heat, the motion of the
engine relative to the firewall combined with Audi's decision to use a
length of larger diameter tubing to somehow protect the smaller hose causes
it to break right near the attachment point on the wastegate cap.  

Some time back there was someone who figured out a formula for making a
eplacement WG diaphragm, but I don't know what happened to that.  It seems
to me I heard a rumor that Ned bought the rights to the design from the
person who figured it out ... which may mean that WG diaphragms are but a
phone call away ...

Steve Buchholz
San Jose, CA (USA)

> ----------
> I am having a low boost problem and several people told me to check my WG
> diaphram.  Where is the diaphram and what does it do?  Sorry, I don't
> know,
> but I haven't messed with the WG much.
> I have a feeling my WG is opening prematurely somehow and bleeding off
> excess boost.  Could this be the diaphram or a bad WG spring???
> Any input or help appreciated.  I may be looking for a new diaphram as
> well,
> once I know what it is and where it is!  :)