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RE: 87' 5KQT turbo overpressure

Wastegate diaphragm.

Remove the wastegate cap by removing the 6 hex bolts that
hold it down. Once the cap is off and the wastegate spring
is removed you can have a look at the condition of the
wastegate diaphragm. It is a rubber diaphragm, you can't
miss it, it wedges in between the wastegate cap and the
lower housing. You'll see the sliding portion of the
gate has the diaphragm clamped between its 2 halves. Feel
around the edges of the diaphragm where it hooks up
to the sliding portion. If there are any tears then you
might just have found the cause of the overboosting.

The diaphragm is used to trap air under the sliding part,
once the trap air's pressure exceeds the pressure from the
top part of the wastegate (control by spring and extra
feed of air) then the gate moves up and the exhaust
gate opens, venting gases away from the turbo and limiting
boost. Once this rubber bit disintegrates the extra that
should be lifting the gate is fed to the back of the
wastegate and forms pressure from the opposite side, this
hold the gate shut. There is also the solenoid valve
on the fire wall that feeds extra air to the back of the
WG, this is ECU controlled. As far as I understand, the
torn diaphragm is now adding to what the solenoid valve
is supposed to do: add extra pressure to the back of the
wastegate to increase boost over that set by the WG

If there are no tears in the diaphragm then you need to
check the solenoid valve on the firewall. Scott Mockrey's
(spell?) website covers this well. The link is on the
quattro list site, I think.

I have no diaphragm in my car. Not a good idea. I've had
1.9 bar show up on the gauge sometimes. Overtaking is
a nightmare, and has to be done quickly before the cut outs
start. I have another diaphragm, but it will only go in when
the 2-piece EM is ready.


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