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On 23 Jul 97, Norman Kraft wrote:
> True, though bad pressure sensors have been known to lead to very
> rich mixtures. The voltage is, of course, the very first thing to
> check, but if that doesn't seem to be the problem check the pressure
> sensor. While this is just a fairly crude test, the resistance
> between terminals #7 and #15 should be about 90 ohms, between
> terminals #8 and #15 should be about 350 ohms. Also make sure the
> diaphragm is not leaking; you can just blow into it to test that.
> Once you know it is working, you can adjust the mixture via the
> pressure sensor adjusting screw. Clockwise leans out the mixture,
> counterclockwise makes it richer.
A leaky diaphram will make it run rich, as you say, but a leaky
bellows will make it run lean. I have only seen destroyed diaphrams
on 914s where the PS is located just below the battery and the owners
or mechanics had lost the battery cover that kept rain water from
filling up the European style battery and overflowing acid onto the
pressure sensor.
I have seen diaphrams where the gasket leaked, and I resealed these
with some light gasket compound, but while I have never found this
to cure anything, I will continue to do it anyway. In the end, it
boils down to a question of how big is the leak at the diaphram
versus the "leak" via the hose back to the intake air distributor.
I would be cautious about suggesting someone try readjusting their
own sensor. The adjustment is very sensitive, and I have seen
several of these ruined when the original adjustment was
irretrevably lost and none that cured the problem. I suspect that
with the proper instruments some people could do a good job. I think
it is also important to understand that the computer uses the PS
information differently at idle than it does off idle, so this
adjustment has the best chance of working if it is done with
instruments and with the throttle valve switch unplugged.
Jim
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Melissa Kepner Jim Adney
jadney@vwtype3.org jadney@vwtype3.org
Laura Kepner-Adney
Madison, Wisconsin
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