From: andrem@pyrtech.mis.pyramid.com (Andre Molyneux)
Newsgroups: rec.autos.vw
Subject: Temp Sensor on '86 GTI
Date: 14 Aug 92 18:18:20 GMT
Originator: daemon@sword.eng.pyramid.com

I had an "interesting" situation come up this last weekend, and thought I'd
turn to the collective wisdom of the net for enlightenment.

My wife and I were driving to Nevada in our '86 GTI last Friday when it
started having problems.  A couple of times, while under a load (accelerating),
the engine appear to miss.  It only happened once or twice while going across
the central valley (CA), but turned into a major problem in the hills.

While going up Old Priest Grade Road (a very steep cut-off heading towards
Yosemite) the car started missing severely.  Changing gears to get the engine
to a higher RPM helped once or twice, but eventually the car died while still
on the grade.  While looking at the instruments when it really started acting
up, I noticed that the temp guage was at the bottom of its range.  Initially
I thought maybe guage had broken and the car overheated without my knowing it.
I quick check under the hood didn't seem to show any excessive temperatures,
and the MAF oil temperature read about what it normally does, so I decided
that overheating wasn't the problem.

I assumed that the computer uses the temperature input as one of the control
parameters for the engine, but figured that it probably wouldn't keep the
car from running if it was wrong.  When attempting to restart the car, we
noticed that the fuel pump was making a strange noise while we were waiting
for the fuel pressure to come up.  "Just great," I thought, "we're at least
30 miles from anywhere that might have a replacement fuel pump."

The grade we were on is so narrow that there's no way to turn around, so we
had to get to the top.  After a few minutes of letting the car sit I managed
to get it started again, and we just barely made it to where the cut-off joins
back with the main road and I could find a place to pull off.  I let the car
sit for a few minutes again, and then started it and began driving down the
less-steep main highway, figuring that I could probably get the car all the
way down the hill even if the engine died again and I didn't have any power
steering.

While we were driving, my wife was going through the borrowed Haynes VW book
we had in the car.  The book claimed that a faulty temperature sensor could
be responsible for stalling.  Once she read that to me, I found a place to
pull over and opened the hood.  I found three wiring connections to the water
outlet from the engine: two on top and one on the bottom.  I disconnected the
two top ones and found that the connections were rather corroded (I couldn't
get to the bottom one due to the engine heat).  I cleaned the connections as
best I could, reconnected the wires, and got back in the car.  When I turned
the key the temperature guage started operating again, and the fuel pump
sounded perfectly normal.  The car started with no problem.  We drove back up
the grade with no further incident so we decided to chance it and continue with
our trip rather than head back home.

The car never gave us any further problems, even though we went up some very
steep grades to Tioga Pass, which is somewhere around 10,000 feet above sea
level.  Obviously a bad connection to a temperature sensor was at fault,
but I'm still trying to understand how it could cause this much of a problem.
I would guess that temperature would be one of the inputs controlling how rich
of a mixture the car runs.  Not being too familiar with VW engines, does it
accomplish this by having some sort of choke, or does it actually have the
injectors spray more fuel?  Or am I missing the point and there's something
else that the temperature would affect?

Also, why would the fuel pump make strange sounds with no temperature input?
That's the one that really has me going.

Thanks in advance for any light you can shed on this subject.

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| Andre Molyneux   KA7WVV     "Insert your favorite disclaimer here"       |
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