In addition to the advice already given on the topic
(Bleed system, check rad fan etc) I can add that not
all thermostats are alike. I've tested several thermostats
in VW diesels, and some have weak springs. What happens is
that the thermostat performs correctly up to some rpm
range. Over this speed the pressure from the coolant pump
will be high enough to force the weak spring open regardless of
temp. This would happen around 3500-4000rpm, which with the
diesel vanagon gearing corresponds to about 60-65mph.
The result would be a drop in engine temp when one goes fast.

I bought and tested both aftermarket and oem thermostats. I forgot
the detail results for each thermostat (Interested parties
can look in the archives for my previous post), but I remember
that the brass "Whaler" thermostats were among the best, and
that's what I've been using since.

/Martin
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Martin Jagersand                 email: jag@cs.yale.edu
Computer Science Department             jag@cs.rochester.edu
Yale University

        Slow down and visit the VW diesel Westy page:
          WWW: http://www.cs.rochester.edu/u/jag/vw
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