Erratic temperature gauge

[Jeff Witwer <jwitwer@intellicorp.com>]

Over the past several weeks, I have been searching the archives and have
posted a couple of questions trying to solve a problem that I have been
having with my 86 5kcstq (105k miles):  for months, the temperature gauge
has been erratic and, more recently, the idle speed has been sporadically
too high (2000rpm).  Based on the archives and responses to my posts, I
focused my attention on temperature sensors and the ISV/throttle switch
system.  This weekend, I realized that it was time to change the coolant
and decided that I should take this opportunity to check the thermostat
itself.  I was amazed to find that the thermostat itself was physically
broken  ..... the "arch support" was broken on both sides of the thermostat
disk.  On one side it looked like fatigue failure of the arch and on the
other side it looked like the solder joint holding the arch in place had
broken.  I never realized a thermostat could fail in this way!  Obviously,
with the thermostat broken in this way, the temperature regulation of the
engine was erratic, as the thermostat would be randomly open and closed
depending upon how the thermostat happened to lodge itself into place at
any given time.  The temperature gauge was right all along!  I changed the
thermostat (using an Audi replacement part) and now the "erratic
temperature gauge" problem is solved.  And the idle speed behavior is 95%
fixed.  Infrequently it still idles higher than it should (1100rpm), so
maybe the ISV/throttle switch still needs attention.  But it is sure a lot
better ...and for only $25.  I will not be so quick to blame the gauge next
time.
Jeff Witwer
E-mail: jwitwer@intellicorp.com