Dear David,
I can sympathize with your experience. A similar thing happened to us some
15 years ago with an older Edwards Oil Diffusion pump as supplied with a
602.  It rurned out that the HEAT SENSING ELEMENT WAS NOT IN CONTACT WITH
THE PUMP WALL BUT WAS FACING OUTWARDS . The unit was monitoring the
surrounding warm air and not the hot pump.That is why the pump did not shut
down when the cooling water stopped circulating.  Interestingly when we
complained,  there were implications  that this had happened elsewhere and
we should have been  aware of this possibility and checked the sensor
before turning on the pump!!!!. I mention this just in case there are some
heat sensors out there that need to be flipped over.
In our case, solvents removed some viscous stuff but some of the oil had
formed a hard cermanic-like coating on the metal surfaces which would only
break up with sand-blasting.  This option slightly pitted the previously
smooth surfaces on jets, etc  and performance was not as good as
previously. Eventually, we installed an ion pump on the 602 analyzer and
have been much happier with this modification.
Roy Krouse