I would check the radiator fan function because this could be your problem.
I was doing the same thing a few years ago in a '87 Westie and we had no over
heat problems (spent an hour straight climing one side of the Sierra Nevadas
only to find out the pass was blocked by snow- hey it was 89 deg in the
valley!)  Just let the van warm up to halfway at idle and then just wait to
hear the fan come on.  If it doesn't come on or comes on when the van is past
3/4 hot then you should replace your switch.  See my website for the
electrical checks.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16

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