I've been wondering about Vanagon Overcooling Syndrome  for a long time now,
and just came up with an idea as I was going downhill in Northern California.

Here's the syndrome: it's cold out, you're going downhill for a long ways,
and there is just nothing you can do to get the coolant warm. It's not a
gauge problem; the coolant really isn't warm, and your heaters don't work,
and the van runs bad.

Must be a thermostat stuck open! you say; but no, at home doing the pot o'
water test, the tstat works perfectly. You put it back in; the van still
overcools. This cannot be; yet it is.

Here's my idea. When you're coasting downhill, you're using the engine as a
brake. This means that above 1500 RPM, with the idle switch closed, the
engine isn't getting any gas, as the ECU has shut the injectors down. If it
isn't getting any gas, it also isn't producing any heat. The engine is
functioning as a brake, but there isn't any combustion, and so, after a
period of 10 minutes or so in freezing temperatures, the coolant loses
temperature, and your gauge is way off the edge of the scale.

I tested this by going downhill, engine warm, and taking my foot off the gas
to coast. I could see the gauge move colder before my eyes. Give it a little
gas, and the engine warms up. This is kind of tricky from a driving
standpoint, but it works.

I'm talking about an '84 1.9 with the twin switches on the throttle
body--maybe in later years VW corrected this with new switch type, maybe a
remap of the ECU?

Anyway, such is my thinking.  Anyone who has experienced this knows it is a
real problem in the winter months. Hope this helps.