I could find little information on replacing the thermostat on my 1985 camper. This is probably because the 1.9L engine in the 83.5 to 85 vanagons has a metal thermostat housing that was replaced by plastic in subsequent years.

Beware: The metal thermostat housing is no longer available. Although I had no problems with removing any bolts, they are known to seize and break off. Use of some kind of penetrant is recommended. I slopped plenty of anti-seize on the bolts when I reassembled everything.

To change the thermostat, I first went under the van and removed the two bolts holding the lower thermostat housing, assuming that I could drop the lower housing and get to the thermostat. This was not to be, as an exhaust pipe prevents removal of the housing even though I turned it in every position possible. I had to remove the two bolts from the upper housing and then was able to lift the upper housing enough to remove the lower housing and thermostat without actually pulling it away from the coolant crossover pipe. I believe the thermostat can be replaced at this point without having to pull the crossover pipe out, possibly damaging the yellow o-ring upon reassembly, but since I wanted to go ahead and replace the small section of hose going toward the front of the van, I removed the whole housing. I didn't have to crawl under the van to get the lower housing bolts out because I could have removed them after removing the whole housing.

When I worked in a laboratory I liberated a tube of silicone grease. I used this where the yellow o-ring goes into the thermostat housing to make sure the o-ring didn't roll back.

One more thing; you're going to lose a good bit of coolant. Be sure there is a large pan under the van before disassembling anything.