Mike,

My '84 1.9 does exactly the same thing. I experienced this last year after
coming down Highway 4 from Calveras Big Trees. Gauge absolutely at bottom,
open loop mode, and--the clincher--no heat from either heater core. It's the
long downhill stretches that get you, and no, it's not your gauge or your
sensor. Your engine really is running cold. What the cause is I don't know.

For me it was not the thermostat. When I pulled it and tested it in hot
water, it worked perfectly, only opening at 195* or whatever it was supposed
to. Other than that it was completely closed. This is theoretically
impossible, at least in a conventional system.

Maybe Dennis could elaborate a bit on how Vanagon thermostats are supposed
to work in the system? I had always thought of thermostats as simple
heat-actuated valves that closed off ALL flow to the radiator when cool.
Apparently this is not the case in the WBX. Is there some way for coolant to
get past the thermostat when it's closed? If not, then why doesn't the
engine heat up the coolant?

For those of you who haven't been through this, it really is a PITA.