Shorting the resistor definitely fixed the problem, and this morning I replaced the resistor. I've driven it twice so far, both of them long enough for the car to heat up, and everything is behaving normally.

Quote Originally Posted by avguy View Post
So I'm thinking you have something going on with the coolant flow. The usual suspects would include a partially stuck thermostat, a blockage in the rad, or a broken water pump. Obviously the T-stat is the easiest (and most likely) so you might want to start there.
This is quite possible, and maybe I've never noticed it because the temperature never rises beyond the normal operating point. It's been probably 3-4 years since I last replaced the radiator, but I flushed the cooling system when I did the water pump/timing belt at 400k.

I can't measure my coolant temperature with your precision. I just have the idiot gauge in the dashboard, and I don't have a fancy scan tool; just a code reader/clearer. Likewise, I can't tell for sure when my fan comes on unless it kicks on at idle, and I can sense the load on the electrical system and engine. Almost all my driving is on the highway, but I do hit miserable standstill traffic in the summers when I travel to Detroit, Grand Rapids and Chicago. I've never had an overheating issue before except that one day when my plastic thermostat housing exploded, but it's possible I'm living right on the edge. It HAS been a while since I replaced the thermostat.