In your newspaper column of July 22, 2007
(Traverse City Record-Eagle) you answered a question of a Pontiac Sunfire (sic) overheating. I had an identical problem (although not with a Sunbird) with identical symptoms. After two years of troubleshooting it turned out that the alternator bracket
(tensioner) was broken and not keeping proper tension on the alternator belt which also drove the water pump.
At idle the water pump was not moving much if any coolant. At highway speeds the pump moved enough coolant even though the belt was not properly tensioned.
A $6.00 weld job fixed a problem that I had poured over $1,000 into!
Funny no one mentioned the water pump. Sometimes the shaft becomes loose from the pump and doesn’t move the coolant correctly. Things like a kinked supply hose at the bottom of the radiator could also be a culprit. Would be interested to know how the water flows across the top of the radiator when observed with the cap off at idle, with the coolant just slightly above the top cooling tubes. Good response re: the bracket being loose/broken for the idler. Even the seasoned guys wouldn’t find that.