Are you SURE the belt is routed correctly? I dont mean to beat a dead horse but you havent yet said if you have double checked it or not.
I dont think you have a blown head gasket, I think that either the belt is routed wrong or your belt tensioner is bad.

 

I am sure the belt is routed properly - I used the diagram from the British Atlantic website. You know - when I was changing the fan clutch I was able to spin the water pump pulley with the belt on it. Not spin it freely but could definitely turn it. If the tensioner is bad - I guess I have to replace the whole unit?

 

If you used AB's diagram then it is routed correctly. If you can turn the water pump with the belt on then I would say the tensioner is bad. If the water pump turns to slow then it wont cool the engine properly.
And yes you have to replace the whole unit.

 

You said thst the back side of the belt was shiny, that makes sense, that is the side that turns the water pump, it is slipping.Forget the exhaust test and replace your tensioner.
I replaced the belt on my DI about 3,000 miles ago and I can still read the writing on the back of the belt, you replaced yours 3 weeks ago and your writing is gone isint it?

 

You might consider picking up the latest issue of Corvette Enthusiast Magazine. They did anice article on solving cooling problems, several I hadn't really considered. Corvettes (older ones) were notorious for bad cooling. Although the article is in a Corvette magazine, the issues and solutions covered can apply to just about any vehicle.

Also, I've had water pumps that were good at low temp but theimpeller would spin on the shaft when the water became warm. Then it wouldn't circulate any water. Tricky to track down.

Good luck,

Mark

 

Have your radiator checked first. If it is ok, for sure replace the tensioner. If your belt is slipping, I would replce the tensioner either way.