I recently bought the car for my son (first car), father-son project. When I rev the engine the temp. gauge goes down, it seems the belt is loose. Where is the belt tensioner? Any adv. is apperciated. btw the heater blows hot on all settings too.

 

The belt tensioner is on the right side of the engine looking at it. It is tucked beside the power steering pump real close up against the engine. It is not adjustable though. If it was bad you would hear a chatter at idle.
The belt just may be stretched. The heater problem is another problem altogether. Someone else in here will have to address that.

 

common problem is faulty mono valve.

if not, then the vacuum and or vacuum element for air flaps.