im trying to guess which one is failing without touching it . heater is hot sometimes but other times blows cold....engine temp. is very high most of the time but does not seem to be leaking water at all........when i turn the car off it bubbles very hot for a few minutes ..any suggestions?

 

Sounds like you have air trapped in your coolant system.

 

Ditto the above. Or, maybe you just have a clogged cooling system.

What model year and how many miles on your truck?