If you've got too much coolant bypassing the thermostat it's possible for it to cause two seemingly unrelated and contradictory problems:
1) engine never completely warms up since the coolant gets into the radiator and gets cooled off too much before it re-enters the engine
2) engine overheats because the coolant doesn't sit in the radiator long enough for it to cool down sufficiently thus causing water that is too hot to come into the engine to soak up any excess heat

 

So, whats the fix?

 

A properly tuned and fully functional engine operating within normal condition such as acceptable compression and camshaft profile and properly adjusted valves, correct timing, operational emissions controls, fully functional induction system, correct thermostat.
Automatic transmission equipped vehicles also have to have the transmission operating properly and not dumping too much heat into the radiator via the transmission cooler.

 

Ah, just to clarify were not talking about my truck...mine is fine...lol