can a thermostat being stuck closed cause the coolant in the radiator to be forced into the overflow reservoir, or does it have to be the HG? also, is there an easy way to check the fan switch, i know that the fan works, but not when its supposed to. thx

 

or, could the HG pressurizing the coolant system make the tstat not open?

 

It is very unlikely a blown HG would keep the thermostat closed, the thermostat opens and closes depending on engine coolant temp.

The engine coolant temperature switch, [fan switch] is pretty simple, it is on or off, it closes when engine coolant temp. reaches 190-200 degrees F.
What exactly do you mean by, "the fan works, but not when its supposed to"

If the HG is blown you will see constant "bubbling in the rad,[ remove rad cap, start engine, look for bubbles] look for coolant in the oil, test for hydrocarbons, [oil] in the coolant, white "smoke" from exhaust.

Coolant will flow into overflow reservoir, [more then normal] if there is a problem with the rad cap.

I would replace the thermostat and the rad cap, they are relitivly cheap and easy enough to swap out, and it can't hurt to replace them anyway. 94

 

thanks for the reply fcm, with jumper wires from the battery, the fan works, but it doesnt come on when the engine gets over temp, and also the lower radiator hose isnt warming up. all i can think is possibly the HG is forcing air into the system and the thermostat wont open because there is air around it instead of fluid, which could also keep the fan from coming on, right? there isnt any oil in the coolant or vice versa, but there is a lot of pressure on both systems. when you remove the oil cap there seems to be way to much air trying to get out, and the radiator is constantly being drained into the overflow. looks like head gasket it is. thanks