this is kind of a stupid question, but what does the thermostat on you engine do? and what happens if it is broken?

 

It regulates the flow of coolant through the engine. It's set to open at a specific temp to allow the coolant to flow through and cool the engine. If the thermostat fails in the open position, the engine is over cooled because the coolant just keeps flowing. If it sticks closed, the engine will overheat because there is no coolant flow.

Generally, if the thermostat is stuck open, you'll notice that the engine is only at operating temp when you are stopped. As soon as you begin to drive again and air flows through the radiator, the coolant temp gauge will start to drop.

 

Aj basically hit the nail on the head but I would like to add one thing. If a thermostat is stuck open, it will only be 'overcooled' IF the radiator is efficient enough to handle the job. It is more likely than not that a thermostat suck open can cause overheating problems. Water circulates too fast and the radiator doesn't get enough time to do it's job. Seen it a dozen times. I have had buddys finally listen to me and get them to install a hotter thermostat to try to fix their overheating problem only to have it work. I had a buddy with a street rod go alll the way down to a 160* and it still overheated. Got him to install a 195* and no more problems.