This is not that control panel on the wall of your house that you argue with your significant other over where the ideal setting should be. The thermostat is a control valve within the cooling system of your engine. Like the control panel for the furnace in your house, the engine thermostat is used to regulate temperature.
When the engine is cold the thermostat is closed, so coolant flow is restricted. This allows the coolant to heat up much more quickly because it cannot make its way out to the radiator where heat is released from the coolant into the outside air. The thermostat is located at the coolant outlet pipe coming from the engine going to the radiator.
There are no fancy computer controls at work here, just old fashioned thermal expansion causing this thing to open. When the coolant gives off enough heat in the radiator to start bringing down engine temperature, the thermostat will react by closing slightly until the engine temperature is maintained at the optimum level for which it was designed.
Two problems result from a thermostat that has malfunctioned, but only one of them is well known because it leads to massive engine failure. The first and most dramatic thing that could happen when a thermostat goes bad is overheating. A bad thermostat can cause this when it simply fails to open at the proper temperature, or it gets stuck while opening and fails to open all the way.
When the engine overheats, the coolant heats up way past 190° to a point where the expansion of the various metal parts of the engine cause the parts themselves to fail. An engine can run safely up to about 230° F with no problems. When the temperature gets much above that the coolant can boil. Once the coolant boils it will build very high pressure in the cooling system and will boil over past the radiator cap or through the coolant expansion tank. This is why you see steam coming from some cars broken down by the side of the road. If temperatures get up to about 250° F the possibility for engine damage becomes a reality. Parts get weak and cylinder pressures go through the roof, literally.
The other thermostat failure that is not as well known but is probably just as common is overcooling. This is of course the opposite of overheating. Instead of the thermostat being stuck closed, overcooling happens when the thermostat is stuck open. If the engine is not allowed to warm up with the thermostat blocking coolant flow, it might not reach operating temperature at all.
Overcooling does not lead to a dramatic meltdown of expensive engine parts, which is why it goes undetected so much of the time, but it is still a problem that should be fixed. When an engine does not reach operating temperature it will not be able to maintain the correct air/fuel ratio. It will run rich, or with too much fuel. This is going to have an impact on fuel economy, but it will also have an impact on how much power the engine produces, and how much pollution will come from the tailpipe.
A typical engine that suffers from a thermostat that is stuck open might run as cold as 140° F. This might not seem that cold but it is. At this temperature you likely will not see your engine temperature gauge move very high off the lowest readings. In addition to this your heater performance will most certainly suffer. On those cold days when ambient temperatures are below 0, your engine will never come close to heating up, and you will probably have frost develop on the inside of the windshield.
Diagnosing a bad thermostat is no problem for an experienced technician. Many times a good repair shop will also recommend thermostat replacement anytime any work is done on the engine near the thermostat, regardless of whether or not it is currently known to be bad. Thermostats commonly go bad at inconvenient times, and considering it’s a $10 part, you should have it replaced anytime your mechanic is in the area.
Keeping the thermostat working will keep your engine running better, keep your fuel economy and power output up. And keep you from suffering a major breakdown from overheating in the middle of nowhere. Don’t forget about your thermostat, it would be a shame for a $10 part to cause so much grief.