Depends on how it breaks honestly. If it gets stuck closed the car gets hot. Stuck open and you will take forever to warm up and in colder areas people seem to notice because of the lack of heat blowing on them
Some people say its bad on the engine over a long period of time, others argue it doesn't do crap. It can cause stress on places where two diffrent metals meet (aluminum head vs. iron block) and can cause problems with condensation not burning off properly during short trips. If any damage is really done by either it takes time.
Doug, you know as well as I do that overheating kills motors pretty quickly.
Replacing the thermostat is a simple job, 15mins of wrenching and 12hours of waiting for the silicon to cure. The directions are on webtech.
Make sure the car is cold, have all the parts and tools, etc. Pull the rad cap, drain the radiator. Unbolt the thermostat housing, pull out the old t-stat and throw it away. Clean the remains of the gasket off of the housing on both sides, put a rag on the engine side so pieces of gasket don't fall in. Slop the new gasket, t-stat, and housings with (blue?) silicon sealant, drop the t-stat in (make sure its right side up!). Put the housing back on, bolt down (I think the torque spec is 25ftpounds, check webtech to be sure), and wait for the sealant to cure (overnight usually). Refill the radiator, burp the cooling system, and you should be good to go.
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