I believe the fan clutch is used to allow the fan to NOT spin when the car is cruising at highway speeds. By not spinning at High speeds, less strain is put on the engine, as the amount of air passing through the radiator is greater then what the fan could possibly deliver.

Sorry to hear about the head gasket, usually head ghaskets go when a car is run hot, but there are other causes of this type failure as well. But there are usually other indications as well. A cooling system pressure test is the best way to check your cooling system. Just pressurize the cooling system and look for leaks. If the pressure doesn't drop, then you have no leaks, if the pressure drops then you have a leak. It will show you where the leak is. If the pressure drops and you don't see where the leak is, then it's usually internal.

I don't think the mirror on the exhaust is effective because one of the by-products of combustion (with a cat) is water. How are you supposed to differentiate this water from the steam caused by a leaking head gasket???

Pressure testing the system you will know for sure. Other symptoms of head gasket failure are 1) a/f in the oil (coffee - with cream - colored oil) or a/f boiling out of the cooling system. If the a/f is being sucked into the combustion chamber, a stop leak should help for awhile, provided the engine doesn't run too hot, but stop leak gets mixed reviews because of it clogging the cooling system.

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