What are the signs of a bad coolant tank cap in 1998 ford contour lx?

Cap seems to be dead and coolant dissapears slowly with no leaking.
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Answers
If you still loose coolant after replacing the coolant tank cap, then do a pressure test on the system and look for external leaks, or (worse) pull the spark plugs and look for coolant on them while the system is still under pressure.
You can also use a simple chemical test to detect combustion gases in the coolant itself. Not all head gasket problems show water in the engine oil.