mazda 626: radiator cap spews coolant (cap is not broken). Engine doesn't overheat.

Take the cap off. WIth engine COLD, does the coolant erupt through the reservoir??? Then you have a HEAD GASKET problem. THe white smoke is most likley coolant getting burned. You might try re-torquing the head, but in the proper sequence. THis MIGHT seal the head gasket. At least, it should reshape the head. This is a GOOD thing. When a head gasket fails, combustion gases can be forced into the cooling system. The result is MOUNT VESUVIUS, because the piston(s) are pushing gas and air past the gasket, and into the cooling system. Good luck!!
 
Blown head gasket. Don't drive it until you have it pressure tested by a mechanic. It shouldn't cost much for the test. The repair wont be cheap. But you could be buying a whole engine if you keep driving it. More than likely compression is leaking into the water jackets because of a faulty head gasket causing it to force coolant out past the radiator cap. The white smoke is actually steam and I bet it smells like antifreeze.
 
Too much pressure in the cooling system from a bad head gasket or cracked head allowing engine compression into the water jacket. The cylinder head will have to come off to be fixed.
 
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