Toyota Repair: 93 previa, blown head gasket, hydrocarbon gases

Question
how can you tell if you have a blown head gasket? while the motor is running it is missing,gets hot and when you open the coolant cap the water blows out.  

Answer
That's a pretty good indication that there may be a headgasket problem or even a crack in the cylinder head but first I would try replacing the thermostat, if the thermostat is stuck closed it will exhibit these symptoms also, if the problem persists I would recommend having a chemical block test done to see if there are hydrocarbon gases in the cooling system.