What causes my Mitsubishi Montero Sport 2001 coolant blowout after engine stops?

Answer 1

If your symptom is that you power the engine off and then, moments later, coolant starts to spray out of the radiator cap/coolant reservoir then you have a cracked cylinder-head and/or broken/failed head-gasket.

What is happening is the crack in the head (or tear in the gasket) is letting combustion gases into the cooling system. These gasses collect at the highest point and effectively "air-lock" that area keeping coolant away from the metal. When the engine is shut off these gasses shrink and allow coolant to touch the blistering-hot uncooled metal surface and instantly boil the coolant into steam which provides the pressure that is driving out your coolant.

If the gasket was the original cause it is very likely that by running it this way the cylinder-head is now ruined and you should expect to have to replace it.

Hope this helped