Cap "blew off" ?
Caps are held in place by a bayonet mount. Short of creative use of minor explosives, they don't blow off. The spring loaded gasket
underneath compresses and releases steam, coolant, and small fish
all over the engine compartment. They do that at a particular over-pressure, defined by the spring. The underlying idea is to allow a higher operating temperature, by raising the boiling point through greater than atmospheric pressure in the coolant.
OK. To spray stuff there's gotta be that pressure. A leaking head
gasket is one possibility, and you hope it's not that, because it means removing the head. The other is boiling even at the set pressure, which means you've not getting enough cooling
. A stuck closed thermostat is one possibility. That's easy to test; heat the thermostat in water
on the stove
, and see at what temperature it opens. If it doesn't open by the time you reach boiling, shout EUREKA, toss it, and start over.
Then there's reduced heat flow through the cooling
system. The heat exchanger
being junked up is one part of it, but the rest of the system may need a thorough flushing
as well. Drain it (lowest hose connection if you don't have a drain plug), flush it, and refill. Messy.
If you found a mess in the heat exchanger, you're on to something: flush the rest of the system and hope that you are home free. Then do the other engine. Good luck with it. You have a messy Saturday in your future.