There are two possible sources of the pressure to drive the water out.

One is gasket failure and exhaust in the water passages. often there will be iridescent/oily sheen material floating on the water if this happens. This is a bit of oil/fuel etc/ blown through the gap. It might also smell different

the other is boiling water inside the engine block due to failure of the water to circulate. So it is cool in the rad and boils in the block. That could be caused by a bad water pump. In this case the water will not get contaminated.

a good mechanic can settle this issue.

I too had an impeller break up on the water pump. Sort-of did the same thing as the OP. Didn't get to the need to change all the gaskets though. It was an older cast iron engine. They were able to withstand more abuse than the modern lightweight ones.

A stuck-closed thermostat will do the same thing. It too can cause the block to boil and put a lot of backpressure behind the pump, eventually ejecting the coolant.

Another clue may be the compressor leaking high pressure air into the waterjacket too.

A coolant gas 'sniffer' will do the trick in identifying just what's in the system. They are available at auto tool and supplies stores, but any good mechanic should have one, and should be happy to help you with this.

Cheers,

Stu

Try removing the radiator cap before starting the engine and after starting the engine watch for bubbles in the liquid in the radiator.

It's possible that there is a crack in the head itself that opens up when the engine is up to temperature. Was the head magnafluxed when you changed the head gasket? If you have to remove the head again I suggest that it be checked for cracks.

Some really good advice already posted, check that out first, but if that does not help then:-.

Did you check the head(s) for flatness when you replaced the gaskets? Its possible that one (I have no idea how many heads this engine has) is warped too much for a gasket to seal for long......

The radiator should have pressure on it if the water circulator is operating properly. If the reservoir is overfilled the coolant tank may over flow. Does the engine temperature increase after the coolant comes out of the reservoir? If no then it is overfull and the system is self adjusting the coolant level in the tank.

What you are describing is not unusual after a repair requiring the coolant be drained from the system. Several heating and cooling cycles maybe required before a proper system coolant level can be achieved.

Pay Attention to the water temperature gage, having a head temp gage attached temporarily after some repairs is a good idea too.

GA.

Stu.

One possibility is an air lock particularly if you have heater pipes that arc upwards creating an air cavity at the top. The air expands when heated causing blowing of the coolant. Some vehicles have heaters higher than the radiator and this can get air locked. An air locked heater will run cold so you can test for this.

I'm also thinking possible thermostat. Try buying one of those flush kits, it's a tee that goes in your heater hose and attaches to a water hose to flush out the system. Also, disconnect the inlet and outlet hoses to your heater core and flush those out individually, you'd be amazed at how much crud comes out. Try that and replace the thermostat and you should be in business. A cracked head wouldn't cause water to back up into the reservoir. Hope this helps.

A cracked head CAN do just that, as several people have already mentioned. The exhaust gases escape through the crack into the cooling water and pressurize the cooling system. Often the bubbles seen are "white" and when they burst, exhaust gas can be seen and smelt.

This is a fairly common occurrence when a cylinder head is cracked where the cooling system is passing close to the combustion chamber. The crack is often so small that water cannot pass back into the cylinder head....though that can also happen....

I stand corrected, I hadn't thought of that. I've experienced a couple of cracked heads and both times had water coming out of the tailpipe. Hopefully a cracked head is not his problem, it's always a bummer.

If the motor is a "Parent Bore" block rather than a "sleeved" block then you might have a condition known as "Porous Core" the correction of which is to bore it oversize and fit an appropriate sleeve.

This was a common problem on 500ci Bedford diesels, as they used a 466ci block without the sleeves to avoid the costs of tooling up for a larger bore sleeved block.

Often this problem wouldn't eventuate for quite some time but often after the first oversize rebuild.

However I'd have the cylinder head(heads) hot pressure tested and evaluated as a first step. Certainly after an over heating incident where you have to replace the head gasket.

Did you ensure that the cooling system was properly purged of air when you refilled it?

Was the block deck and head mating surfaces "square" and clean before you fitted a new head gasket?

Did you torque down the head bolts to the proper tension using the right sequence?

Did you try to reuse the "stretch" style head bolts? Many mechanics have fallen foul of these one shot bolts. In Australia, Ford started using these on the E series falcon cars but didn't tell anybody. A lot of taxi owners needlessly replaced heads and gaskets due to not knowing about this style of head bolts.

Not knowing what model truck and engine makes it a little difficult to be more specific.

Cheers

Perry