Fresh water overheat

[John G]

Bavaria 30 Volvo  D 1-20. Engine running fine for 2 hours then overheat alarm. Coolant forcing its way out of expansion tank. Clearing very hot. Raw water circuit working normally with water exiting the exhaust and exhaust temperature sensor (i fitted) normal. Any help gratefully received

[SYJetzt]

Maybe one or more of the following issues:
Not enough raw water flow, raw water circuit partially blocked,
impeller ripped,
filter clogged .
Thermostat not working.
Head gasket blown.

[Diverphil1]

as sy posted but add to check exhaust elbow, a pinhole in the raw water to exhaust manifold would let exh pressure into the system. before it gets bad enough for your coolant to escape.

[sunshine]

I think some people have reported an air lock in the coolant side, leading to overheating. Cure was to make up a little bracket to raise the expansion tank to make sure its the highest point in the system.  I've never had this problem, so cant comment on how real a problem this is, or how effective a solution.