Does the engine consume coolant
If so it could be a head gasket.
After checking the coolant
for oil and the oil for coolant....
Check the riser elbow
, where the raw water
enters the hot exhaust pipe.
Perhaps not 'enuf' water is entering the pipe to cool the gases.
Disconnect the raw water
feed to the elbow
, start the engine and measure/check the flow...
It should match the water coming out the exhaust pipe after reattaching.
If not the riser is corroded and clogged; holding back the raw water from the pump.
The riser may have to be replaced.
I've ever had much luck cleaning/scraping/boiling them to clear the flow....