yanmar overheating

[tjandoverguy]

i have a yanmar 4 cylinder diesel no terbo 1300 hours 2008
last week overheated it was low on coolant put allmost a gallon in ,motered
for 6 hours ran perfact
this week coolant was full overheated in 1 hour
sea water intake clean impeller was fine
remover coolant pump all was fine with that boiled thermostat opens at 165 full open at 190
put it togeather ran for 1/2 hour and ok temp 175
any suggestions what to look at next

[JKMilligan]

Ideally get a temp gun and check actual temp of relevant eng components. May not be overheating and it’s the sending unit or gauge.

[Captain Bill]

check the heat exchanger, it may be clogged. You may be loosing coolant through a leak in the exchanger as well.

[tjandoverguy]

i put a heat gun on it and it was 200

[Attikos]

Originally Posted by tjandoverguy

i put a heat gun on it and it was 200

That, or not far above it, is when the overheating alarm goes off, at least on mine.

Someone mentioned the heat exchanger. Have you also considered checking the mixing elbow? I know that's what everyone says to the point it must sound trite, but there's a reason they do.