I have a 3306PC no. 66D01651 in a boat. For the first time I noticed a loss of coolant, didn't think much of it and topped it up with about 6 cups.

The next time I was on the boat I checked the coolant and again it was down so I topped it up with about a litre or 2 pints of coolant.

Today I was on the boat and the motor hadn't been run since I last topped up the coolant but again the coolant was down and the engine oil was up. I sucked some oil out of the sump and the first thing that came out of the pump was coolant.

I ran the engine for a minute to see if there were any bubbles in the coolant or in the overflow pipe to try and see if it was a head gasket (youtube) but nothing.

So, am I in trouble? Any suggestions? Engine hour meter stopped at 6,000 hours but the engine was used for harbour cruising and would have had a pretty easy life. Except for being forty years old.

I think next step will be a pressure test but any input would be appreciated. Engine has an oil and gearbox cooler.