Originally Posted by transmitterdan
From your description it appears your generator
does not now have an expansion tank. It must be very old if it did not come with one from the original manufacturer. You may have an issue where the water
you are adding is simply replacing what has been forced out the expansion type pressure cap. I would recommend that you verify whether or not the generator
requires an expansion tank and proper expansion type pressure cap. If it was designed to have an expansion tank then seriously consider restoring that feature.
I agree on the recommendation. I have been on two very expensive boats that had a similar issue with a Cummins genset that was supposed to have an expansion tank (provided by manufacturer) installed separately but the builder
did not install it. If you do not let the genset cool down without load for a few minutes the lack of expansion tank will cause loss of coolant
every time it boils over upon shutdown..