I have a Universal M2-12 diesel
in my Pearson
27. When I bought the boat, it had an 11-gallon hot water
heater in the lazarette, along with a remote coolant
expansion tank. The heater was plumbed into the engine coolant
system, as designed. The pressure cap on the remote
tank is 15 psi, and the pressure cap on the expansion tank at the engine
is 13 psi. There is an unpressurized plastic coolant recovery tank in the engine compartment. The recovery tank has a vent hose at the top, which empties to the sump under the engine.) The recovery tank was always dry when I ran the boat with the hot water
system in operation, since coolant expansion was handled by the remote tank in the lazarette.
According to the Universal manual specs, the coolant system holds 2.3 quarts and operates at 14 psi. I assume this is without the hot water circuit plumbed.
About two years ago, I took the hot water system off-line and placed a 6-inch loop on the inlet/outlet. Ever since, I have had overheating
problems due to gradual loss of coolant. Everything will run fine for awhile, then the temp starts shooting up. I shut it off, add more coolant, and it goes back down.
I suspect it is because the cap is rated at 13 psi, and that too much coolant is boiling into the recovery tank. I just checked online, and the replacement pressure cap is rated at 14 psi.
I feel rather sheepish that the solution is this simple, I was worried about a blown head gasket
or something else... but does it look like I diagnosed the problem correctly?