Odd coolant leak issue
We are prepping the boat for launch on Monday and noticed on Wednesday that on on Westerbeke W-52 diesel, the coolant expansion tank for the heat exchanger was empty of its customary green diesel coolant. I filled it and returned to the boat yesterday. The coolant was down below the "max" line about 1/2" (1 cm.) I started the engine on the cradle (I have the ability to fill the sink and to run water into the raw water circuit) and it started quite well.
I ran it for about 45 seconds, which drained the sink and sent the pink winterizing antifreeze out of the boat.
I then examined the expansion tank. No change.
We examined the dipstick. It is blissfully free of water or coolant, although a problem with the anti-syphon loop spring plunger last year caused many oil changes. I am doubtful that the two issues are related, as I didn't note any loss of coolant in the closed circuit last year.
We eventually found the missing green coolant in the bilge under the engine...about a litre and a half (just over a quart).
So now I have an obvious leak that occurred over the winter, but which I can't replicate now. Is it possible I have a gasket or a seal failure in the heat exchanger, the hoses or...and I hope this isn't the case...the block itself that has "healed itself" with the arrival of warmer weather?
We have to get our boat back to our club on Monday, about a six-mile motor and/or sail. It has been suggested to me that after I am dropped into the marina basin that I borrow a dock for a few minutes and fire up the engine and watch the coolant levels and the temperature closely. I will also purchase a few jugs of coolant in case I have to replenish en route. When we get to our home dock, I will take off the heat exchanger and have it examined and pressure-tested. At this point, I just want to get back to that home dock. Monday will be a light-air day, so I will rig all sail. If necessary, I can likely just run the motor for a couple of minutes at either end of the trip.
Any suggestions on how a coolant leak in winter can just "go away" are welcome. I would like to eliminate some possible causes before I call in a mechanic.