Mysterious Winter Coolant Leak

[Marc14857]

During the past 5 years I've owned two new boats an H356 with a 3GM Yanmar and an H44 with a 4JH Yanmar. Each engine experienced the same coolant leak during the winter storage. In the fall, I would perform all of the routine winterization on the engine and ensure that the coolant level in the expansion tank was somewhere between the low and high marks. When I would come back in the spring to recommission the boat I would find the coolant expansion tank empty and a puddle of coolant 4 or 5 ounces in the engine bilge just below the engine. No tell tale indication of leakage path on the engine or the expansion tank. Taking off the radiator cap I verified that the level in the radiator was at the top or very close to the top of the radiator neck.

I just opened my boat this weekend to start commissioning and not surprisingly the same think happened, empty expansion tank and small puddle of coolant under the engine and no tell tale signs of a leakage path. As this seems to happen every year and with two different engines, I was hoping that someone might be able to figure out what was happening. I should also note that once I refill the expansion tank the engine is good to go for the season.

 

[Bill Roosa]

A long shot but the engine water pump (the antifreeze one not the salt water one) shaft seal is one of those "O-ring" type. With cold weather and no engine run time the seal can (a big can here) allow coolant to ooze out over time. the daily heat/cool cycle works the shaft seal allowing the oozing. Once you start the motor or it get warmer out the seal closes up and the leak automagicly goes away.
Pull the water pump pulley and check under the water pump shaft right at where the water pump shaft comes out of the pump proper for wetness. As this is hidden from view normally it would escape detection under just an eyeball inspection.

 

[Rich Stidger]

Marc,

I have the same issue with my Volvo MD22L-B in my 1997 h40.5. In my case I can detect drips of antifreeze from some of the rubber hoses- not near the water pump. I have replaced hoses though I've never found a defect in them. I even have suspected a leaking freeze plug but I have not confirmed this as a source.

It is bizarre- the leak only occurs over the winter....

 

[Ivan]

Overflow hose?

Marc:

Another long shot, but it could be as simple as a marginal seal between the hose from the engine to the overflow tank. Cold winter temperatures would definitely cause the metal (copper or brass, or whatever it is, but metal for sure) nipple sticking out from the engine to shrink more than the rubber hose due to their different thermal expansion coefficient, and could cause the seal to be loosened enough to cause a very slow leak. Slow, but over several months maybe enough to empty the tank???

 

[Marc14857]

Re: Overflow hose?

Ivan,

I did think of that one last year and put a small hose clamp on the nipple. Did not make a difference. It is strange though that the loss seems to be limited to just the expansion tank

 

[Don Lucas]

The anti-freeze does freeze. Just not solid, more like slurry. Are you sure it isn't just backing up in the expansion tank and leaking out. Then when it warms up the level is just less.

 

[captron]

Expansion tank leak

Marc,

I have the same issue with my Volvo MD22L-B in my 1997 h40.5. In my case I can detect drips of antifreeze from some of the rubber hoses- not near the water pump. I have replaced hoses though I've never found a defect in them. I even have suspected a leaking freeze plug but I have not confirmed this as a source.

It is bizarre- the leak only occurs over the winter....

This is too strange, I have the same Volvo engine as Rich, only my issue is slightly different, meaning if I fill the expansion tank 1/4 to 1/2 full it will leak out to about 1/4 inch from the bottom, but not just in the winter, most any time makes no difference if the engine is running or not.
I pulled the expansion tank filled with soapy water and pressurized it to see if there was a crack in the tank, not evident, also checked the hose fitting at the bottom of the tank and secured a zip tie, still does it, puzzled!

Capt. Ron
Wind Dreamer
H 40.5
New bern, NC

 

[Capt Mike]

Must be a common occurance, my H386-3JH3E does the same thing. Had to do some work on the engine this week, noticed my expansion tank was empty, two or three ounces were in the engine bilge! Seems to be some residual fluid around the thermostat housing or engine water pump.

 

[Rich Stidger]

Capt Ron,

Ah ha! Another fellow 40.5 leaker has surfaced! Check the large hose in the center of the picture below for wetness. This is the point where I can find the leak, but I can't find the real source. It seems like the leak is from above the hose. This is the hose I have replaced and I have even used Permatex on the hose surface where it pushes onto the fitting.

Still leaks.

 

[chuckwayne]

Marc,
I've got the exact same issue on our 2002 356! also tried the hose clamp on the expansion hose/nipple without success. I also go aboard during the winter and there was no sign of the slurry backing up into the tank, nor is there any wetness around any of the hoses, the water pump, or the sides of the engine! a real mystery! after spring arrives, no problem.

 

[John T1594]

Weird!

I too must join the winter leaking engine club. My 3GM30F leaks about an ounce over every winter except the past one. Yes, the expansion tank is down a little. I have placed paper towels and searched and searched but can't find the source. The engine runs well with no leaks and no loss of coolant during the boating season.

 

[Bill Roosa]

Expansion tank loss is explained as follows.
The leak is clearly a "one way - out" one. that is to say that (going hypothetical here) as the engine warms the anti expands and forces fluid out the leak. As the engine cools the anty contracts and the leak proves sufficiently unleaky in that “sucks air” mode so as to create enough vacuum to operate the coolant cap valve on the engine which then dutifully sucks a little anty into the engine from the expansion tank.
So you are looking for a leak that is really one way out and only occurs upon slight pressure or slow pressure increase.
My guess is that the pressure is coming from the warming of the engine and I'd predict that you don't see it in the fall and winter but do in the spring as the interior of the boat warms up. You could try a pressure tester on the coolant cap hole.
What should really frost your cookies is the amount of anty gone from the expansion tank is larger than the amount found under the engine!!!
But I can’t tell all my professional secrets. ;-)
All this goes away when you start the engine though so I'm becoming more confidant it is the water (anti) pump shaft seal.

 

[Solace H340]

Sure it is coolant?

I had a leak under the engine and it turned out to be diesel fuel. The fuel line banjo fitting to the engine wasn't as tight as it should have been. At first I thought it was coolant because it was yellowish and the level in the coolant tank goes down significantly when the engine is cool. Looked the same color as the picture in the postings. Might be worth checking this possibility out.