Thermostat and Cooling System

Does the Euro-Yanmar 3GM30F cooling system (engine loop) have a thermostat? I checked the manual and list archives. No mention.

I drained the antifreeze (3/4 gallon) from the top and bottom petcocks. When I refill will I have to run the engine and wait for a t-stat to open? If not, I should be able to refill in the boatyard prior to launch and avoid one more last minute job at the dock.

JeffC

I've never disassembled my engine far enough to get a good look at it, but according to the Yanmar Diesel Engine Parts Catalog the engine does have a thermostat: part no. 121750-49800. It lives under the elbow on top of the mounting bracket for the fresh water circulating pump.

The Yanmar Service Manual warns to remove the coolant filler cap when draining the system. Otherwise it won't all drain. There are three petcocks to check. My recollection is that after draining the two with short lengths of tubing on them, nothing was left to come out of the one on the aft end of the heat exchanger. But I wouldn't skip the middle" one that is just above the oil dipstick.

BTW, I splurged on a purchase of the catalog and service manual from a Yanmar dealer recently. The catalog is especially handy as it has pretty much every part number. But more importantly it has lots of nice big blow up diagrams of all of the engine subsystems.)

Scott Thompson

Surprise, #653

Scott,

Thanks for the reply. I wish the Yanmar manual had instructions for filling the cooling system, or anything else for that matter.

Do I need to remove the t-stat and fill the block before I run the engine? I've never done that with an automobile engine. I guess I'll just fill the heat exchanger, run the engine and wait for the t-stat to open.

BTW, the antifreeze that I drained from the system was very thick. It seemed as though it was not diluted 50%. That could explain why the engine ran a little hot all the time.

JeffC

Coolant is pumped from the heat exchanger, through the pump, to the bottom of the block. From there it goes to a chamber in the pump mounting bracket at the top/front of the engine. The thermostat is in this chamber. When the thermostat is cold, coolant is diverted straight back into the suction side of the pump. When the thermostat heats up and opens, this diverter passage is blocked and the passage through the elbow on top of the mounting bracket opens, letting the hot coolant flow to the heat exchanger instead. (I'm not on the boat now, and this wouldn't be in the manual, but my recollection is that it goes to the hot water heater first.)

Since the pump draws coolant directly from the heat exchanger and pumps it directly to the block, the block should fill quickly once you start the engine, even with a cold thermostat. But you probably can't get all of the bubbles out until the thermostat opens, letting coolant circulate through the hot water heater, etc.

The method that you have in mind worked fine for me when I did this job last year. Fill it up, run it a bit until the thermostat opens, then top off. Repeat until it won't take any more. Finally, close it up and add a small amount to the reserve tank. It's really just like an auto system. The principle is the same, except you have a heat exchanger instead of a radiator.

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T. Scott Thompson

sthompson@toad.net