Yanmar 18hp over heating

[steveebryant]

Anyone know anything at all about small diesel engines?
I had to replace a hose that ran from the engine to the domestic water heater on the boat. Of course a lot of coolant ran out. When I reconnected the hoses and filled that part of the water system, the engine started to overheat.
On this 18 hp Yanmar, there are two separate water systems. One for heating domestic water; this is a closed system which employs a heat exchanger of sorts and is the system that I was working on. The other water system is use for cooling the engine; this system takes water from the lake to cool the engine. To my knowledge, the two systems are not connected.
The engine has been run a total of 20 hours since I have owned it and it has never over heated. After I changed the domestic hot water hoses, the engine will get hot after running but a few minutes.
Anyone got a clue what might be going on?
Thanks,
Steve

 

[coleman]

Have you checked the coolant level? It might be that you just have air in the system. If you have a flexible impeller pump, which are fluid-tight for self priming while turning, you may need to turn the engine a few times to prime out the system. The system is not necessarily like your car where coolant and can just trickle to the low points and push the air out (though some of the newer Yanmars have centrifugal pumps more similar to that of a car).

If you have topped off the coolant after overheating, you may need to check and see if the pump was damaged by running dry and is no longer functional. I recently did this to my raw water pump by forgetting to open the thru-hull (d'oh!!!). Not a hard item to replace, usually not a pricey one either.

Also, do you have a coolant overflow container set up to burp any air out and draw in coolant? It's a good idea because you can see air bubbling out before the boil over occurs. I need to add one to my own boat.

 

[John T1594]

A very common problem with these engines is that an air bubble or air lock forms in the system when you change coolant. It usually occurs around the water pump or any high place in the system. You have to find a way to burp that air either by removing a hose or with an air bleed screw on the pump - if you have one.
Sometimes the air will clear after the engine cools down and you can add more coolant through the fill port on the heat exchanger. I'm sure others will have some more specific advice.

 

[steveebryant]

John,
I believe that my problem was that I did have a bubble of air in the closed cooling system and after I tightened the engine belt, that water pump was sufficiently strong enough to force the air out.
Thank you for responding to my question,
Steve

A very common problem with these engines is that an air bubble or air lock forms in the system when you change coolant. It usually occurs around the water pump or any high place in the system. You have to find a way to burp that air either by removing a hose or with an air bleed screw on the pump - if you have one.
Sometimes the air will clear after the engine cools down and you can add more coolant through the fill port on the heat exchanger. I'm sure others will have some more specific advice.