Yanmar 3YM30

[tomperanteau]

Recently while motoring we had a belt break on the engine. I didn't realize that it had broken for a minute or two, and the engine had overheated. I shut her down and replaced the best. She started right up and sounded great. I thought I had made it through a potential catastrophe.

Now it seems that, after filling her with water to test her, the engine almost immediately blows the top off the overflow tank. I can run it from cold, for about 10 seconds, and pressure builds up in the overflow and the top pops off. The overflow will slowly fill and I have to shut it down. I noticed that if I have the radiator cap off, water fill spray out of the opening when I crank the engine.

Anyone seen this before? The engine is practically new with around 180 hours on it. Could it be that the water in the engine is almost immediately boiling and building too much pressure?

[dabnis]

Sounds like a blown head gasket with the combustion gases blowing into the water jacket?

dabnis

[tomperanteau]

Geez, I hope not! She is running really smoothly for the short duration that I have run her. Doesn't seem to be any mismatch in the cylinders, but then it is difficult if it is a small blow out.

[JimsCAL]

I agree with dabnis. Sounds like a head gasket. Hopefully the head isn't warped.

[smurphny]

Hope you didn't add cold water when the engine was shut off and hot. This can cause the casting to crack. Got my fingers crossed you don't have compression entering from cylinders.

[dabnis]

Originally Posted by JimsCAL

I agree with dabnis. Sounds like a head gasket. Hopefully the head isn't warped.

A compression test would probably show which cylinder(s) are low, indicating a leak into the water jacket. I can't think of anything else that would pressurize the coolant system that fast. Head gasket isn't too severe to change as long as the head is sound and not warped. A machine shop can check the head for being warped and if not too severe it may be trued up.
If so, valve clearances should be re-set, even if you just replace the gasket with no head work being done. Not likely the block deck is warped but it can be checked with a good sharp straight edge, at least for starters. A dial indicator, if there is a solid surface to mount it on? might show any variences?
Not sure exactely how to mount it, maybe someone with machine shop experience could advise?

Dabnis

[tomperanteau]

I'm thinking you guys are right. The engine is in a really tight place in my old Columbia. Plenty of room around it, just no access to the top unless it is pulled out. Guess it's time to break out the tools.

[dabnis]

Originally Posted by tomperanteau

I'm thinking you guys are right. The engine is in a really tight place in my old Columbia. Plenty of room around it, just no access to the top unless it is pulled out. Guess it's time to break out the tools.

Ugh!! sounds ugly. Good luck and let us know what you find. Before you rip it out you might want to do a compression test just to make sure, but as I mentioned before I can't think of anything else that would cause the cooling system to pressurize that fast? Especially after it overheated.

Dabnis

[fryewe]

I've seen this before. Pressure in the cooling system means exhaust gases in the water jacket = blown head gasket if you are lucky; warped head or cracked head if you are not. Sorry, Tom. Good luck.

[centaursailor]

A head gasket is not such a big job if you can get access.
The fact shes running well before she blows the water fill tank is a good sign, may be you will be lucky. Go with the advice and get the head skimmed. Check if the Yanmar supply thicker gaskets to allow for the skim.
Safe sailing