Yanmar 2GM20F Coolant - SailNet Community
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post #1 of 10 Old 06-14-2015 Thread Starter
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Question Yanmar 2GM20F Coolant

Hey everyone,

I have my first diesel - a yanmar 2GM20F.

Yesterday the exhaust stopped producing water and the WC Alarm sounded within minutes of starting the engine before I even got out of the marina. I brought her back into the slip immediately and here's what I observed:

The Coolant overflow tank cap had blown off and the overflow tank was over full (above the "FULL" mark).

1. I checked the impeller and it's fine. I made sure that they water pump belt was tightened appropriately.
2. I installed a brand new mixing elbow a month ago - so that's not the problem.

What could it be? I motored 5 days ago without any problems. I am wondering if maybe the belt on the water pump was too loose and maybe wasn't turning the pump, causing the engine to overheat and the pressure built up in the overflow and exploded the cap off? I have no idea. I am the least mechanically inclined person I know (although I did successfully change the impeller and the mixing elbows myself).

Should I pour the coolant back into the top of the engine (there is a cap that looks like a radiator cap on my car)?
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post #2 of 10 Old 06-14-2015
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Re: Yanmar 2GM20F Coolant

Intake water strainer clogged?
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Re: Yanmar 2GM20F Coolant

Starting from where raw water enters, is there any flow restriction (something clogging the intake outside on the hull), is the thru hull all or partially closed, the hose to the pump clear? When you remove the hose from the pump with the thru hull open does water flow into the boat? if not, there is a clog somewhere. You can try blowing things out with either a water hose or just blowing air out the hose. If that is not the issue and the impeller is sound, could an old impelled have broken and pieces traveled to the riser? What is the age of the engine? Could carbon have accumulated in the riser restricting exhaust flow?
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Re: Yanmar 2GM20F Coolant

Thanks for the responses!

Pardon my ignorance, but where will I find that water strainer?

Water flows from the raw water when I took apart the pump. As for the flow going to the riser, what is a riser and where would I find it?

I do not know the age of the engine, unfortunately.
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Re: Yanmar 2GM20F Coolant

I have a yanmar 2GM20F and after motoring for a short time (10-15 minutes) the cap to the coolant expansion tank blew off and filled up above the "full line" it boiled over and I lost some coolant into the bilge.

also water stopped flowing out of the exhaust. I've checked the impeller and it's fine. I also replaced the mixing elbows a month ago.

is it ok to restart the engine to get the coolant back into the engine?
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Re: Yanmar 2GM20F Coolant

If your exchanger is clogged, it may be the cause of the overheating and lack of water flow. As mentioned, check the outside strainer cap, then the valve, then any inside strainer, right up to where the hose enters the pump. Check the impeller again. If still in doubt, start/stop the engine quickly with the hose off the downstream side of the pump to see if the thing is pumping. If it is, that narrows it down to flow through exchanger and riser. The riser is easy enough to check by just blowing through it. If the riser is clear then it's time to clean the exchanger.

The water enters the boat through the seacock. That almost always has an outside strainer which can get slogged from seaweed, etc. Once through the seacock, there can be another inside strainer which looks like a clear cylinder or a saucer-like device. That may be clogged if you have one. From that filter, the water goes to your seawater pump and then to the engine.
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Last edited by smurphny; 06-14-2015 at 03:00 PM.
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Re: Yanmar 2GM20F Coolant

OK - so I have resolved the water exhaust issue. Must have been as simple as the water pump belt not being tight enough - because water is exhausting properly now.

I ran the engine at 900 RPMs for about 5 minutes then cranked it up to about 3000 RPMs in the slip to see what would happen. Sure enough the warning buzzer sounded and the CW warning light comes on.

There is coolant in the overfilled reservoir above the "fill line" so I am guessing the problem is somewhere with the coolant (it's the older green stuff in case that matters - i have read that I may want to change to red).

So do I pour the coolant out of the reservoir and into the top of the engine into the cap (that looks like a radiator cap on my car)?
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Re: Yanmar 2GM20F Coolant

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Originally Posted by dbw_kairos View Post
OK - so I have resolved the water exhaust issue. Must have been as simple as the water pump belt not being tight enough - because water is exhausting properly now.

I ran the engine at 900 RPMs for about 5 minutes then cranked it up to about 3000 RPMs in the slip to see what would happen. Sure enough the warning buzzer sounded and the CW warning light comes on.

There is coolant in the overfilled reservoir above the "fill line" so I am guessing the problem is somewhere with the coolant (it's the older green stuff in case that matters - i have read that I may want to change to red).

So do I pour the coolant out of the reservoir and into the top of the engine into the cap (that looks like a radiator cap on my car)?
If you are still overheating, it could be that the thermostat is not opening fully when the engine gets hot or you are still not getting adequate water flow even though some is coming out the exhaust. A semi-clogged heat exchanger could be the problem. There are many other causes of overheating but these are the most likely and easy to check. Sounds like you need a service manual:-)

Open the automotive type cap to check that there is fluid in the exchanger. Then fill and check that the expansion reservoir stays at the right level when the engine is hot. If the pressure cap blew, you lost some unknown amount of coolant. If there was still fluid in the reservoir then the system is probably still full but check it anyway. It is VERY unlikely that the problem is the type of fluid.
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Re: Yanmar 2GM20F Coolant

It could be that when the engine overheated the first time and the coolant boiled over, there is now some air in the system. If that's the case, it will take a bit of bleeding to get it out, but its a pretty simple fix.
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post #10 of 10 Old 06-15-2015 Thread Starter
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Re: Yanmar 2GM20F Coolant

thanks @JimsCAL and @smurphny

I seem to have resolved the issue.


It appears that the coolant boiled over and I need to add some. I just have to figure out what kind I can add (I want to wait to drain this winter if possible). But if I am not allowed to just add some, I will probably switch from the current green to the red that yanmar apparently is now recommending.
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