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Discussion Starter #1
In a modern yanmar engine with 75 hours, I observed what looked like low level of coolant. In the plastic expansion tank (not sure of proper terminology), it was right on the mark for low. The boatyard doing the 50h service said this was normal. Now when I look at the boat with the engine cold, that plastic tank is empty. The engine is not overheating or anything (yet?).

Questions: is this normal, at least as it gets cooler? I am worried I might be loosing coolant (no leaks in sight around the engine) or at least that I can no longer tell the level at all (it could have a gallon left or a quart and I wouldn't know?). Thanks!
 

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At the very least you should fill the tank to where you can see the level at all times.Then keep a close watch. Slow loss into the salt side of heat ex is hard to detect unless all other possibilities are eliminated. Is hot water heater in the circuit?
 

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When the engine is cold the coolant should be at or just above the low mark. You can add a 50/50 mix of coolant and water to the expansion tank.

You might also open up the cap on the heater exchanger WHILE THE ENGINE IS COLD and check the level. If you cannot see into the exchanger, you can use your finger to check. You should feel coolant before you feel the top of the coil inside the exchanger.
 

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Kynntana (Freedom 38)
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This has been a constant source of weirdness for me because the manual doesn't specifically state at which level the normal cold engine should be. I have a Yanmar 3GM30F with about 2600 hours. I keep forgetting to look at that tank when it has been running to see if it is filling up to the high mark. I just keep the level of 50:50 coolant at the low mark. Everything seems to be fine. This is one of those many things I plan to inquire about when I take a diesel class one of these days. Also, my manual says you should be able to touch all the engine parts by hand and if anything is too hot, it should be checked out further.
 

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Be sure to only top up with the right coolant, which is whatever coolant is in the engine. Start with whether its green or red...better to the exact brand and product.
 

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Discussion Starter #6
Yes, water heater is in the circuit, would that affect the symptoms or possible problems? I can't tell the kind of coolant from the color, I would assume red because that's what the yanmar manual calls for, but I did not have a chance to open the heat exchanger yet.
 

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My general take on this (cars too) is keep the coolant level at about the low mark when cold. Don't really worry about the level when hot.
 

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In a modern yanmar engine with 75 hours, I observed what looked like low level of coolant. In the plastic expansion tank (not sure of proper terminology), it was right on the mark for low. The boatyard doing the 50h service said this was normal. Now when I look at the boat with the engine cold, that plastic tank is empty. The engine is not overheating or anything (yet?).

Questions: is this normal, at least as it gets cooler? I am worried I might be loosing coolant (no leaks in sight around the engine) or at least that I can no longer tell the level at all (it could have a gallon left or a quart and I wouldn't know?). Thanks!
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I think of this as just like the expansion tank in your car. It should be no less than the low mark when fully cold. If it drops out of sight when cold, it is allowing air to be sucked into the cooling system, which the overflow tank is designed to prevent by keeping extra fluid available to be drawn in. My concern would be the fact that the level was at the low point 25 hours ago, and has now dropped out of sight. In a car with an overlflow tank, you would expect to operate for thousands of miles without any significant change in the level (when cold) unless something was wrong. Same thing in a boat, I believe. The 1st question is just how much you need to add to bring it back to the low point. Once you know that you will know the extent of the potential problem. Sometimes it may just need a pint or less, if the tank is small. If it needs much more than that, I would be watching the level carefully after filling it to the low mark (cold) to see just how fast it is dropping. I don't beieve that the change from summer to fall temperatures should be an issue, since the volume of water at 70 degrees or 40 degrees really doesn't change much. If you just need a small amount to bring the level to the mark, then it might just be temperature.
 

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If you suspect a coolant leak, try pressure testing it with a bicycle pump to make sure it holds pressure. Also consider installing a float sensor with an audible alarm so you'll never let it run out of coolant.
 

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Discussion Starter #10
Ok, the coolant level is all the way to the top on the heat exchanger, and it is greenish yellow. The expansion tank is completely empty, and I know it wasn't back in the summer. Confirmed there are no visible leaks.

Sounds like we could pressure test it now (without draining it?). It is also safe to use for occasional limited distances. Would be ok to wait to drain/replace it by long life coolant in the spring, and pressure test it when it is empty?
 

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You may have an air "bubble" somewhere in the system? You could try removing the cap when it is cold, then fire it up to where it is warm. After it cools, add coolant if necessary.
Some engines have an "air bleed" screw near the top of the block, if so, loosen it until coolant flows out.

Paul T
 
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