Engine Coolant Level

[dmksails]

Hi All - I have a Yanmar 3GM30F. I normally will make sure the coolant is about 1/2-in below the fill cap. I just ran the engine for 6 hours and at the end I noticed that the coolant was down about a cup - and about that same amount was in the overflow tank.

The book is not clear on how full to fill it with coolant - it just says fill it. So how full is full?

[Cheechako]

Well, my experience is that it will offload into the overflow tank. I would say it seems to stabilize maybe 1/2" below the cap, if you fill it up it will end up there anyway. But this may be variable based on engine and other things...

[tkeithlu]

General case, not specific to your engine: The cap is spring loaded to seal until the coolant reaches a particular pressure, based on not boiling until reaching a particular temperature somewhat in excess of 212F/100C. Some engines then vent into the overflow tank, and suck it back in when the temperature/pressure drops. Cars do that. Some vent into the overflow tank and leave it there, which sounds like your engine. Mine dumps it all over the engine.

So, if you have found some coolant in the overflow tank, the engine has dumped some excess, and that's OK. If it's a lot, or it throws steam all over the compartment, you're boiling coolant under pressure, or have a defective cap, and your heat exchanger may not be doing its job.

The experience you've had with it does not sound like a problem. Keep an eye on it.