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The expansion bottle on the cooling system of an engine is there to catch coolant if over filled and it expands ? Yes or No? 
If yes, why does it have a min and max line etched on it? 
If no, is it a top up reservoir ? And if it is a top up reservoir and the level drops, where is the coolant going, if not found in the bilge?

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It's where you fill the system from. Max line is to show where you should fill to when cold. If you overfill it, then as the coolant gets hot and expands it may crack the tank and leak.

Min mark is there to show the minimum serviceable level of coolant. In practice it also shows whether you're losing coolant e.g.if you fill to max and two months later you find it at min you know you're losing fluid.

As for where it goes, if it's not lost via a hose leak into the bilge then the next place to look would be the cylinder head gasket. A gasket leak can allow coolant from the water jacket into the combustion chamber where it will get pushed out of your exhaust as steam.

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The coolant could be leaking anywhere it travels, be that hoses, heat exchanger, thermostat housing or head gasket.  Is there pressure in the reservoir after the engine's been run?  Check hose clips carefully, especiall V clips as even marine grade stainless clips still have mild steel bolts.

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It was explained to me that the bottle was an overflow tank. There is no pressure in it because it’s not sealed, it has a push top lid but that lid is just pop off pop on tight not screw on. I always fill from the top of the engine, like you would on a car, unscrew the cap and poor the coolant in. Could it be that there is air circulating that once finds its way out, it sucks some more coolant in? I’ve noticed a drop in level ever since I re fitted the turbo.

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Is the turbo liquid cooled, or did you have to remove coolant hoses to change it?  If there's air in the system there will probably be a bleed screw somewhere to get rid.  Is the screw cap on top of the heat exchanger?

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16 minutes ago, SiDfish said:

Is the turbo liquid cooled, or did you have to remove coolant hoses to change it?  If there's air in the system there will probably be a bleed screw somewhere to get rid.  Is the screw cap on top of the heat exchanger?

This is the turbo in question, there were a couple of coolant pipes attached that I had to drain so yeah I guess it’s water cooled. 1F348B61-761D-42C4-8A83-945CE26EB1E8.jpeg.20c0874fc4f355621c38a2b870efb3f1.jpeg

 

And this is the bottle and cap

 

496B8852-1846-4D2D-844A-953DA70DAFE9.thumb.jpeg.97fe9ccad8e3b289382d6b2b5ff323cf.jpeg

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33 minutes ago, Saintly Fish said:

It was explained to me that the bottle was an overflow tank. There is no pressure in it because it’s not sealed, it has a push top lid but that lid is just pop off pop on tight not screw on. I always fill from the top of the engine, like you would on a car, unscrew the cap and poor the coolant in. Could it be that there is air circulating that once finds its way out, it sucks some more coolant in? I’ve noticed a drop in level ever since I re fitted the turbo.

If there's no pressure cap then it can only be to catch any excess coolant if it overheats or too much was put in. Better to catch it than spew it into the bilge.

It's like an oil catch can then, but for coolant.

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2 minutes ago, Andy135 said:

If there's no pressure cap then it can only be to catch any excess coolant if it overheats or too much was put in. Better to catch it than spew it into the bilge.

It's like an oil catch can then, but for coolant.

That’s what I was thinking, plus it’s made of plastic so won’t hold up to pressure. But, it does have a min and max measurement. If I fill to max then go out , when I get back it’s on minimum. Well has done for the past two trips. 

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8 minutes ago, Saintly Fish said:

And this is the bottle and cap

496B8852-1846-4D2D-844A-953DA70DAFE9.thumb.jpeg.97fe9ccad8e3b289382d6b2b5ff323cf.jpeg

The inner circle is your pressure cap, the square white reservoir is connected to the overflow outlet on your pressure cap. As I said in my previous post it will catch any overflow/excess coolant.

If it has min and max markings on it then I guess that reservoir may have been used as a header tank on whichever car it was originally designed for and is being used in a non-pressurised application on your engine.

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Just now, Saintly Fish said:

That’s what I was thinking, plus it’s made of plastic so won’t hold up to pressure. But, it does have a min and max measurement. If I fill to max then go out , when I get back it’s on minimum. Well has done for the past two trips. 

If it's for catching fluid, then I'm not sure you should be filling it. If your system is working as it should then it should be empty and no fluid should get into it past the pressure cap.

Where does the translucent hose that runs out of your reservoir cap go to? The bilge??

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2 minutes ago, Andy135 said:

The inner circle is your pressure cap, the square white reservoir is connected to the overflow outlet on your pressure cap. As I said in my previous post it will catch any overflow/excess coolant.

If it has min and max markings on it then I guess that reservoir may have been used as a header tank on whichever car it was originally designed for and is being used in a non-pressurised application on your engine.

So why does the level drop?

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1 minute ago, Andy135 said:

If it's for catching fluid, then I'm not sure you should be filling it. If your system is working as it should then it should be empty and no fluid should get into it past the pressure cap.

Where does the translucent hose that runs out of your reservoir cap go to? The bilge??

Yes straight to the bilge

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Looks like it from the pics.  Sounds like you've got either air in the system (Yanmar manual will give bleed point) or a leak on one of the pipes or unions you moved when fitting the turbo.  With everything getting so hot you can have quite a leak that evaporates without getting to the bilge, which itself is pretty warm when running.  Can't believe how clean your boat is below decks, big respect.

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6 minutes ago, Saintly Fish said:

So why does the level drop?

Because your pressure cap opens to allow air (from your recent system refill) out into the catch tank, which then pushes the fluid you just put in it, out into the bilge.

Once the system has been fully bled it will settle down and the fluid level will stabilise, but I don't reckon there's any need to fill the reservoir to max as that will just take up space intended for any overflow coolant that comes past the pressure cap.

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16 minutes ago, Saintly Fish said:

Yes straight to the bilge

Might be worth checking in the lowest point of your bilge then for the coolant you put into the catch tank. Or do you have an auto-bilge pump that would have discharged it already?

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8 minutes ago, Andy135 said:

Might be worth checking in the lowest point of your bilge then for the coolant you put into the catch tank. Or do you have an auto-bilge pump that would have discharged it already?

My bilge lowest point is just below my sump. It’s easily visible and dry. 

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7 minutes ago, Saintly Fish said:

My bilge lowest point is just below my sump. It’s easily visible and dry. 

Is it the lowest point whilst underway though? Might the coolant have trickled astern whilst underway and is now trapped elsewhere? Can't remember the layout of your engine room floor so not sure if this is even a possibility, but it has to have gone somewhere.

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Could you lay something absorbent under your engine? (Wings maybe) the water will evaporate but the coolant will stain the wings, if it is a slight leak. Even better get a big man nappy off Andy for the job. 

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5 minutes ago, JonC said:

Could you lay something absorbent under your engine? (Wings maybe) the water will evaporate but the coolant will stain the wings, if it is a slight leak. Even better get a big man nappy off Andy for the job. 

Yes I suppose I could mask some blue paper roll down, that’ll give an indication. I do regularly check the bilge and it’s always dry. 

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1 minute ago, Saintly Fish said:

Yes I suppose I could mask some blue paper roll down, that’ll give an indication. I do regularly check the bilge and it’s always dry. 

Why not stick the outlet end of the translucent hose into a water/fizzy pop bottle. It'll either be empty at the end of a run, or full. Either way it avoids man nappies, blue roll or making a mess of your engine room floor.

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You would notice some residue from the coolant if you was weeping some. 
Get a refractometer and check the gravity of your coolant, if water is evaporating the coolant will be more concentrated. 

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