Yanmar 3 Gm 30

[Jacana]

When engine heats up coolant expands and flows into overflow tank however when engine cools coolant doesn't get sucked back .I have replaced radiator cap,checked hose and fittings for blockage and or leakage ,all appear good.When I remove cap you can feel and here suction is still there.Any ideas why coolant won't return?

[Wotname]

Is the vent in the overflow tank clogged?

[a64pilot]

Bad cap, there should be NO suction, it's sort of a one way valve, will hold pressure, but open when there is any suction.
Or wrong cap

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[Jacana]

Wotname - weren't aware of a vent in overflow tank ,I will see if I can find it. a64pilot-cap not faulty as apart from buying a new one which didn't solve the problem ,also tried the cap from my other engine still no change.Also suction is part of the way the system works as when the coolant cools down it creates a vacuum which sucks the coolant up from the overflow tank back into the heat exchange tank and that is what 1 of my engines isn't doing.

[Wotname]

Originally Posted by Jacana

Wotname - weren't aware of a vent in overflow tank ,I will see if I can find it. a64pilot-cap not faulty as apart from buying a new one which didn't solve the problem ,also tried the cap from my other engine still no change.Also suction is part of the way the system works as when the coolant cools down it creates a vacuum which sucks the coolant up from the overflow tank back into the heat exchange tank and that is what 1 of my engines isn't doing.

A couple of things to note:

You got two engines - right?
One works properly - right?

If so, keep swapping parts until the fault transposes - I thinking it is the overflow tank or piping between HE and the overflow tank.

The overflow tank itself must either open to the atmosphere or have a vent.

[Jacana]

Have swapped cap then hose then tank and no change to either engine,same one is still pushing coolant into overflow tank and still not transfer back while engine cools.What should I try next?

[guyfromiowa]

The coolant recovery system requires that the antifreeze side of the system must be airtight. As the coolant in the engine heats it will expand. The expanding antifreeze mixture raises the pressure enough that the pressure valve in the cap pops off and allows the antifreeze mixture to flow into the coolant recovery tank. As the engine cools after shutdown the antifreeze mixture in the engine shrinks. Here's the tricky bit - if the engine is not completely airtight the shrinking antifreeze mixture draws air back into the engine instead of drawing the coolant from the recovery tank. The air leak might be so small that you can't see it. A loose clamp, a tiny crack in a hose or a failing seal on the water pump shaft can be repaired pretty easily after you finally discover it. If the air leak is a gasket on the cooling system hardware or a failed head gasket it will be harder to find. But you must find and fix the leak in order to restore the system to full function.

If the leak is the water pump shaft you will see a trace of antifreeze at the 'weep hole'. Usually you can find THAT on the pump casting behind the pulley, (if belt-driven), or at the drive end if shaft driven.

If the system is not air tight what will happen is that the pressure from the expanding coolant will force the valve of the cap open as well as forcing a tiny bit of coolant out of the leak, so find the leak and fix it. Once the system is airtight again the coolant will go back into the engine after shutdown.

Guy

[Sardean]

Double check all hoses for leaks or a pinhole in hoses, as this will stop water returning to motor. Another test to do is to pressure test system, an easy way is to pump up system to cap pressure using a bike pump. Disconnect the hose that goes to overflow tank and use it to pump up cooling system.

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[Jacana]

Problem now solved.Hope the answer assists someone else .
Couple of years ago I had overheated the engine and a mechanic removed head to shave it then replaced everything.
Recently had another mechanic pull the head to see why the coolant wasn't flowing correctly -the head gasket was put on upside down by the previous mechanic-which blocks some of the coolant flow holes.
After a year or two of using the engine cautiously I am now back to full revs.and running like a Roller presently up to 7000hrs.

[Cheechako]

Thanks for the "answer". So much for hiring a mechanic huh?

[Wotname]

Yes, thanks for the update. Now that is a problem most of us won't diagnose over the interwebby