Expanssion tank/ blown head gasket question

[mimosa]

My expanssion tank looses water each time i drive it, and I basicly have to fill it every 3rd day as the water level falls to the bottom of the tank.
I have now noticed that after a long enough drive to reach normal operational temperature, once the engine is switched off the water slowly overflows out the expanssion tank.
I have seen a way of checking if the head gasket is blown and air is blown into the cooling system, is to have the engine running with the cap off the expanssion tank. I did this and no bubbles it seemed fine, however I let the engine run until it warmed up and the thermostate opened, the water started to rise to the the top of the tank and flow out with a big air bubble straight after. By the time the engine warmed up the water was bubbling vigoursly and lots of water flowing out of the expanssion tank. At this point I turned the engine off. There is no signs of water in the oil system.

So my question is do I have a blown head gasket, or is this normal for the expanssion tank to behave this way because it is a pressurised system and I am running the engine with the cap off. Is it normal to loose coolant once the engine is switched off?

Thanks in advance

[Mk3 AAA]

Water pump maybe, or aux water pump.

My aux was making a grinding sound, and it was just shot inside.

Also, if you're loosing water, try and look where it's going.

[wai]

...however I let the engine run until it warmed up and the thermostate opened, the water started to rise to the the top of the tank and flow out with a big air bubble straight after. By the time the engine warmed up the water was bubbling vigoursly and lots of water flowing out of the expanssion tank. At this point I turned the engine off. There is no signs of water in the oil system

This inicates that the cooling system is losing pressure and so allowing the water/coolant to boil. I take it that when you have to top up so much that you are adding coolant and not just water? The coolant does two things. First it is an anti-freeze, and second, it is anti-boil.

So, two things.

First check that the cooling system is holding its pressure. Any radiator specialist will be able to put an adaptor, reattach the radiator cap, and pump up the cooling system up. This will tell you if the cap is working correctly, and then whether the system is holding pressure.

If this checks out OK, then get a compression test done.

While you are at it, check the oil as you can end up with water in your oil.

If it is a head gasket, it all depends on where the failure has occurred. Ic could be between two cylinders, or between a cylinder and an oil gallery, or between a cylinder and the water jacket, or between the water jacket and an oil gallery. The last is very rare.

An old trick to overcome combustion gasses "pumping" the coolant out is to remove the thermostat, however this is not recommended. It is only a "get you home" fix where you are unable to get things fixed in an emergency.

[dark]

Sounds to me like you have no pressure in your cooling system, is the water leaking out of the cap? Could be as simple as the cap is gone /or like me your whole tank is done. I changed the tank first and the problem persisted, changed the cap as well and it hasn't lost a drop since! I will do all my hoses next, just to be safe as well.

[VR28LT]

Sounds to me like you have no pressure in your cooling system, is the water leaking out of the cap? Could be as simple as the cap is gone /or like me your whole tank is done. I changed the tank first and the problem persisted, changed the cap as well and it hasn't lost a drop since! I will do all my hoses next, just to be safe as well.

Same problem I had a while ago, changed a few things and problem was still there, it ended up being a $10 cap

[Golf Loon]

Its a pressurised water system. You will never get it full if you run it with the cap off.
Get a new pressure cap, we stock them if you cant get one yourself.
Overfill the bottle to the top, put the cap on tight and it should bleed itself down to the max level.
You could also have a leak somewhere and should get that checked.

[mimosa]

Thank you all for your assistance.

[G_1991]

Yeh check the o ring and thread on the cap. I replaced my whole hose kit with a racing. Fitted a new expansion tank new crack pipe and water pump to fix the problem and turned out it was the cap fml. Hahah least I won't have any cooling issues for the next 10 years