Thread: Should the coolant tank cap be completely air tight?

Mine does the same thing but it never uses water or runs hot when offroading so i never worried about it. But in saying that i try to keep my engine and engine bay clean so i can see any problems before they become expensive ones. I'm not saying its normal because i dont know, I would keep an eye on it

Well it didn't look like it had lost any water nor did it seem hot or anything like that, just heard the noise.

Should be air tight as this tank is part of the sealed cooling system and under pressure.

Originally Posted by Young Angus

Just drove the Disco back from my parents place tonight and popped the bonnet when I got back and noticed a bit of an air leak type sound coming from the cap of the new coolant tank I just had installed during my recent service. Just sounded like there was a tiny bit of pressure escaping from the cap but I'm not sure if it's meant to do this or if it's meant to be completely air tight...or perhaps it doesn't matter?

Is this normal?

The radiator cap is a pressure release valve, so if there is air being released it's because the pressure in the cooling system has exceeded the pressure relief valve in the cap and is venting, Regards Frank.

Originally Posted by Tank

The radiator cap is a pressure release valve, so if there is air being released it's because the pressure in the cooling system has exceeded the pressure relief valve in the cap and is venting, Regards Frank.

Is this a bad thing though? I just had the coolant tank replaced with a brand new clear plastic one (as opposed to the original black plastic one) by my garage so if it's not sealing properly I'll just take it back and tell them to fix it up.

have the cap and the tank pressure tested...

if they used the old cap on a new tank the rubber seal may be sitting in a different place and not sealing OR there is a casting defect on the lip of the tank that needs a touch of love with some 1200 grit wet and dry.

Sorry to HiJack, but this just sparked my interest.

I am getting an aluminium header tank make to replace the dodgy black one on my Disco, I have used a normal radiator style "neck" with 15Lb pressure cap (normal style rad cap), will this cap be able to release the air pressure built up inside the new tank even though its designed to release water like it would if it were on a radiator?

Back on track, Dads range rovers allways used to vent pressure when the vehicle was parked after a drive, usually took a few minuets of sitting for the pressure to build up enough to bleed off.

Originally Posted by Phat-Customs

Sorry to HiJack, but this just sparked my interest.

I am getting an aluminium header tank make to replace the dodgy black one on my Disco, I have used a normal radiator style "neck" with 15Lb pressure cap (normal style rad cap), will this cap be able to release the air pressure built up inside the new tank even though its designed to release water like it would if it were on a radiator?

Back on track, Dads range rovers allways used to vent pressure when the vehicle was parked after a drive, usually took a few minuets of sitting for the pressure to build up enough to bleed off.

As said earlier a radiator "cap" is designed to release excess pressure above the pressure designated by the factory, a 15lb cap will release pressure (and sometimes water, if it is overfull) to maintain factory pressure of 15lb. When you turn the engine off the water temp. rises because it is no longer travelling through the radiator, pressure may rise along with temp. and a correctly operating cap will release excess pressure.
On the other hand if you have a blown head gasket excess pressure and water will be pushed past the cap by the combustion pressures, you will know the difference quite readily, a little pressure relief after a run is normal, bubbling in the header tank and steam and water gushing out is usually a blown headgasket, Regards Frank.