Coolant Temperature

sierrafery · Friday at 13:47

marjon said:
Pressure is only applied to the cooling system (all being well) by the heating up of the system and expansion of the water. Overnight it cools so should be back to zero.

When you top up your cooling system and put the cap back on, you don't pressure the system.

J

Maybe we understand differently how this works, once the air in the tank was pressurised to 1.4 bar due to the expansion of coolant i dont think that pressure will drop to zero once the coolant cools down cos then the level in the tank would drop more that it used to... and from my experience there is always some residual pressure in the tank, if it wasnt i would susspect the cap's seals or valve

kermit_rr · Friday at 13:54

yes theres often a 'little bit' of pressure when you take the cap off when its cold. gases dissolved in coolant.. coolant vapourising in local hot spots etc.
yes coolant (and the air in the expansion tank) gets hot and tries to expand. it can't expand much so the pressure goes up.. once it cools down everything goes back to how it was and the pressure is back at where you started

marjon · Friday at 14:03

sierrafery said:
Maybe we understand differently how this works, once the air in the tank was pressurised to 1.4 bar due to the expansion of coolant i dont think that pressure will drop to zero once the coolant cools down cos then the level in the tank would drop more that it used to... and from my experience there is always some residual pressure in the tank, if it wasnt i would susspect the cap's seals or valve

I don't often disagree with you, But you are wrong.
If you put the cap on at zero pressure , the only way pressure can build up is because of expansion and heat. Your coolant level will rise slightly (if you look at the expansion tank when hot) but the air pressure in the air space will also be higher.

How is that when the system started at zero?
So when its back to cold why shouldn't things go back to the way they were, I.E zero pressure and same coolant level?

The pressure cap rating is to allow excess pressure beyond the system design to relieve pressure, I.E head gasket problem, blocked rad etc. This should not happen under normal conditions.

J

kermit_rr · Friday at 14:07

marjon said:
I don't often disagree with you, But you are wrong.
If you put the cap on at zero pressure , the only way pressure can build up is because of expansion and heat. Your coolant level will rise slightly (if you look at the expansion tank when hot) but the air pressure in the air space will also be higher.

How is that when the system started at zero?
So when its back to cold why shouldn't things go back to the way they were, I.E zero pressure and same coolant level?

The pressure cap rating is to allow excess pressure beyond the system design to relieve pressure, I.E head gasket problem, blocked rad etc. This should not happen under normal conditions.

J

the cap is a built in failure point like a shear pin. under normal conditions it just sits there, but in an emergency it will do its thing and prevent a more catastrophic failure,, like a hose or the radiator splitting.
unlike a shear pin, the spring in a cap means it can be reused

tom1979 · Friday at 15:40

kermit_rr said:
the cap is a built in failure point like a shear pin. under normal conditions it just sits there, but in an emergency it will do its thing and prevent a more catastrophic failure,, like a hose or the radiator splitting.
unlike a shear pin, the spring in a cap means it can be reused

That's interesting as I've never trusted a cap after it's vented!

tom1979 · 2025-01-29T16:16:29+0000

tom1979 said:
Although i think I also have another issue now as it's holding pressure in the cooling system overnight so possible head gasket time anyway!

I think this will be my first job before worrying about the engine running too cold! Run up to only 75*C and the top hose is fairly hard (plus the waterfall noise around 1200rpm). Will sort that out and then come back to look at the temperature issue!

kermit_rr · 2025-01-29T16:33:11+0000

Hoses should be easily squeezed at 75c

tom1979 · 2025-01-29T17:08:26+0000

kermit_rr said:
Hoses should be easily squeezed at 75c

yeah

Coolant Temperature

sierrafery

Well-Known Member

kermit_rr

Very

marjon

Well-Known Member

kermit_rr

Very

tom1979

Well-Known Member

tom1979

Well-Known Member

kermit_rr

Very

tom1979

Well-Known Member

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