Cooling system stays pressurised?

payydg · Jun 18, 2017

post head gasket change I've noticed the cooling system stays pressurised (or maybe it's caused a vacuum). Basically when I take the header tank cap off it hisses.

Is this as simple as a new cap needed? Or something more sinister... it still seems to be using a bit of water post head gasket change, which is a bit annoying although I'm planning on getting a refurbed head in the coming months.

200tdi defender 90

wammers · Jun 18, 2017

payydg said:
post head gasket change I've noticed the cooling system stays pressurised (or maybe it's caused a vacuum). Basically when I take the header tank cap off it hisses.

Is this as simple as a new cap needed? Or something more sinister... it still seems to be using a bit of water post head gasket change, which is a bit annoying although I'm planning on getting a refurbed head in the coming months.

It entirely depends how long you are waiting before removing the cap. Cooling systems run under pressure. If you remove the cap before the system has cooled down you will get pressure release. If the system was over filled and coolant has been ejected when the system cools you may get an hiss as air is sucked in.

payydg · Jun 18, 2017

wammers said:
It entirely depends how long you are waiting before removing the cap. Cooling systems run under pressure. If you remove the cap before the system has cooled down you will get pressure release. If the system was over filled and coolant has been ejected when the system cools you may get an hiss as air is sucked in.

Should have said happens once comepletly cool. For example - got back from a 150mile trip. Then went to Bulgaria for a week. When I got back it did the hissing thing. Bit odd as it never did this before.

payydg · Jun 18, 2017

wammers said:
It entirely depends how long you are waiting before removing the cap. Cooling systems run under pressure. If you remove the cap before the system has cooled down you will get pressure release. If the system was over filled and coolant has been ejected when the system cools you may get an hiss as air is sucked in.

Also, why and where would the coolant be ejected from?

200tdi defender 90 btw.

lynall · Jun 18, 2017

If theres still pressure after a week i would say its a little unusual but not unknown, as you have had the gasket done it at least proves you have a watertight system as before it may have been weeping out of the blown/blowing gasket.

Rough rule is the seam on the header tank is the correct level;, if you look in the cap hole you will see a little cross this it the level marker, cant see it you have to much coolant in there which may also explain your pressure as there wont be enough expansion room.
Plus of course if you are overfilling the system that would explain why its losing coolant as its being over pressurised.

payydg · Jun 18, 2017

lynall said:
If theres still pressure after a week i would say its a little unusual but not unknown, as you have had the gasket done it at least proves you have a watertight system as before it may have been weeping out of the blown/blowing gasket.

Rough rule is the seam on the header tank is the correct level;, if you look in the cap hole you will see a little cross this it the level marker, cant see it you have to much coolant in there which may also explain your pressure as there wont be enough expansion room.
Plus of course if you are overfilling the system that would explain why its losing coolant as its being over pressurised.

Helpfully I swapped it out for an alisport one cos I'm a pimp. Level is what I always put in. I did the gasket myself so may have not cleaned surfaces properly but still unlikely to have failed again so quickly. Maybe I had been over filling it and now cos the gasket is sealed its forcing water out and causing a vacuum. I'll try running it a bit lower and see what happens.

wammers · Jun 18, 2017

payydg said:
Also, why and where would the coolant be ejected from?

200tdi defender 90 btw.

System must be filled to max line with engine cold. If max line is exceeded, when coolant warms and expands any excess will be ejected from the overflow.

payydg · Jun 18, 2017

wammers said:
System must be filled to max line with engine cold. If max line is exceeded, when coolant warms and expands any excess will be ejected from the overflow.

Where does the overflow end up? Must have a one way valve to keep pressurised?

Alan Crossley · Jun 18, 2017

lynall said:
If theres still pressure after a week i would say its a little unusual but not unknown, as you have had the gasket done it at least proves you have a watertight system as before it may have been weeping out of the blown/blowing gasket.

Rough rule is the seam on the header tank is the correct level;, if you look in the cap hole you will see a little cross this it the level marker, cant see it you have to much coolant in there which may also explain your pressure as there wont be enough expansion room.
Plus of course if you are overfilling the system that would explain why its losing coolant as its being over pressurised.

Good afternoon Lynall, if, as you say, an overfilled system is effecting excessive pressure, when the coolant is expelled, can that create a siphon which expels additional water? Leaving the level much lower than it should be.

lynall · Jun 18, 2017

payydg said:
Where does the overflow end up? Must have a one way valve to keep pressurised?

Its all in the header tank cap, valve seal etc
So long as theres adequate room for coolant expansion (at least 2 inches from bottom of the neck) and the hoses arent solid hard when its running I wouldnt worry to much.

lynall · Jun 18, 2017

Alan Crossley said:
Good afternoon Lynall, if, as you say, an overfilled system is effecting excessive pressure, when the coolant is expelled, can that create a siphon which expels additional water? Leaving the level much lower than it should be.

It could be possible as I guess just about anything is as coolant systems can be funny buggers, but general rule is once pressure has gone the valve in the cap should shut and reseal the system again, might be worth getting a new cap?
But a system acting as you say woulld have me looking for an air lock first,
I dont know if you know but best to fill 200 and 300 a certain way as just dumping the coolant in can lave the head unprotected until the stat opens for the first time and then it bleeds itself?

Alan Crossley · Jun 18, 2017

lynall said:
It could be possible as I guess just about anything is as coolant systems can be funny buggers, but general rule is once pressure has gone the valve in the cap should shut and reseal the system again, might be worth getting a new cap?
But a system acting as you say woulld have me looking for an air lock first,
I dont know if you know but best to fill 200 and 300 a certain way as just dumping the coolant in can lave the head unprotected until the stat opens for the first time and then it bleeds itself?

I recently fitted a new cap and thermostat, so they should be fine. My car is a Td5' and when I fill the coolant I always take out the bleed screw to ensure that the coolant has filled the system. Is that the correct method or is there an alternative which might ensure that the total system is free of air? Appreciate your kind help.

payydg · Jun 18, 2017

lynall said:
Its all in the header tank cap, valve seal etc
So long as theres adequate room for coolant expansion (at least 2 inches from bottom of the neck) and the hoses arent solid hard when its running I wouldnt worry to much.

So where would the coolant be going? Is it lost as vapour through the header cap?

flat · Jun 18, 2017

payydg said:
So where would the coolant be going? Is it lost as vapour through the header cap?

Nah it literally spills out the sides of the cap (it has an inner seal and an outer one with a run off pipe) - on a standard header thank that is

Cooling system stays pressurised?

payydg

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