brianp38dse
Well-Known Member
When you take the cap of and refill you let air in coolant will run out till the cap goes on and keep leak in till it has let up a vacuum in the system then engine gets hot expands and seals leak.
Datatek
Well-Known Member
There is never a vacuum in the cooling system, it runs under pressure when hot. The leak may seal due to expansion of metal parts.
brianp38dse
Well-Known Member
What I was saying is having just topped up and put cap back there will be no pressure in system and with leak opened due to contraction coolant will leak out till it creates a parcial vacuum unless it sucks air in elsewhere, I work on heating systems cutting into pipe work using this method create a vacuum no need to drain system to work on a pipe.
Datatek
Well-Known Member
The only time there is less than atmospheric pressure is as the engine cools. When hot there is 14psi of positive pressure.
brianp38dse
Well-Known Member
I was trying to explain the op`s bit ( I top up reservoir when cold It leaks out from behind the engine, then stops leaking and still runs at running temp.) that is how it may leak when just topped up and cold then stop when still cold, the coolant dripping out will create a negative pressure so will stop, unless you open cap again to equal atmospheric pressure then in will drip again.
Datatek
Well-Known Member
Removing the cap on a hot engine apart from being dangerous, lowers the pressure to atmospheric but does not create a vacuum.
brianp38dse
Well-Known Member
We are talking about when everything is cold not hot.
