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[Rods2]

The fact that releasing the coolant header cap after the engine has cooled down releases an airlock suggests you do not have any leaks in the cooling system and the head gasket is fine as this would release the pressure from the system.

[Andy H]

The fact that releasing the coolant header cap after the engine has cooled down releases an airlock suggests you do not have any leaks in the cooling system and the head gasket is fine as this would release the pressure from the system.

I would agree that when a head gasket has failed sufficiently to allow coolant into the cylinders overnight then you would expect the pressure to be released.

The issue I have is that something seems to be pressurising the system over and above the thermal expansion of the coolant. A pressure of hundreds of psi in the combustion chamber trumps 15 psi in the expansion tank hence it is possible that early stage head gasket failure could allow small quantities of hot gas to be forced one way while prevent coolant flowing back the other way.

Also we don't seem to be losing coolant (yet)...

[Andy H]

The fact that releasing the coolant header cap after the engine has cooled down releases an airlock suggests you do not have any leaks in the cooling system and the head gasket is fine as this would release the pressure from the system.

I would agree that when a head gasket has failed sufficiently to allow coolant into the cylinders overnight then you would expect the pressure to be released.

The issue I have is that something seems to be pressurising the system over and above the thermal expansion of the coolant. A pressure of hundreds of psi in the combustion chamber trumps 15 psi in the expansion tank hence it is possible that early stage head gasket failure could allow small quantities of hot gas to be forced one way while prevent coolant flowing back the other way.

Also we don't seem to be losing coolant (yet)...

I knew I would regret typing that last line.

Got home from work to discover that it lost coolant while SWMBO was driving it. Coolant sprayed all over the top of the  expansion bottle.

Looks like I get to drive the MV6  and she can borrow the Galaxy / Mondeo Van I bought yesterday until a full post mortem on her Galashambra.