coolant loss question

[Dash01]

Suppose that coolant is disappearing from an M30 engine. Heater core and head gasket are typical suspects.

But, with a new HG, if coolant is disappearing through the heater core, why would there be a brief hissing sound as the reservoir cap is being removed?

In other words, the hissing sound is that of vacuum being relieved in a cold engine. If the heater core or elsewhere is leaking, how could there be vacuum or pressure differential in the system?

Likewise, if the integrity of the system is compromised, how could pressure build up in a hot engine? Wouldn't any internal pressure simply escape through the leak?

[bill g]

A tiny leak in a headgasket can leak coolant into the combustion chamber and also allow combustion gas to leak into the coolant passages - I had this problem when my M30 gasket was failing, evidenced by loss of coolant and excessive pressurizing of the system - the hoses were still rock hard a day after turning the engine off. So obviously some types of leaks can open and close depending on operating conditions and temperature change.

Bill G

[Ross]

Yep, the firering of the HG acts like a one way valve. It allows very high pressure combustion gas to pass into the cooling system but is still sealed well enough to keep the modest pressure in the cooling system at bay.

[Dash01]

Hmmm. This is a brand new HG. After running just a few miles, perhaps I should make sure the head bolts are all uniformly and properly torqued.