Jp83

Does the 2004 a4 have a over pressuring cooling system

lyleswk

Not exactly sure what you mean. The system does operate under pressure just as all automotive coolant systems operate.

turbo510

Correct, because it is inefficient to run an engine with comparatively cool cylinder walls- heat going to the coolant is energy that doesn't do useful work. Allowing the coolant to pressurize, by sealing it from the atmosphere, allows the liquid to reach a much higher temperature without boiling. If it wasn't for limits on how hot oil and mechanical parts can work, the coolant would be run even hotter