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Pressure when hot? No pressure when cooled off?
Guess what?
That's normal.
A physical property of water is that it expands when heated.
Hence the pressure in the system after you run the car.
As the car cools down, the water volume (pressure) decreases, ideally back to ambient pressure (no pressure differential on either side of the cap).
As long as the pressure does not exceed the rated pressure on the cap, no coolant will be lost. This is how a system in good working order will operate.
Generally, a compression leak into the coolant system will be noted by pressure remaining in the system even after an overnight cooloff. If you have a pressure differential on a completely cooled system, this is most likely a combustion leak into the coolant system, pressurizing the system beyond it's original atmospheric pressure (at rest). Conversely, a coolant leak into the combustion chamber is noted by white exhaust and a sweet smelling exhaust.
It's also a bad idea to remove the pressure by loosening the cap. While this may help a terminally ill system, it hinders effective heat transfer within a good system. Water's boiling point is raised as the pressure increases. This fact helps keep subnucleate boiling reduced, greatly increasing the heat transfer to the water. Additionally, growth in the water's volume (remember, heat causes this to happen) will cause a loss of water as it is ejected from the (formerly closed) system.
Hope this helps explain what you are seeing on your car.
Jim
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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles
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