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if there was a block problem or other breach between the engine and cooling system, typically the engine would overheat rapidly and blow the cap.
if there is speculation that there may be such a breach, then a chemical test or leak down test should be carried out to verify.
if no breach is found, then there are only so many things which can cause the system to pressurize (water pump, thermostat, blockage, air in the system, failed fan clutch), and really if it isn't overheating or blowing the cap off, I find it hard to believe it is in fact over pressurizing. The cap for the cooling system is designed to let go when the system develops too much pressure. If the cap is not letting go to release pressure, then the pressure cannot really be that high (the cap is rated at 2.0bar)
so, I would advise a chemical or leak down test, as well as a check of cooling system components. If all turns out okay, you are not overheating or losing water, then I can't see how there could be an issue.
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