Hi Amarin,

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New green cap has double the pressure capacity than the old grey (green cap 150 kPa, grey cap 75kPa).
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Actually no, the grey cap is also 150kPa, it was replaced by the green one.

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If its original tank (thicker walls) it shouldn't warp as whatever excess pressure should've been vented out by functioning cap leaving only the residual pressure which still within the design capacity of the tank.
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I agree with you, but this is the original Volvo tank, I never changed it, so this gives you an idea on the level of pressure I see.

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Where this excess pressure came from? I suspect faulty headgasket.
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Which is my original suspect in my first post.

For sure, I will replace the gasket, but I am sure there is a leak in the water pump gasket too, when I replace it, how can I check if the water pump itself is good or not?

Thermostat: Is there a way to test it too?

Water tank: I guess now I have to replace it as well, I cannot keep it warped.

Alessandro