I agree, two caps both allowing coolant past would be very unlucky, so if it is bypassing the cap it is doing so at more than 1.5bar pressure.
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Air in the system causes steam, which raises the pressure beyond the cap rating.
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If there was that much air I would also expect you to have a bouncing idle as the waxstat cold idle valve would be stuck open for too long.
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so that kind of rules airlocking out.
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There is a definitive head gasket test I use which is 100% guaranteed to diagnose failure.
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It consists of a hollowed out spark plug welded to a tube, which is then welded to an air line fitting.
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I pressurize each cylinder in turn to 60psi at TDC with the pressure cap removed.
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If any bubbles come up then the only place they can do so is at the head gasket.
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You can also use a leakdown tester kit instead of making your own tool.
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Check the thermostat bleed O rings.
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