The header tank mainly contains air. The free volume accomodates the volume expansion of heated coolant and so the air compresses and vapour pressure rises. If the pressure increases to the cap's vent pressure, it will be mainly hot air that comes out and there's a good chance the coolant will be retained. It depends on how full and how hot the coolant is. Filling the system to the MAX marker is recommended, but it will tolerate somewhat more providing the engine isn't prone to overheating. Under normal running conditions with a leak-tight system, it will accomodate 10-15mm higher than the Max marker. Filling it to the top of the fins or higher is wasteful as thermal expansion will force some out.
TC
Last edited by T-Cut; 24th August 2014 at 14:53..
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