Cooling system pressure

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Wed 2 Jun 2004 12:11
Cooling system pressure - Andrew-T
Can the experts out there (=in here) say whether there is anything for or against venting the cooling system 10 mins after stopping a hot engine, to purge most of the 'air' above the coolant? Seems to me that the running pressure would then be slightly lower, presumably a good thing? There would then be a partial vacuum when cold - good idea or not?
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Wed 2 Jun 2004 12:16
Cooling system pressure - Sooty Tailpipes
Not very good, due to the system being most efficient when under pressure.

(Boyles' or Charles' Law - can't remember which!)
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Wed 2 Jun 2004 12:26
Cooling system pressure - Peter D
Not good, The pressure lifts the boiling point of the water thus avoiding bubbles to be generated at hot spots in the engine, normally in the head. What you see on the gauge is the avarage water temp i.e. 88 or so. but areas of the head excess 100 degrees and unpressurised water woulf 'Kettle' and cause air to be generated in the water system. When the engine cools a vacuum is not created and most rad caps have a one way valve in them to avoid this as it would collapse the hoses and stop water circulating in a cold engine. Regards Peter
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