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post #20 of (permalink) Old 16-03-2017, 21:54
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Join Date: Jul 2012
Location: Hunstanton
Car: MGF
Posts: 3,337
"the pressure within the cooling system varies with revs"


so that means if you have a leak that only happens at certain pressure... then driving at the revs that turns the pump at that required pressure to cause the leak... will happen.. ie a hose that only leaks at certain pressure..

in effect.. you cool the engine because the pump pushes more coolant round the engine... but at the same time... you increase the leak.. ( more pressure ) and loose idle pressure when you stop driving... so if you loose pressure.. the coolant that wasnt boiling... suddenly starts to boil, as it reaches atmospheric pressure..

its a hideous cycle.... if you rev the engine... you increase the pressure... the water that was boiling.. now stops boiling, its under pressure... but the leak gets worse... more pressure more leak.. and what leaks.. is at atmospheric pressure.. so you get clouds of steam...

now.. so far - you are not loosing any significant quantity... and i suspect your pressure is still behaving normal.. but if the leak gets worse.. and you loose pressure.... you will make it worse by idling the car.. yet see some short term benefit if you drive it , ( you increase the pressure and stop the water boiling ) but in the end.. the pump will push coolant out... and you get air locks... boiling water in the head... etc etc.. bad news..

i saw nearly 1 bar pressure at 4000 rpm... certainly over 0.8 bar... i only tried that once.. this was a shoehorn example...

at that pressure the coolant wont boil... but its temperature might be over 100 degrees C at that pressure.. certainly over 98 degrees C..

so if it it leaks.. into the atmosphere... it will boil violently..

and why folks report clouds of steam when the vent cap vents..

so knowing that the system pressure varies with engine revs... is very important. and i might make a video to show the scale of revs, verses pressure and temperature...

as an aside.. i dont know yet that the pressures and temperatures around the engine head and the rad are the same in entirety... the head exit pipes might be very different to head entry pipes, and the header tank... one might not have the same temperature and pressure everywhere..

Last edited by Incony; 16-03-2017 at 22:08.
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