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Old 06-04-2019, 03:48 PM   #9
Killer Bee
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Join Date: Jan 2016
Location: Barber City, CA
Posts: 2,432
Re: Coolant level question

sounds like everything is ok to me.. from what I'm reading anyway..

you are correct about the pressure caps.. an open or vented system does not need the vacuum seal in the cap to draw coolant back into the system obviously because it doesn't have a reservoir to draw it from

so the principle difference is, in a closed or reservoir system, the vacuum seal in the pressure cap will open when the engine is off and coolant temperature is lowering which creates a vacuum.. this keeps the system completely full or flooded for optimal heat rejection and also gives the operator a visual indicator of coolant level, as long as the system is tight, by observing the overflow bottle.. also more friendly to the environment

please note, this is not the same as an expansion tank system where the reservoir is pressurized..

in an open system, the pressure cap only has the pressure seal.. this system will operate with an air gap along the top of the top tank which is normal.. this is easily established by filling the system level full, bringing the engine up to temperature and allowing the excess to purge from the overflow bib, hopefully into a hose pointed into a pan..

this process is however a normal product of actual fluid pressure developed by expanding coolant, not overheating, boiling over, etc. once the engine is off again and the coolant temperature decreases, the vacuum may remain in a tight system, slightly collapse larger hoses, etc.. you may recall at some point in your experiences, removing a radiator cap from an older model vehicle and it slurps in a little air.. this is normal and indicates a tight system.. you may also recall the coolant level is approx. an inch from the top, also normal.. when it's at operating temperature, the coolant expansion will nearly or completely fill the system again..

little rule of thumb for y'all, 1 inch of vacuum equals about 2 psi of sealing integrity..

if a closed system vacuum seal pressure cap is used in an open system, the only result is the atmosphere will replace the contracting coolant as it returns to ambient temperature..

so for your situation, you may have to drive it a few times reaching normal operating temp before it settles.. it may purge a few dribbles here and there when at it's highest temps, but eventually it will level off..

if you're conscientious of the actual level, when it's cool or ambient temp, for instance after sitting overnight, take a popsicle stick or similar and dip the tank at the pressure cap and make a reference mark.. if repeated checks under same conditions are consistent, call it done and mark it for duty..

hope that helps, good luck!
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