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Purging Air out of Cooling Systems
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Oct 31, 2008, 11:43 AM
Post #1 of 2
(5764 views)
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Purging Air out of Cooling Systems
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This is a common problem for techs and DIYers so I'm locking this rough draft up top for reference. Please feel free to comment or re-write to something more concise explaining this process if you can do better I'd be thrilled. I find myself explaining this same issue many times and have never found just one neat explanation of this common issue. I/we can add, edit and try to get this into one common sense article that explains the issue if possible. Thanks - Tom Greenleaf
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Vehicles differ in the ways you purge/bleed out air. Many will just self purge over several cycles by themselves if not allowed to boil in cycles of cool/cold to full warm ups with thermostat open as it's a restriction between whatever is in engine and what is on radiator side of a system. When cold it's shut of course and open when hot. The idea is for systems to push air/gasses out to recovery tanks which goes in thru the bottom and return just liquid in the expansion and contraction of warming up and cooling down. Run heaters, full temp low fan - feel temp output - once warm as expected you are on the home stretch.
When a system is low for any reason you need to know that it's truly full and some only give you a recovery tank to watch which is fine but isn't the whole story till all is known purged - then it means something. When recovery tanks have the pressure cap there's usually no other cap on radiator.
When found low or after an overheat that loses coolant you fill where you can when engine is cool. Some will have bleeder plugs (plumbing plugs) at or near thermostats and some don't. Some you may need to remove the highest hose and fill from there and allow several cycles of warm ups and cool downs - observing temps, pressure build up and loss when cooling noting heater can throw heat and temp needles stay stable. That's a good sign that the air is out.
When air/gasses are recurrent then a problem may exist with combustion gasses getting into the engine side which doesn't transfer heat well - causes late thermostat reaction and is constantly overwhelmed with air that just can't cool the engine and symptoms include.... heat not working, fast rise at recovery tanks and pressure building up sooner than normal from just expansion as it warms. When a system is opened and warm (CAUTION FIRST!) it shouldn't build up pressure so fast again and if so the gaskets (head and intakes sometimes) can be the fault.
When dealing with a system with a cap on the radiator you would fill it as much as possible when cold - wait as it will drop some, fill again, then start till it warms to open thermostat with rad cap off (spills can happen and always use common sense with warm/hot coolant AND note pressure as flash boiling can occur) - and then coolant level will drop - sometimes out of sight and slowly refill with engine running in that type usually finishes the job - cap can be put back on and let the rest purge out thru the cycles or warm and cool - observed over several cycles - heater running too as an indicator an helps to get air out when flowing.
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Some vehicle just behave the first round and that's the end of it and some are a real pest. Some so difficult that you can have to jack up the vehicle such that the fill cap is the highest possible so air will go to the top from where it hides.
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There are other things that confuse the issue too: Fan(s) electric or by fan clutch must operate, debris must be clear for air thru radiator and of course the components must all be good.
Some rare problems include water pump impellers that are weak and some are strong and can collapse inlet hose from lower radiator - those subject to that usually put a spring inside that hose to prevent that. Springs can be missing from new hoses and should be reused from old ones.
The air needs to be known out for further diagnosis if a problem still exists. Each vehicle is a little different on how to properly fill them up and know it so there's no good "one size fits all" description for all cases. It takes time and patience to fill a system and rushing it isn't helpful. It can help to take vehicles for a run around a block - stopping, starting up, steering left and right which swishes the coolant around inside to get the air in place to escape out thermostats to recovery tanks not return again. The only thing we can count on is that air will rise to the top of it's area and as said at the beginning could be trapped inside engine locked by a shut thermostat. Hope that helps you understand how this is such an issue to at least rule out before more can be diagnosed if needed.
(This post was edited by Tom Greenleaf on Jul 11, 2009, 1:18 AM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jul 24, 2010, 1:53 PM
Post #2 of 2
(4483 views)
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Re: Purging Air out of Cooling Systems
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BTT^^
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