What happens to air when bleeding cooling system?

[J]

When you drain the coolant and then refill, the head is the highest part so as the coolant fills doesn’t this trap air in the jacket? What I’m wondering is when someone drains at the radiator and fills at the radiator, where is the air in the system? What does getting it up to operating temperature do to this air?

I did a coolant change recently and noticed that until I got to operating temperature the lower radiator hose was not hot but once it got up to OT, the thermostat opens and that hose gets really hot. I don’t really know why this is other than the coolant is in the radiator and both upper and lower hose but the heating of the coolant only occurs at the block and the heat doesn’t reach the lower hose. When the block coolant gets hot enough the thermostat opens and the coolant now has a circuit?

In ETCG’s video about bleeding the cooling system, he raises the RPM at the throttle to warm the engine up to operating temperature. I always wondered whether it was safe to do this? If there’s air in the head or block water jacket, could this caused localized overheating in these air pockets that could warp or head? I would think localized hot spots would not impact overall coolant temperature, thus not showing up on coolant tmperature gauge but still cause damage.

Idling takes forever to get the car up to operating temperature. I would think even if there was an air pocket the idling would have the same problem? Where exactly is the air and how does it “bleed” out?

Topic

[terry]

It is really hard to tell where the air pocket’s at? But what you can do is get a jack and raise the front of your car as high as you can get then start the engine again with the rad cap off and heater knob to hot section and so on………

[Richard Kirshy]

Raising the engine rpm does two things:
Accelerates the warm up
Increases the circulation of coolant by spinning the water pump faster

With having the radiator cap removed ( with the use of a spill free funnel )and the T-stat open, the air pocket is able to move and in most cases vent out to the open cap.

But there are exceptions to every rule:
Some coolant systems are more difficult to bleed than others.
In such cases there is a vacuum device that allows you to put the coolant system under vacuum, once proper negative pressure is reached, coolant is added through a valve.
Forward to 38:00 minutes

[Mike]

[quote=”KnowNothing” post=174649]I did a coolant change recently and noticed that until I got to operating temperature the lower radiator hose was not hot but once it got up to OT, the thermostat opens and that hose gets really hot. I don’t really know why this is other than the coolant is in the radiator and both upper and lower hose but the heating of the coolant only occurs at the block and the heat doesn’t reach the lower hose. When the block coolant gets hot enough the thermostat opens and the coolant now has a circuit?[/quote]

Yes, that’s correct. On most cars, the coolant flows up through the thermostat, into the upper hose and into the top of the radiator. It then flows to the bottom of the rad, out the bottom rad hose to the pump and into the block and heads. Then the cycle repeats. When the engine is below operating temperature and the thermostat is closed, there’s no flow to the rad and the lower hose remains cool.

This flow pattern is a relic of early automotive engineering, called thermo-siphon cooling, when cars typically didn’t have thermostats or water pumps. The hot water in the engine would simply rise to the highest point in the system, (the top rad hose), and flow into the rad where it cooled down, became denser and flowed to the bottom of the rad, into the bottom hose and back into the block. The entire system was a convection loop.

[Author]

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