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Old 06-28-2009, 09:11 PM
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How to purge air from cooling system

I am going to replace my radiator and want to make sure I get all the air out of my system so how is the best way to do it? My radiator sits a little higher than my thermostat housing on my motor and I have been told that that makes it harder to get the air out.

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Old 06-28-2009, 09:13 PM
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Quote:
Originally Posted by redride
I am going to replace my radiator and want to make sure I get all the air out of my system so how is the best way to do it? My radiator sits a little higher than my thermostat housing on my motor and I have been told that that makes it harder to get the air out.
Keep a cooling port open on your intake while you fill your new radiator.
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Old 06-28-2009, 09:18 PM
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A trick I was shown years ago, is to push open the thermostat, and put 2 aspirin in it to hold it open. They will melt after the system is full. Another is to drill a 1/8" hole in the thermostat to let the air bleed out.
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Old 06-28-2009, 09:21 PM
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I had already drilled the hole a while back but did not know if that would be enough to get it out or if I needed to do anything additional.
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Old 06-28-2009, 09:25 PM
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I fill them up as much as i can,Then while it's running,When the thermostat opens...I fill it very slowly..Work's most of the time.If you try to fill it to fast,It will close the thermostat..
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Old 06-28-2009, 09:40 PM
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I've found an 1/8" hole in the flange to be enough. Fill. Drive. Cool and check. Refill as needed.

I always use an overflow plumbed to the radiator as well, kept about 1/2 full of 50/50 coolant/antifreeze- or what ever ratio you use.
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Old 06-29-2009, 01:38 AM
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Quote:
Originally Posted by redride
I had already drilled the hole a while back but did not know if that would be enough to get it out or if I needed to do anything additional.
When I replace radiator I also drain system as much as I can.
On my vehicles I remove the block drain plugs to allow engine to drain.
Replace block drain plugs.
Not sure how to drain heater core.
I have pulled heater core hose and introduced 10 psi air to remove some liquid from heater core.
I don't like to flush with tap water due to possible scaling issues; anal.
I look up capacity for my ride so I will have a rough idea when it is full.
The air can escape thru your 1/8" hole but give it a few minutes.
Some cars have a bypass and don't require a hole in the T-stat.
I just add the 50/50 coolant/distilled water then start car.
When t-stat opens you may see radiator level drop and the top hose will get hot.
Keep an eye on the temp gauge for anomalies.
Recheck level and temp gauge post test drive.

There is probably a much better way to do it.
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Old 06-29-2009, 05:33 AM
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The fact your radiator is the highest point of the cooling system will make it self bleed the air. The problem is when the fill point is not the highest point of the cooling system such as many modern cars.Fill it run it recheck good to go.
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Old 06-29-2009, 05:51 AM
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Best way to avoid air in the system is to fill it from the top down. Connect the bottom hose and fill thru the manifold, add about 1.5 gals to the block, put on the inlet and t-stat,then connect top hose and fill the rest of the radiator.
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Old 06-29-2009, 04:20 PM
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Quote:
Originally Posted by barnym17
The fact your radiator is the highest point of the cooling system will make it self bleed the air. The problem is when the fill point is not the highest point of the cooling system such as many modern cars.Fill it run it recheck good to go.
This is correct, you should have no issues as long as the stat opens, the air will all end up at the top of the radiator. One of those cheap yellow fill kits makes filling a dry system a lot easier also. Dont remember who makes them but it is basically a big funnel and a radiator cap with a hole that the funnel goes into. It will hold about a half gallon of coolant to fill the system as the air is purged.
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Old 06-29-2009, 10:06 PM
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No worries unless its an oddball setup. 99% of them draw water in the bottom hose and push it out the top hose after the thermostat opens. That means as you fill the radiator (or surge tank) the water will fill in the lower hose, dump into the water jacket of the block, and continue filling up the engine. Usually you'll find that the radiator will fill, then slowly drop. That is because of air/water sneaking past the thermostat. Then as you fill the radiator, usually the water falls down the upper hose to meet the thermostat. As long as you're sure you have water up as high as the stat you should be good. When the stat opens, water will dump into the top of the radiator and get sucked in the bottom. In this way, all air gets purged into the radiator, but since the water getting sucked in comes from the bottom, the block won't suck in any more air. Once you've run it a couple times to open the stat, just refill the radiator and you're done.

If you have a simple overflow tank (not a pressurized surge tank) go get one of those Lev-r-vent radiator caps. They're not a high-quality cap so you might want to just use it for bleeding. What I do is just lift the vent once a day allowing the air to get forced into the overflow. Then when it cools it draws water back in. Do this when its hot. Out with the air, in with the water.

Last edited by curtis73; 06-29-2009 at 10:11 PM.
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Old 06-29-2009, 10:26 PM
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Quote:
Originally Posted by T-bucket23
This is correct, you should have no issues as long as the stat opens, the air will all end up at the top of the radiator. One of those cheap yellow fill kits makes filling a dry system a lot easier also. Dont remember who makes them but it is basically a big funnel and a radiator cap with a hole that the funnel goes into. It will hold about a half gallon of coolant to fill the system as the air is purged.
That is what we used at the dealership or just jack up the front end of the car till the radiator cap is the higher than the intake manifold. It also helps to run your heater.

FYI those funnels work great for checking suspect head gaskets. You can see the exhaust burbling out of the radiator.
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