Coolant question
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Senior User
Joined: Feb 2009
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Coolant question
Could you guy's get me straight on something real quick....I'm about to do a coolant system flush on my 07 F250, and have a question....looking back about a year ago, while i was changing my air filter I remember someone saying that it would be easier to remove by taking the small coolant hose off the top of the degas bottle to give more room...well I tried doing this with the engine hot..and when i pulled that hose off coolant shot out all over the place....what causes that, and when doing the flush, removing the degas bottle cap to fill and flush the coolant will I have to worry bout this happening again???? Should i wait til the truck is cool before removing the degas cap????
Hotshot
Joined: Aug 2006
Posts: 17,398
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From: Whittier, CA
Yes. The degas cap is the equivalent of a radiator cap on old style radiators. You can get seriously burned removing the cap under pressure. But if you must remove it while hot, turn the cap slowly and carefully, and let the pressure vent slowly.
Hotshot
Joined: Aug 2006
Posts: 17,398
Likes: 317
From: Whittier, CA
Safety is the main reason to not open that cap while hot. But there is another reason - you may overheat and boil your coolant.
Water boils at 212°F at sea level, as we all know. But it boils at an even higher temperature when pressurized. This is why we have 16 PSI caps on our coolant system. The pressurized water, combined with the properties of the coolant, gives a boiling point above 265°F, which we hope is higher than our cooling system will reach.
But if you run your engine up to normal operating temperature of 220°F, the coolant expands and the cap vents to hold 16 PSI. If you then release the cap and vent to atmospheric pressure, then start a heavy pull on a depressurized system, you may boil your coolant. Particularly if you happen to be in the mountains at high elevation. Once the pressure is lost, it will not be regained until the system has cooled and then heated again.
Water boils at 212°F at sea level, as we all know. But it boils at an even higher temperature when pressurized. This is why we have 16 PSI caps on our coolant system. The pressurized water, combined with the properties of the coolant, gives a boiling point above 265°F, which we hope is higher than our cooling system will reach.
But if you run your engine up to normal operating temperature of 220°F, the coolant expands and the cap vents to hold 16 PSI. If you then release the cap and vent to atmospheric pressure, then start a heavy pull on a depressurized system, you may boil your coolant. Particularly if you happen to be in the mountains at high elevation. Once the pressure is lost, it will not be regained until the system has cooled and then heated again.
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Senior User
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Well the only reason I ask this is because i am wanting to do a coolant system flush and in order to fill with new coolant i must remove degas cap obviously, and to drain coolant I am going to have to remove the lower hose, and I don't want coolant blowing out all over me....but I plan on removing the t-stat so i can just leave truck in the driveway, instead of taking it for a drive to get it up to operating temp to open t-stat...i'm just gonna rev it up in the drive and let the distilled water cycle...to flush it...
Post Fiend
Joined: Oct 2009
Posts: 12,123
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From: N. Fort Worth, tx
if you are going to do it in the driveway there is no need to install the cap at all. yes its normal to have the hose spray water if the system is not vented first. I dont know how sound this advise is but I will never open a cap on a hot motor that is not running. hot engines that are not running build more pressure then one that is.
also a little math for you 1psi = 3*f of change in boiling temp so a 16 psi cap adds 48*f to the boiling point
also a little math for you 1psi = 3*f of change in boiling temp so a 16 psi cap adds 48*f to the boiling point
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Posting Guru
Joined: Oct 2009
Posts: 1,340
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From: Bradenton, FL
I agree i've added coolant while the engine was still hot, just becareful taking of the degas cap, turn till you hear it hissing(thats the pressure equilizing) as soon as it starts to hiss, turn even slower until it stops. getting burnt doesn't feel good
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Hotshot
Joined: Aug 2006
Posts: 17,398
Likes: 317
From: Whittier, CA
Good point, and that's because the coolant temp increases temporarily when you shut the motor off. Motor off = no water pump/fan = no radiator flow = cooling stops. The engine block is still hot and superheats the coolant for a few minutes. On cars with a real coolant temp gauge (not Fords) you can actually see the temp rise if you leave the key on.
