Coolant reservoir pressurized? -> sprayed everywhere when opened...

[wild.coast]

Curious question: sled has been sitting in the garage for a week (i.e. dead cold), I noticed that the coolant level in the reservoir was low (well below the cold level mark), when I twisted the reservoir cap coolant sprayed everywhere - is the system somehow pressurized??

I can see there is a coolant overflow hose coming out of the reservoir bottle, you'd think with it being vented any pressure would equalize??

2016 Summit 800, but I doubt that matters.

 

[Jerry2star]

Coolant systems rely on pressure to raise the boiling point of the liquid, the overflow is only utilized when the pressure exceeds what the cap is rated for, so if you overheat and the liquid boils or expands the cap opens up to release pressure, its not uncommon to maintain pressure positive or negative upon cooling.

Think of it like a bag of chips (mostly air) you buy a bag at a gas station in idaho falls @4700' altitude then drive up Sawtell peak @9800' and the bag of chips burst open from positive pressure.

Same thing happens in your cooling system under heat or elevation changes, you top off coolant while the motor is hot and are on a hilltop then ride home and the motor cools at a low elevation creating negative pressure which would cause to fluid to spew out when the pressure is equalized.

But yes they are pressurized.

 

[wild.coast]

Coolant systems rely on pressure to raise the boiling point of the liquid, the overflow is only utilized when the pressure exceeds what the cap is rated for, so if you overheat and the liquid boils or expands the cap opens up to release pressure, its not uncommon to maintain pressure positive or negative upon cooling.

Think of it like a bag of chips (mostly air) you buy a bag at a gas station in idaho falls @4700' altitude then drive up Sawtell peak @9800' and the bag of chips burst open from positive pressure.

Same thing happens in your cooling system under heat or elevation changes, you top off coolant while the motor is hot and are on a hilltop then ride home and the motor cools at a low elevation creating negative pressure which would cause to fluid to spew out when the pressure is equalized.

But yes they are pressurized.

That makes sense to me, thx!

But how come the reservoir doesn't equalize to ambient pressure via the overflow hose?? (the black hose coming out of the top of the reservoir). That hose is just an open connection, doesn't have any check valve or anything... I just don't get how the physics works, how does it hold pressure? Does it somehow create a vacuum? But that sounds wrong because the hose is empty of fluid..

 

[Jerry2star]

The seat/seal that your radiator cap contacts is below that point , the attached picture is of a cap in a system that uses and overflow tank (car) and utilizes a low pressure valve to draw in when low or negative pressure is present. Most sleds and say dirtbikes dont have an overflow so it just lets pressure out but will not suck air in. Thats why vehicles have a hot and cold fill line in the overflow tank.

 

[wild.coast]

The seat/seal that your radiator cap contacts is below that point , the attached picture is of a cap in a system that uses and overflow tank (car) and utilizes a low pressure valve to draw in when low or negative pressure is present. Most sleds and say dirtbikes dont have an overflow so it just lets pressure out but will not suck air in. Thats why vehicles have a hot and cold fill line in the overflow tank.

Aha! Makes total sense. Thank you!!