Coolant being pushed into overflow, pinch the line??

[Pathofinder]

Im having some troubles with my car, I think I have an air bubble in my coolant system and I just found out I have a bad rad cap/ The problem is when I drive, coolant is getting pushed out of the rad and into the overflow.

I think I have air in the system and I can hear my rad cap *very slightly if you put your ear to it* fizzle and make a little noise, to me like its not sealing. So I just went and bought a new rad cap.

My problem is I have to take the bumper off the empty the overflow, and its getting anooying, until I wait for my coolant vacuum tool to arrive, can I pinch the line going to the overflow to keep the coolant in the rad so I can still drive the car??

[Rev_D21]

The overflow is a give/take tank. You don't need a bleed tool to get the air out, the pathfinder has either a 10mm bleed screw by the throttle body or a corked bleed tube between the firewall and the intake manifold, it has a rubber condom on it. Get the truck up on ramps and open which ever bleed method your truck has until coolant with no air bubbles runs out. If you block the overflow tank the system will find another, usually more expensive, way of getting the coolant out.

[PapaSmurf2k3]

You put the new radiator cap on and are still having the same results?

[Pathofinder]

I just put the cap on, havent driven it yet

I dont get how the radiator draws from the overflow though, being as the overflow isnt sealed and has a hole in it for when it gets to full

Would a faulty rad cap allow coolant to go into the overflow? Im unsure of why coolant is even going in there. I had my husband put a drain plug on the bottom of the overflow so we dont have to take the bumper off the drain it anymore, but Imhoping I never have to again

I dont get how coolant gives/takes from the resevoir, especially since the resevior isnt sealed and has that vent hole

[PapaSmurf2k3]

You don't want to drain your reservoir, that's what its there for. There should be a "min/max" level on it. That's where the coolant should be with the engine cold.

The way it works is as follows:
Like pretty much everything in this universe, coolant expands when it gets hot, and contracts when it gets cold. Once it expands enough to hit a certain pressure (usually .9-1.1 bar in Nissans), the radiator cap essentially opens a valve to let some coolant escape to the overflow reservoir in order to maintain its pre-determined pressure.

The reason its important to maintain that specific pressure is because under pressure, fluids have a higher boiling temperature. BUUUT, if the pressure gets too high, you can blow out other stuff (like gaskets and whatnot).

[CanuckQx4]

I dont get how the coolant get sucked back in myself

Like it drops so far down there into the tank does it not? How would it get sucked back up??

[Dattebayo]

The key to why it works is because of where the overflow line is attached to the coolant system.
It's because the tank is several inches above the rest of the coolant. This means that coolant can only enter or leave the tank when the pressure is too high or too low. It's not like there is a whole lot of pressure in the system to begin with, that's why the system doesn't suck in air.

[PapaSmurf2k3]

I dont get how the coolant get sucked back in myself

Like it drops so far down there into the tank does it not? How would it get sucked back up??

The hose is connected to the bottom of the reservoir tank. When the engine cools, and the coolant contracts, it creates suction, which sucks the coolant back into the system via the overflow tank/hose.

[KGB]

reveredd21 is right. the overflow creates suction only when the system is completely sealed. thats why you heard the gurgling with the old radiator cap.the one creating the flow of coolant and ultimately the suction is the water pump. if you want to test this out outside of the car, take a straw cut a slit or make a small hole above the drink.