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YACT, how does the coolant flow fromt he overflow back into the radiator?

C

coolred

Diamond Member
I know whent he pressure in the cooling system builds up enough it can push the spring up on the radiator cap thus releasing water into the overflow. Then when it starts to cool back down the water from the overflow returns to the radiator. But what causes that to happen. I mean the pressure lifting the spring on the cap cause the water to flow into the overflow, but I am baffled as to how it can flow back into the radiator. Yes I am just that stupid, thank you.


Anyways I am having a cooling issue, and it appears that as the water tempature and pressur eincrease the water flows into the overflow, and often times spills over the overflow at the top. This leads me to belive it is not circulating properly. But even after the car has cooled off completely the water remains in the overflow, leaving it full and the radiator not full.
 
As the water cools in the radiator and engine it contracts and creates a vacuum which draws the apporpriate water back into the radiator thru a 1 way valve in the bottom of the radiator cap.
 
Originally posted by: Ronstang
As the water cools in the radiator and engine it contracts and creates a vacuum which draws the apporpriate water back into the radiator thru a 1 way valve in the bottom of the radiator cap.
Click to expand...

winnar
 
It's the same forces at work, only backwards.

The water heat, it expands. The water cools back down, it contracts.

The contraction creates a vacuum. The water flows back out of the overflow tank to relieve the vacuum.
 
Well, there are different setups. On my Mustang, there is simply a nipple on the side of the radiator fill neck. There is a hose connecting that to a nipple on the bottom of my overflow tank. When the pressure in the system rises, it pushes coolant out into the overflow tank, and when it cools, the vacumn pulls the coolant in the overflow back in. At least in 5.0 Mustangs, there is no valve involved...
 
Originally posted by: Insane3D
At least in 5.0 Mustangs, there is no valve involved...
Click to expand...

Yes there is. It is built into the part of the cap that seals to the inside of the neck of the radiator below the nipple for the overflow hose. There has to be or it would be an open system and you would boil over at ~212 degrees F.

 
Originally posted by: Ronstang
Originally posted by: Insane3D
At least in 5.0 Mustangs, there is no valve involved...
Click to expand...

Yes there is. It is built into the part of the cap that seals to the inside of the neck of the radiator below the nipple for the overflow hose. There has to be or it would be an open system and you would boil over at ~212 degrees F.
Click to expand...

Ahhh....I'll have to check that. The way it seems to work to me is the lower part of the cap you are referring to is pushed up enough when the pressure reaches a certain point, and allows the coolant to flow into the overflow tank. I guess I'm thinking of a valve in the traditional sense...
 
Originally posted by: Insane3D
Originally posted by: Ronstang
Originally posted by: Insane3D
At least in 5.0 Mustangs, there is no valve involved...
Click to expand...

Yes there is. It is built into the part of the cap that seals to the inside of the neck of the radiator below the nipple for the overflow hose. There has to be or it would be an open system and you would boil over at ~212 degrees F.
Click to expand...

Ahhh....I'll have to check that. The way it seems to work to me is the lower part of the cap you are referring to is pushed up enough when the pressure reaches a certain point, and allows the coolant to flow into the overflow tank. I guess I'm thinking of a valve in the traditional sense...
Click to expand...

No, it is not a valve in the traditional sense. Next time you have the cap off look at the part that seals against the inner throat of the radiator. In the center is a smaller circle or flap that creates the one way valve I speak of. Pull back on it a little and you will see what I am talking about.
 
I believe the car's head gasket is blown. The exhaust gas enters into cooling system and force the coolant out of the system.
 
Originally posted by: Nerdwannabe
I believe the car's head gasket is blown. The exhaust gas enters into cooling system and force the coolant out of the system.
Click to expand...

I was actually kinda wondering that myself, doesn't a blown head gasket often produce white smoke from the exhaust?


But anyas, I understand about how the pressure opens the vavle whic allows the water to flow to the overflow. But once the water cools, the valve closes, so I don't see how the water can reenter the radiator if the valve is closed. I'm sure you guys are explaining it correctly, but I am apparently missing something about the process.
 
Originally posted by: coolred
Originally posted by: Nerdwannabe
I believe the car's head gasket is blown. The exhaust gas enters into cooling system and force the coolant out of the system.
Click to expand...

I was actually kinda wondering that myself, doesn't a blown head gasket often produce white smoke from the exhaust?


But anyas, I understand about how the pressure opens the vavle whic allows the water to flow to the overflow. But once the water cools, the valve closes, so I don't see how the water can reenter the radiator if the valve is closed. I'm sure you guys are explaining it correctly, but I am apparently missing something about the process.
Click to expand...

Vacuum sucks it back through a different one way valve that's built into the cap. There are two different one way valves built into the cap, one for each direction. The spring loaded outflow valve is easy to see (it seals against the inner neck of the filler hole), but the inflow valve is small and not very obvious; it's in the center of the cap--you should hear a tiny rattle from that valve if you shake the cap.

At any rate, try a new radiator cap first. The boiling may be caused by a cap that's not allowing the system to pressurize sufficiently. IIRC, operating pressure for most cooling systems should be around 15 PSI. Also inspect the inner neck of the filler opening. If the plastic is deteriorated, then the spring loaded valve can't seal, and the system won't pressurize.
 
Yeah, a new cap is a good try. Might get lucky. But I think the radiator is plugged. Warm the car up to OT, shut it down, and feel the radiator. I should feel evenly warm on the entire surface. Any cold spots are an indication of poor circulation. Pull the rad. and take it to the local radiator shop. They should be able to core it for $80 or so.
 
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