I was thinking about this just now and I want to know if I'm right.

Working on my radiator today got me thinking my rad cap was off and I was purging the air and I thought if the coolant was being circulated by the pump after the thermostat opened why didn't the flowing coolant just gush out through the open filler neck?

I was thinking that with this in mind because I know that water under pressure always wants to take the easiest path (to an area of lower pressure).

So I thought that if the coolant entered from the right tank it would much rather come out the filler neck (path of least resistance) then deal with getting forced through those tiny fins.

I promptly forgot about it but tonight out of nowhere it made sense.

The pressure end of the rad (the side without the filler neck) is on the left the coolant enters the left tank under pressure and is forced through the fins and comes out the other side to the right tank. On this side of the tank the discharge hose is on the bottom so the coolant simply flows down from the fins and into the lower hose due to gravity.

The only way that coolant would flow up through the neck is if there was a serious blockage at the lower hose right?

While on that topic I understand the the rad cap is a calibrated spring that presses on a round metal and rubber gasket and gives when the pressure rises over 16 PSI and deposits the coolant into the recovery tank.

But when the pressure drops a vacuum is drawn and the coolant it drawn back into the rad.

How is this done? The original valve is a one way deal coolant can leave the rad but not re-enter.

Is there an additional valve for this process in the cap?

Thanks in advance.

 

You have it mostly correct. The missing piece is the vacuum valve on the underside of the radiator cap that is also a 'one-way' valve that only opens under negative pressure (vacuum). This is the part that allows the coolant to flow back into the radiator as the engine cools down.

Here's a link for the curious: FAQ : The Pressurized Cooling System

 

Krankshaft,

There is quite a bit of suction on the lower outlet of the radiator due to the suction of the waterpump. That makes the coolant recirculate, not gravity.

That's why some of the lower radiator hoses have a coil of steel on the inside to keep them from collapsing under suction.

 

Hi Tung,

Gravity does help though. In the "olden days" of the Model-T Fords, they did not have water pumps. 'Thermo-siphon' only was used to circulate coolant. Hot water rose to the top of the radiator and gravity plus sinking tendency of cooler water pushed the coolant back through the engine block.

 

Hi Dave,

How's everything? I never knew that old cars relied on natural convection like that. Do they have big radiators too? I imagine it couldn't have been very efficient.

 

no fuel pump in Model T either. pickup in front of tank.
low fuel level required backing up a steep hill or one didn't make it.

wonder if backing up a hill helps coolant flow also?

maybe an example of the Second Law of Thermodynamics

 

Yep, small motor and big radiator. Model-T's lasted a long time with that simple set up but yes, they couldn't increase the horsepower much beyond without better cooling. Strangely, the replacement, Model-A, only had 40 hp and a water pump but were plagued with overheating problems. One option was radiator cap mounted temperature gauge so the driver could keep an eye on things.

Krankshaft: Sorry about hijacking your thread!

 

Suction I forgot that piece you can't have a pump without suction. Coolant has to be drawn to the pump before you can get the coolant moving.

Thanks for clearing that up I always wondered what it mean't when coolant hoses said doesn't contain spring.

An additional valve on the cap for vacuum only that makes sense. I imagine it wouldn't have to be as large either.

Nice link Aero I like detailed sites like that.

By the way Rock auto is fast I'll have all my coolant system parts on Friday and Saturday via ground. Unfortunately they will have to wait until after my two flushes are complete.

Can't they ship any slower ?

Thanks again for the replies.

 

Water is sucked from right (lower hose) to left of the rad (upper hose return) via the water pump. You should feel a surge in the upper hose or hose above the water pump if you squeeze it & the thermostat is open. I'm surprised modern engines do not have a flow meter included in the coolant system. These can also have coolant temp combined.

Aeroman.