Radiator Expansion Tank - How it works...

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jondee86
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Radiator Expansion Tank - How it works...

Postby jondee86 » Mon Jun 30, 2014 2:56 pm

You can see the expansion tank fixed to the strut tower on the left
of the picture (stolen at random from the interweb)...

Image

So, how does it work then ?

When the engine heats up the water expands and pressure builds up
inside the cooling system. Then the radiator cap lifts a bit against the
spring to allow some water to escape and prevent the pressure getting
too high (otherwise it will blow the hoses off). Thus, during normal
operation, you can expect the level in the expansion/overflow bottle
to change from cold to hot... maybe by half an inch or so.

When the engine cools down the water starts to contract, and creates
low pressure inside the cooling system. When this happens, the radiator
cap acts like a one-way valve, and lets the low pressure draw water
back into the engine from the expansion tank. You will know that this
system is working if you open the radiator cap and find that the radiator
is full right to the top with coolant.

The expansion tank is marked with high and low water marks. When
the engine is hot the water level should be under the top line. When it
is cold the level should be above the bottom line. Water will not come
out the overflow tube and dribble down the inner guard until the level
is above the top line. But if you fill the tank to the top line when the
engine is cold, you can have some come out the overflow as the engine
heats up and pushed extra water into the tank.

Steam, foam, froth or creamy liquid in the tank, or steam/hot water
puking out the overflow tube, is always a bad sign. You should take a
look to find the cause and get it fixed.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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idreamofdrifting
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Re: Radiator Expansion Tank - How it works...

Postby idreamofdrifting » Tue Jul 01, 2014 10:52 am

jondee86 wrote:You can see the expansion tank fixed to the strut tower on the left
of the picture (stolen at random from the interweb)...

Image

So, how does it work then ?

When the engine heats up the water expands and pressure builds up
inside the cooling system. Then the radiator cap lifts a bit against the
spring to allow some water to escape and prevent the pressure getting
too high (otherwise it will blow the hoses off). Thus, during normal
operation, you can expect the level in the expansion/overflow bottle
to change from cold to hot... maybe by an inch or so.

When the engine cools down the water starts to contract, and creates
low pressure inside the cooling system. When this happens, the radiator
cap acts like a one-way valve, and lets the low pressure draw water
back into the engine from the expansion tank. You will know that this
system is working if you open the radiator cap and find that the radiator
is full right to the top with coolant.

The expansion tank is marked with high and low water marks. When
the engine is hot the water level should be under the top line. When it
is cold the level should be above the bottom line. Water will not come
out the overflow tube and dribble down the inner guard until the level
is above the top line. But if you fill the tank to the top line when the
engine is cold, you can have some come out the overflow as the engine
heats up and pushed extra water into the tank.

Steam, foam, froth or creamy liquid in the tank, or steam/hot water
puking out the overflow tube, is always a bad sign. You should take a
look to find the cause and get it fixed.

Cheers... jondee86


+1
Thanks jondee86, your knowledge of the 4AGE is always informative, and very helpful!
Image

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Grant
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Re: Radiator Expansion Tank - How it works...

Postby Grant » Tue Jul 01, 2014 7:46 pm

Yea I need to get the correct size tube for that, hose clamps don't work if the hose is too big haha, my coolant doesn't go back after the engine cools down.

Does the coolant level really change an inch in the expansion tank? I've never measured it, but I don't think it's that much...
Last edited by Grant on Wed Jul 02, 2014 7:38 am, edited 1 time in total.

ritz
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Re: Radiator Expansion Tank - How it works...

Postby ritz » Wed Jul 02, 2014 12:26 am

The coolant does flow back into the raditor when it is cold.

Coolant cap serves two main purposes.
#1. Relieve pressure to prevent engine damage due to prolonged high temperatures.

#2 water expands, so pressure is relieved and goes to overflow

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Re: Radiator Expansion Tank - How it works...

Postby ritz » Wed Jul 02, 2014 12:28 am

Once water is cooled, it contracts and causes a vaccuum and will siphon water back into the radiator.

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Re: Radiator Expansion Tank - How it works...

Postby ritz » Wed Jul 02, 2014 12:29 am

Sorry redundant information . past bedtime

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Grant
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Re: Radiator Expansion Tank - How it works...

Postby Grant » Wed Jul 02, 2014 7:43 am

ritz wrote:The coolant does flow back into the raditor when it is cold.

Coolant cap serves two main purposes.
#1. Relieve pressure to prevent engine damage due to prolonged high temperatures.

#2 water expands, so pressure is relieved and goes to overflow

Oh yea I worded my post poorly sorry. I meant the hose connecting my radiator to the overflow doesn't seal perfectly, so my coolant isn't pulled back into the radiator... also I believe my overflow tank has a small hole at the top which would also cause any vacuum to be lost... :(

How much is a new overflow? 8-)

ritz
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Re: Radiator Expansion Tank - How it works...

Postby ritz » Wed Jul 02, 2014 8:18 am

Grant wrote:
ritz wrote:The coolant does flow back into the raditor when it is cold.

Coolant cap serves two main purposes.
#1. Relieve pressure to prevent engine damage due to prolonged high temperatures.

#2 water expands, so pressure is relieved and goes to overflow

Oh yea I worded my post poorly sorry. I meant the hose connecting my radiator to the overflow doesn't seal perfectly, so my coolant isn't pulled back into the radiator... also I believe my overflow tank has a small hole at the top which would also cause any vacuum to be lost... :(

How much is a new overflow? 8-)


ohhhh!

The overflow or coolant reservoir tank on our cars is not a pressurized system. If you notice there's a vent where the cap is. The tanks go for about $30 depending on condition.


Expansion tanks are for vehicles that have a pressurized overflow system (mainly Germany vehicles and some domestics).

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jondee86
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Re: Radiator Expansion Tank - How it works...

Postby jondee86 » Wed Jul 02, 2014 7:27 pm

Pleased to see a bit of interest in this topic :)

@ Grant. Yes, I was shooting from the lip, but today I measured the difference
from cold to hot, and it was 12mm (1/2").

It is correct that the expansion tank/overflow tank/reservoir/whatever is not
pressurised. And it is also correct that the water returns to the radiator because
the low pressure in the radiator when it cools siphons the water out of the tank.
Any air leakage at either end of the overflow hose will break the siphon effect
and water will not get sucked back into the radiator.

And yes again, there is a small vent hole on the underside of the lid of the tank,
that connects to the short overflow hose that sends any excess water down the
inside of the guard. So, you will only push water out there if you fill the tank
pretty much right to the top.

Last, to the reason why there is an expansion tank. Back in the day when the
world was a far simpler and happier place, the overflow hose from the radiator
neck just went to the side of the radiator, and then dumped any overflow on
the ground. This meant that there was always air in the top of the radiator, and
when you looked inside the correct level was "covering the top of the tubes".

But that pocket of air meant that there was always a lot of disolved oxygen in
the water, and disolved oxygen promotes corrosion. So the expansion tank and
siphon system was introduced to keep the radiator always full, and eliminate
the air pocket.

Cheers... jondee86
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.

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Re: Radiator Expansion Tank - How it works...

Postby phanist » Thu Jul 03, 2014 1:01 am

+1

i love this section of the forum

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