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Expansion tank


fifthwheel
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I have seen plastic expansion tanks on all kinds of engines from contractors plant to bikes over the years. What I could never understand is how the coolant gets back into the rad after the spring cap has opened and lets the hot water out. If it never needs to be put back why the tank?

My Honda scooter has this set up, The plastic tank is the only way I can put coolant into the system as the rad cap is hidden under the bodywork. I cant see the point topping this low pressure tank up if it cant get in to the system. Thanks John.

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I understand how the pressure cap system works but on my Honda the pressure cap is on top of the rad under all the bodywork. Daily checks can only be done by looking at the low pressure overflow tank. This coolant to my mind cannot ever return to the main system. The pressure cap lifts and coolant is passed to the low pressure tank with a rubber cap. When the engine cools the pressure cap rests back on its seat and will not as I see it let coolant back to the rad if needed.

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How can it flow back when the pressure cap is closed? Vacuum would pull the pressure cap on to its seat even more.

I am not being argumentative but just cant see the point of the flimsy low pressure tank with a rubber filler flap.

Is it just to stop the coolant running on to the road.

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How can it flow back when the pressure cap is closed? Vacuum would pull the pressure cap on to its seat even more.

I am not being argumentative but just cant see the point of the flimsy low pressure tank with a rubber filler flap.

Is it just to stop the coolant running on to the road.

 

It's an overflow tank IF your coolant expands or boils and its also a top up or reservoir. Fluids always find their lowest level so if you take off the body work and look at the bike from the side it will become clear to you how the system works. Maybe this will help . Notice that Kawasaki call it a Reserve Tank and not an Overflow Tank .

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I understand how the pressure cap system works but on my Honda the pressure cap is on top of the rad under all the bodywork. Daily checks can only be done by looking at the low pressure overflow tank. This coolant to my mind cannot ever return to the main system. The pressure cap lifts and coolant is passed to the low pressure tank with a rubber cap. When the engine cools the pressure cap rests back on its seat and will not as I see it let coolant back to the rad if needed.

What does the Honda Workshop Manual have to say about how the system works ?
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How can it flow back when the pressure cap is closed? Vacuum would pull the pressure cap on to its seat even more.

I am not being argumentative but just cant see the point of the flimsy low pressure tank with a rubber filler flap.

Is it just to stop the coolant running on to the road.

 

It's an overflow tank IF your coolant expands or boils and its also a top up or reservoir. Fluids always find their lowest level so if you take off the body work and look at the bike from the side it will become clear to you how the system works. Maybe this will help . Notice that Kawasaki call it a Reserve Tank and not an Overflow Tank .

Sorry Bob, is it Bob? The only way coolant can return to the system as I see it is if there is a one way valve in the radiator cap that works when the pressure cap is on its seat. If this is the case then I have never seen or heard of such a thing. They say you learn something new every day. john

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When the system expands and pushes water out it is still a sealed system, if it pushes water out and the system cools it creates negative pressure which the cap allows to be sucked back in, your rad cap is an important little device

Are you saying the pressure cap opens, then the water flows into the low pressure container, then when cooled vacuum draws coolant back into the rad. If so what makes the pressure cap open to do this, or is it always held off its seat? or are you winding me up. john.

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How can it flow back when the pressure cap is closed? Vacuum would pull the pressure cap on to its seat even more.

I am not being argumentative but just cant see the point of the flimsy low pressure tank with a rubber filler flap.

Is it just to stop the coolant running on to the road.

 

It's an overflow tank IF your coolant expands or boils and its also a top up or reservoir. Fluids always find their lowest level so if you take off the body work and look at the bike from the side it will become clear to you how the system works. Maybe this will help . Notice that Kawasaki call it a Reserve Tank and not an Overflow Tank .

Sorry Bob, is it Bob? The only way coolant can return to the system as I see it is if there is a one way valve in the radiator cap that works when the pressure cap is on its seat. If this is the case then I have never seen or heard of such a thing. They say you learn something new every day. john

 

Cheers John , you can call me Bob if you like . Have you read the text ? Apparently it is not a one way valve, it is a two way valve . Have another read . Bear in mind this is just my ER5, other bikes may differ but I think the general principal is probably the same .Tis is my bike , just to confuse matters I have bypassed my carb heating system.

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No sorry Bob cant read it text too small. john.

 

I have an ER500 and am a member on Facebook group, there is a file section and I have pinched a bit of info from it.

Is this the text you were trying to show me?

This explains the valve in the rad cap and I can only assume the Honda being the same. This is the only way it can return to the rad, thanks for the link if it is the same.



The system is pressurized by the radiator cap t

o suppress boiling and the resultant air bubbles

which can cause engine overheating. As the engine warms up, the coolant in the radiator and the

water jacket expands. The excess coolant flows through the radiator cap and hose to the reserve tank

to be stored there temporarily. Conversely, as t

he engine cools down, the coolant in the radiator and

the water jacket contracts, and the stored coolant flows back to the radiator from the reserve tank.

The radiator cap has two valves. One is a pressure valve which holds the pressure in the system

when the engine is running. When the pressure exceeds 93


123 kPa (0.95


1.25 kgf/cm², 14


18

psi), the pressure valve opens and releases the pressure to the reserve tank. As soon as pressure

escapes, the valve closes, and keeps the pressure at 93


123 kPa (0.95


1.25 kgf/cm², 14


18 psi).

When the engine cools down, another small valve (vacuum valve) in the cap opens. As the coolant

cools, the coolant contracts to form a vacuum in

the system. The vacuum valve opens and allows the

coolant from the reserve tank to enter the radiator.

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Fastbob is quite correct in what he says,rad cap has two valves ,one to let coolant out when it reaches a set pressure,the other smaller valve in the centre of the cap to return the coolant as the system cools down.This is to maintain a constant volume of coolant.

Cars used to be like this until they put a header tank on so there is an air gap which is compressible, no need for an expansion tank.I guess these are not generally used on a bike due to space.

In theory if your bike is not using any coolant, the level in the expansion tank should remain constant when checked when cold.

If the level drops then you have a external leak or an internal issue such as a head gasket.

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I couldn't read it it was too small and I couldn't see any way to enlarge it, will try again.

 

Is this any better ? I forgot that it was a screenshot of a PDF , I keep all my manuals in my phone but of course I can zoom in on a PDF . I can read this on a standard phone but it helps if you rotate to landscape configuration.

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I couldn't read it it was too small and I couldn't see any way to enlarge it, will try again.

 

Is this any better ? I forgot that it was a screenshot of a PDF , I keep all my manuals in my phone but of course I can zoom in on a PDF . I can read this on a standard phone but it helps if you rotate to landscape configuration.

Thats looks the same as I posted above thanks Bob.

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I couldn't read it it was too small and I couldn't see any way to enlarge it, will try again.

 

Is this any better ? I forgot that it was a screenshot of a PDF , I keep all my manuals in my phone but of course I can zoom in on a PDF . I can read this on a standard phone but it helps if you rotate to landscape configuration.

Thats looks the same as I posted above thanks Bob.

 

Well I don't know then , I'm reading it perfectly easily on my phone right now . Never mind, Bender's link in post #16 explains it brilliantly .

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I understand how the pressure cap system works but on my Honda the pressure cap is on top of the rad under all the bodywork. Daily checks can only be done by looking at the low pressure overflow tank. This coolant to my mind cannot ever return to the main system. The pressure cap lifts and coolant is passed to the low pressure tank with a rubber cap. When the engine cools the pressure cap rests back on its seat and will not as I see it let coolant back to the rad if needed.

 

Hi mate, have you got it now ? I only ask because a number of us have been to some trouble to help you out here but so far you have not confirmed your understanding. Thanks.

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I think the problem is lack of understanding due to parts with unfortunate and misleading names and manufacturers who only give the bare minimum of information which then is inevitably cloudy given that common knowledge is often incorrect knowledge and assumptions are made about how unfortunately named parts function.


The information already given here by members is correct but the issues are complex and hopefully I can help with clarity by means of a necessarily very lengthy description.


First of all there are different approaches to cooling system setups and parts and it depends what choices the manufacturer has made as to what these do and what happens to the coolant.


There are historically many setups of cooling systems with old names that are to be honest misleading these are:-


cooling systems with no additional tank beyond the radiator and a conve3ntional one way pressure cap fitted to the radiator.

cooling systems with a rubber sealed cap on the radiator and an additional tank beyond the radiator with a radiator cap one way calve fitted to that.

cooling systems with a radiator cap one way valve fitted to the radiator and an additional tank beyond the radiator with a cap on it.

cooling systems with a radiator cap two way valve fitted to the radiator and an additional tank with a cap fitted to that.


There are many choices a manufacturer can make but they all have to deal in some way with the problems a water cooled system creates. Water is not compressible and yet it expands when it is heated. This allows a pressure cap system to function and also allows the water to flow into and out of a cooling system that has an additional tank, and this then allows the cooling system to deal with the unfortunate complications that a water cooled system is prone to.


The problems are basically trapped air in the radiator matrix and trapped air in the small and remote parts of the water jacket due to poor filling after the system has been drained and refilled. There has to be some way of expelling this trapped air which can only happen after the system is filled and the engine has run up to temperature and the water pump has had a chance to purge the air and move it to the top of the radiator.


This means that the system is going to be only partially filled until the air can find its way to the radiator and then the top of the radiator and then out of the radiator either to the environment or into an additional tank.


In the old days the excess water was allowed to drop onto the road and then the radiator must be manually inspected for level and topped up to replace the air which now is in the radiator and easily replaced, if you can get access to it.


Given that bikes have enclosed radiators with no easy access, most manufacturers use an additional tank with a two way valve in the radiator and a minimalist cap in the additional tank.


The way this works is, as the temperature climbs the water expands and pressure builds, The pressure cap in the radiator creates an overpressure state in the system and this prevents the water from boiling when it exceeds 100 degrees. The boiling point of water is varied depending upon the pressure it is under.


When there is unwanted air in the system due to air locks after draining the system, the cooling system routinely expels this air after it reaches the radiator and it then flows into the additional tank it then rises to the top of the additional tank and can be replaced with water there thus topping the system up and purging the air.


The cap that allows this air and water to exit the radiator and enter the additional tank is not a one way valve on motorbikes. It is two, one way valves each looking after a different function.


When the engine is hot water and air or water alone passes into the additional tank through the first one way valve in the cap. This keeps the pressure correct for the system. The second one way valve does not operate yet.


When the system cools down after the engine stops the coolant contracts and then a vacuum is created. By now all the air has been expelled from the radiator the second one way valve opens in the radiator cap and this allows whatever is in the bottom of the additional tank to be drawn back into the radiator, and by now that will only be water because the air is at the top of the additional tank.


It is a neat system that allows the transfer of unwanted air trapped in the system to be safely expelled into an external tank which then is conveniently topped up returning the cooling system to peak efficiency.


The common view of a radiator pressure cap is a one way valve. That would not function in a modern system because it would not allow the return of water to replenish the expelled air.


The core of this is the two one way valves, one valve keeps the system under pressure and the second valve allows the controlled replenishment of the radiator with water to replace previously expelled water or previously expelled air.


I know this has been a very lengthy description but it has to be, there is a lot of misunderstanding and confusion with some riders and this has been created by an unwillingness to fully and completely describe what is a complex system suffering with poorly and misleadingly named parts as I said at the beginning.

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