Coolant goes back up when cap removed

[Aaron2410]

Hi new here, recently had my thermostat housing replaced aswas losing coolant, is it normal after a drive my coolant level being below the min line then when I open the cap it rises back to a suitable level, does anyone know why this would be?

 

[Neb_engineer]

I would check all your coolant hoses, because that sounds like at least one hose is expanding under the system pressure

[Luke4efc]

Or the expansion cap has failed and is pressuring the system.

[st line x 140 driver]

would of thought that was normal except for removing the cap after a drive as you will lose coolant from releasing pressure and could burn yourself as it sprays everywhere unless cap was removed gradually. Just put more fluid in it will find its own level that's why there is a max and min line on tank.

[Aaron2410]

1 hour ago, Luke4efc said:

Or the expansion cap has failed and is pressuring the system.

How will I tell if the cap is faulty? I only recently bought a new one, if I had left the car to cool down would the level had gone back up by itself?

[st line x 140 driver]

Yes thats why its called an expansion tank!

[st line x 140 driver]

9 minutes ago, Aaron2410 said:

How will I tell if the cap is faulty? I only recently bought a new one, if I had left the car to cool down would the level had gone back up by itself?

Yes thats why its called an expansion tank! When cold it should be at max level or close too in use it should not go below min level

[Aaron2410]

13 minutes ago, st line x 140 driver said:

Yes thats why its called an expansion tank! When cold it should be at max level or close too in use it should not go below min level

Yeah I thought so, just concerned why it went between min and max once I took the cap off

[TomsFocus]

The expansion works the other way round.  Hot coolant expands and would overflow the reservoir when the engine's hot if there wasn't a built in air gap for the coolant to expand into.  This is why you mustn't fill higher than the max mark.  As it cools, it contracts again and shrinks back down but should remain higher than the min mark.

You've got an air lock in the system.  Fluids can't be compressed so if the coolant disappears then it has to be into a pocket of air.  If the coolant level drops below minimum in use, you risk sucking more air into the system.

Ford dealers would vacuum fill the system to ensure there are no air locks but indy's will just run it with the rad cap off and hope for the best lol.

[st line x 140 driver]

Tom think you have it the wrong way round.

When the engine is at full operating temp, the process is static. As the engine cools, first the pressure drops in the system, next the pressure falls below zero to a slight vacuum. This vacuum draws coolant from the tank, back into the system. ... The lowest level will occur when at full temperature and at highway speed.

Lucky you can see the level older cars had steel tanks.

[TomsFocus]

3 minutes ago, st line x 140 driver said:

Tom think you have it the wrong way round.

When the engine is at full operating temp, the process is static. As the engine cools, first the pressure drops in the system, next the pressure falls below zero to a slight vacuum. This vacuum draws coolant from the tank, back into the system. ... The lowest level will occur when at full temperature and at highway speed.

Lucky you can see the level older cars had steel tanks.

I haven't.  That quote started correct but then contradicted itself.  As the engine cools, the pressure drops and and draws coolant back into the engine from the tank.  That's correct. 

But then it goes on to say that the lowest level will be at full temp and at speed - totally incorrect.

[st line x 140 driver]

Agree to disagree.

Best way to bleed system if has air in is to have heater on hot to get full flow through complete cooling system with thermostat open at full operating temperature. Run with cap off header tank which will remove air lock. Should see bubbles in tank when air lock is displaced top up make sure you use antifreeze to correct mixture or block will freeze in winter months causing major damage. 

[Aaron2410]

4 minutes ago, st line x 140 driver said:

Agree to disagree.

Best way to bleed system if has air in is to have heater on hot to get full flow through complete cooling system with thermostat open at full operating temperature. Run with cap off header tank which will remove air lock. Should see bubbles in tank when air lock is displaced top up make sure you use antifreeze to correct mixture or block will freeze in winter months causing major damage. 

I drove home from work this morning it had done it again so I tried the method of running engine and cap off, did see a few bubbles in the coolant, it has seemed to stay just below the min line for now so whether it just needs topping up, they have replaced the coolant with a blue mixture rather then pink which is in there beforehand

[st line x 140 driver]

colour of antifreeze doesn't matter just the strength👌

[Aaron2410]

9 minutes ago, st line x 140 driver said:

colour of antifreeze doesn't matter just the strength👌

Cool thank you 

[fleamoo]

Does sound like a big airlock to me. As far as I'm aware blue and pink antifreeze don't mix but if they drained it thoroughly then I think you should be ok but best check with an expert!

[fleamoo]

Sorry, hang on I've just had another think.... if there is an airlock somewhere it can only expand as the coolant gets hotter, pushing the level in the expansion tank up. 

[Aaron2410]

2 hours ago, fleamoo said:

Sorry, hang on I've just had another think.... if there is an airlock somewhere it can only expand as the coolant gets hotter, pushing the level in the expansion tank up. 

So what would this mean? 
 

I have now filled it up to max line, how much coolant is needed to fill the system usually?

[st line x 140 driver]

8 hours ago, Aaron2410 said:

So what would this mean? 
 

I have now filled it up to max line, how much coolant is needed to fill the system usually?

That's what you are meant to do remember as engine runs level goes down. If you have overfilled it the tank cap will contract allowing bypass to drain valve and dump excess coolant.

[fleamoo]

Think I may have misinterpreted some of the wording in your posts. If we aren't talking big movements in level then a small drop in level as the engine heats up, cap on, is normal.This is due to the various rubber hoses expanding. If left alone, when the engine is switched off and allowed to cool, the level will return to its original position.I think you've complicated things a little by topping up, so you may find that the cold level drops over time to a new position somewhere around Min. It's fine to leave it there cos that's it's 'natural' level. Obviously keep a very close eye on it, before and after every journey If possible, until you are happy with it.

 

[fleamoo]

Missed Simon's post.I think we agree.

[TomsFocus]

So much confusion over a simple system!  I take it none of you are plumbers or heating engineers?  It works exactly the same way as your hot water tank or central heating radiators at home.  

It's simple physics:

1. Hot water expands

2. Fluids can't be compressed, but gasses can.

 

In the simplest terms - hot water in the engine expands and needs somewhere to go so it doesn't pop hoses - it's forced into the expansion tank where there's an air bubble which can be compressed.  The level in the tank rises slightly.  It's that simple.  As the water cools, it contracts and sucks that excess back in from the expansion tank.  The level drops back to original line.

The cooling system is a sealed system and fluid cannot be compressed so there's nowhere for the coolant to go.  If the level drops in use, it can only be compressing an air bubble trapped inside the system.

 

[Martin-Fiesta-MK6-1.4]

On 11/11/2020 at 9:29 AM, TomsFocus said:

The expansion works the other way round.  Hot coolant expands and would overflow the reservoir when the engine's hot if there wasn't a built in air gap for the coolant to expand into.  This is why you mustn't fill higher than the max mark.  As it cools, it contracts again and shrinks back down but should remain higher than the min mark.

You've got an air lock in the system.  Fluids can't be compressed so if the coolant disappears then it has to be into a pocket of air.  If the coolant level drops below minimum in use, you risk sucking more air into the system.

Ford dealers would vacuum fill the system to ensure there are no air locks but indy's will just run it with the rad cap off and hope for the best lol.

Is running the engine with the coolant cap off not common practice to get rid of air bubbles after a coolant change mate? need to do ine and most sources say that its fine?

[Martin-Fiesta-MK6-1.4]

On 11/11/2020 at 9:56 AM, st line x 140 driver said:

Agree to disagree.

Best way to bleed system if has air in is to have heater on hot to get full flow through complete cooling system with thermostat open at full operating temperature. Run with cap off header tank which will remove air lock. Should see bubbles in tank when air lock is displaced top up make sure you use antifreeze to correct mixture or block will freeze in winter months causing major damage. 

Correct antifreeze mixture? off the shelf antifreeze is pre mixed isnt it? https://www.eurocarparts.com/p/triple-qx-red-ready-mixed-antifreeze-coolant-5ltr-523770511

[st line x 140 driver]

40 minutes ago, Martin-Fiesta-MK6-1.4 said:

Correct antifreeze mixture? off the shelf antifreeze is pre mixed isnt it? https://www.eurocarparts.com/p/triple-qx-red-ready-mixed-antifreeze-coolant-5ltr-523770511

Bit like windscreen wash can buy it concentrated or ready mixed strength depends on environment. Advantage premix you don't have to test it with an antifreeze strength tester.

Concentrated is always cheaper. Using antifreeze increase boiling point too. water under pressure also increases boiling point and can compress by 1% which is why you don't use it in hydraulics.