Coolant level question

[canis]

The water becomes pressurised when hot precisely because it can't expand, the volume of the coolant system doesn't increase, save for the hoses being flexible and so might stretch a bit.

Therefore, opening the cap when anything other than cold gives an erroneous reading. The only way to properly check water level is when cold, before starting, even for a second.

If there is air in the system somewhere, it too will reach the same pressure as everything else. Air compresses far more than water can, so that is where your level drop is going (in other words, the bubble(s) gets smaller).

The way to remove this is:
Top up the water to max first thing in the morning before starting the engine.
Close the lid.
Take it to a car-park and drive in tight right-hand circles. This centrifugal action purges the heater matrix, you might hear it bubbling from inside the car.
Drive home, park but leave the engine running, open the bonnet.
Feel the top hose with your hand (watch out for the fan, it can start without warning!)
Be careful of all moving parts. Obviously.
It should be hot by now (means the thermostat is working, confirm by feeling the top of the radiator, cold spots will reveal your air bubble)
Reach down to the bottom hose (again be very careful of the fan)
Squish the top hose with your left hand.
Squish the bottom hose with your right hand.
Release the top hose.
Release the bottom hose.
Keep performing this pumping action until the level in your bottle drops and stays dropped. If after several of these pumps (say, ten, maybe) nothing has happened, wait for a few minutes.
Repeat the pumping procedure until you are confident the air is purged, or there never was any.

You can reverse the pumping direction alternately if necessary. The aim is the break up whatever air is in the system and get it past it's capture. Usually once it gets moving it's just a matter of time before it ends up in the expansion tank.

This works for all cars, not just volvo.

It's called an expansion tank because originally cars didn't have them. They weren't under pressure. Any excess water dripped out onto the road underneath. The problem with that is when the water contracts it will draw air into the top of the radiator. By capturing it in a bottle, the water (and any air) could be discharged, and during contraction (cooling) it could refill itself with the water previously discharged and not the air which simply bubbles harmlessly to the top of the bottle. As cars got more efficient the manufacturers began designing them to operate at higher temperatures, and pressurisation prevents boiling. So now it is a closed system.

Opening the cooling system when hot is dangerous and pointless. By releasing the pressure, the water can boil. It will boil in the engine block, where it is hottest, and it's only route to relieve it's pressure is by forcing all the other water out of it's way. The only escape is through the filler-cap, and orange rusty water will spill out of the top. Not only is this a scalding hazard, it also stresses components, and you'll be left with no clue about water levels anymore. So it's useless to do this.

[colnet]

Great post Canis

[960kg]

Originally Posted by canis;2289496

The water becomes pressurised when hot precisely because it can't expand, the volume of the coolant system doesn't increase, save for the hoses being flexible and so might stretch a bit.

Therefore, opening the cap when anything other than cold gives an erroneous reading. The only way to properly check water level is when cold, before starting, even for a second.

If there is air in the system somewhere, it too will reach the same pressure as everything else. Air compresses far more than water can, so that is where your level drop is going (in other words, the bubble(s) gets smaller).

The way to remove this is:
Top up the water to max first thing in the morning before starting the engine.
Close the lid.
Take it to a car-park and drive in tight right-hand circles. This centrifugal action purges the heater matrix, you might hear it bubbling from inside the car.
Drive home, park but leave the engine running, open the bonnet.
Feel the top hose with your hand (watch out for the fan, it can start without warning!)
Be careful of all moving parts. Obviously.
It should be hot by now (means the thermostat is working, confirm by feeling the top of the radiator, cold spots will reveal your air bubble)
Reach down to the bottom hose (again be very careful of the fan)
Squish the top hose with your left hand.
Squish the bottom hose with your right hand.
Release the top hose.
Release the bottom hose.
Keep performing this pumping action until the level in your bottle drops and stays dropped. If after several of these pumps (say, ten, maybe) nothing has happened, wait for a few minutes.
Repeat the pumping procedure until you are confident the air is purged, or there never was any.

You can reverse the pumping direction alternately if necessary. The aim is the break up whatever air is in the system and get it past it's capture. Usually once it gets moving it's just a matter of time before it ends up in the expansion tank.

This works for all cars, not just volvo.

It's called an expansion tank because originally cars didn't have them. They weren't under pressure. Any excess water dripped out onto the road underneath. The problem with that is when the water contracts it will draw air into the top of the radiator. By capturing it in a bottle, the water (and any air) could be discharged, and during contraction (cooling) it could refill itself with the water previously discharged and not the air which simply bubbles harmlessly to the top of the bottle. As cars got more efficient the manufacturers began designing them to operate at higher temperatures, and pressurisation prevents boiling. So now it is a closed system.

Opening the cooling system when hot is dangerous and pointless. By releasing the pressure, the water can boil. It will boil in the engine block, where it is hottest, and it's only route to relieve it's pressure is by forcing all the other water out of it's way. The only escape is through the filler-cap, and orange rusty water will spill out of the top. Not only is this a scalding hazard, it also stresses components, and you'll be left with no clue about water levels anymore. So it's useless to do this.

Not arguing with anybody just stating this is precisely what happens......therefore if your coolant level alters it`s height enough in the expansion reservoir the system will contain air and should be purged.





.

[bobthecabbage]

It's a bit confusing when people say the water can't expand because the system is pressurised.

The system becomes pressurised by the water expanding and compressing the air in the expansion tank.

There is no restriction on the water expanding in the system. There is very little that will stop water expanding save a massively thick pressure vessel.

If you accidentally trap water in a bolt hole when working on your engine block then when it heats up the block will crack, that is the force of expanding water!

So, as you can imagine, a few rubber hoses and plastic are not going to resist the expansion of water.

So in conclusion, the water does expand, the air compresses (in the header tank) and the pressure in the system is provided by the compressed air.

If you want an illustration of the incompressibility of water then consider this.
4 km down in the deepest oceans the pressure is 400bar or 5801psi and yet the water down there is only compressed 1.8%.

In my other post the expansion of water I estimated equates to 2.75% so extrapolating from this in order to prevent expansion of water in the cooling system the pressure would need to be about 8000psi whereas I believe the relief valve in most cooling systems opens at about 14psi.

[clarkey1984]

Originally Posted by bobthecabbage

It's a bit confusing when people say the water can't expand because the system is pressurised.

The system becomes pressurised by the water expanding and compressing the air in the expansion tank.

There is no restriction on the water expanding in the system. There is very little that will stop water expanding save a massively thick pressure vessel.

If you accidentally trap water in a bolt hole when working on your engine block then when it heats up the block will crack, that is the force of expanding water!

So, as you can imagine, a few rubber hoses and plastic are not going to resist the expansion of water.

So in conclusion, the water does expand, the air compresses (in the header tank) and the pressure in the system is provided by the compressed air.

If you want an illustration of the incompressibility of water then consider this.
4 km down in the deepest oceans the pressure is 400bar or 5801psi and yet the water down there is only compressed 1.8%.

In my other post the expansion of water I estimated equates to 2.75% so extrapolating from this in order to prevent expansion of water in the cooling system the pressure would need to be about 8000psi whereas I believe the relief valve in most cooling systems opens at about 14psi.

Yes! It's the expansions of the water, and thus the compression of the air that creates the pressure in the system, knew I wasn't going mad haha

[canis]

Originally Posted by bobthecabbage

It's a bit confusing when people say the water can't expand because the system is pressurised.

The system becomes pressurised by the water expanding and compressing the air in the expansion tank.

There is no restriction on the water expanding in the system. There is very little that will stop water expanding save a massively thick pressure vessel.

If you accidentally trap water in a bolt hole when working on your engine block then when it heats up the block will crack, that is the force of expanding water!

So, as you can imagine, a few rubber hoses and plastic are not going to resist the expansion of water.

So in conclusion, the water does expand, the air compresses (in the header tank) and the pressure in the system is provided by the compressed air.

If you want an illustration of the incompressibility of water then consider this.
4 km down in the deepest oceans the pressure is 400bar or 5801psi and yet the water down there is only compressed 1.8%.

In my other post the expansion of water I estimated equates to 2.75% so extrapolating from this in order to prevent expansion of water in the cooling system the pressure would need to be about 8000psi whereas I believe the relief valve in most cooling systems opens at about 14psi.

Yeah, true enough. I did say the bubbles get smaller, and air inside the expansion tank is basically the same. Naturally, both air and water in a closed system will equalise in pressure, and air is more compressible than water. The same phenomenon keeps your cola fizzy in transit.

You're also quite right about the damage water pressure can exert, including your bolt hole example. Water to steam produces a 1:9 expansion ratio (if memory serves from my traction engine volunteer days). Oil can do the same, and it's why head bolts need to be inserted dry. And left to heat indefinitely (as in a bolt hole) it certainly does blow hoses, circulation keeps it cool enough, but you know that of course.

I'm not arguing either. I think we're both explaining the same thing.

[clarkey1984]

We use water at work as I work for a blasting company, our top spec uhp guns can run up to 55,000 psi, that will blow thick paint off metal in about half a second, its crazy what water alone can do.