Why you should have air in your expansion bottle.

[icosahedron]

I'm very reluctant to enter the air/no air in expansion bottle debate but: You cannot compress a liquid. With no air in the system there is no "give" and the expansion of coolant inflates the silicone hoses and expansion bottle. The system gets over-pressurised and the expansion bottle eventually succumbs to stress cracks. Sounds familiar?

With a little air in the expansion bottle the increase in coolant volume is accommodated by the reduction in air volume as it is compressed. The level of coolant rises in the expansion bottle and hence the name.

I've only been able to maintain this with the original black dalek cap. All the blue ones allow the air to slowly escape to the top-up tank and be replaced by coolant.

Ever noticed the MIN mark on the expansion bottle? It is there for a reason: The more air in the bottle the lower the eventual pressure will be.

[itchyfeet]

Header tank ( inside engine bay) has the pressure cap and should be full, expansion tank ( behind flap) has to be half full, when the coolant expands it flows out into the expansion tank and suck it back as it cools, your cap should be working or this doesnt happen and thats when you get over or under pressure as you may be getting

it is suposed to be under a set amount of pressure dictated by the cap ( about 1 bar if i remember correctly) and designed for that, the pressure increases the boiling point, if things crack its because they are old or you cap isnt venting

http://www.brick-yard.co.uk/forum/bleed ... 29710.html" onclick="window.open(this.href);return false;

put your new blue cap back on the old black one isnt working

if you want to run an old broken cap fine but its not right to encourage other people to do that

http://auto.howstuffworks.com/cooling-system7.htm" onclick="window.open(this.href);return false;

[mark]

the system is the result of vw throwing untold millions trying adapt a flat four air cold into a flat 4 water cooled with the engine in the ass end. the position of the radiator means that any air in the system will travel to its highest point, a rad full of air=no cooly, no cooly= untold tech pages of "my van keeps overheating" etc (see a pattern forming) when working correctly and filled correctly the system will never hit the min line, ever, running at the min line is the equivalent of running your engine oil at the same level...... a short trip away from disaster.

but anyhoo its your van and ill keep an eye out to see how you get along

mark

[itchyfeet]

A bit of contradiction here...

http://forum.club8090.co.uk/viewtopic.p ... 9#p7950759" onclick="window.open(this.href);return false;

Have heard of many ways of bleeding the cooling system and still won't follow any of them. I use a pressure plant sprayer. Fill rear with coolant until full. Remove bleed screw and fill radiator with thin tube using the plant sprayer until full. Replace bleed screw, open heater control and start engine. Let it IDLE, no need to annoy everybody around. Keep expansion bottle topped up as engine heats up, replace cap when full and go for a drive. Top up expansion tank when cooled down to get rid of remaining trapped air.

Other uses for plant sprayer: Filling gearbox with oil and pressure bleeding braking system by yourself.

[ghost123uk]

I've only been able to maintain this with the original black dalek cap. All the blue ones allow the air to slowly escape to the top-up tank and be replaced by coolant.

Bit confusing there. the last bit of that sentence (italics by me) is exactly what is supposed to happen. Any air in the expansion tank gets forced into the top up tank as the whole system expands due to heat, then as it shrinks on cooling, it sucks coolant from the top up tank. That's VW's way of making sure there is no air in the expansion tank !

Ever noticed the MIN mark on the expansion bottle?

No, I haven't (There is one on the top up tank of course )

The more air in the bottle the lower the eventual pressure will be.

Rubbish, (meant in a nice jokey sort of way ) You cannot alter physics. Pressure is what it is, 1 bar is 1 bar, regardless of what gasses or liquids are present.

[icosahedron]

It is amazing how poorly the cooling system is understood. The outlet from the expansion bottle is situated at the bottom and the inlet at the top, the air remains trapped at the top. There is no way that it can be transferred to other parts of the cooling system and accumulate. The amount of air in the system should remain constant with any loss of coolant being replenished from the top-up bottle. The air is only there to cushion the expansion and prevent over-pressure. If the coolant level is below the minimum mark there is too much air in the system. The coolant will only expand by a fixed volume and the air will be compressed by that volume. Not too difficult to understand that the amount of air will dictate the cooling system pressure.

The dalek cap is supposed to be a one-way valve. The pressure release feature is there to prevent a calamity, not to regulate the pressure. Fortunately it seems to work well as it is being put to use a lot.

As for how I'm getting along with my van: Unlike many on here I've never had cooling problems since fitting my 1983 Aircooled with a DJ engine in 1989. Always had air in the expansion bottle until I had to replace it and the cap due to age. Since then I've had to replace stressed bottles every few years until I've found a way of maintaining the air again.

I repeat, I was very reluctant to enter this debate. Hopefully someone will find the information useful. I'm well aware that mobs rule so feel free to ignore it.

[ghost123uk]

Fear not icosahedron, a friendly debate, like one might have in the pub etc, is no problem (and gives us summat to do when it's too hot outside to do much!)

Now, I cannot see how your system can maintain any air in the expansion bottle. The Dalek cap vents any air, and then some expanding coolant, into the top up tank when the engine is hot. So at that time there cannot be any air in the expansion bottle. Then, as the engine cools down, the shrinking volume of coolant causes a negative pressure which sucks pure coolant from the bottom of the top up tank, thus maintaining the "no air" in the expansion bottle. VW designed it to do that for a good reason. Self bleeding is what it is doing.

If you personally have a system that always has air in the expansion bottle, that just means it is not working the way it is supposed to.

[ghost123uk]

And I have to repeat :-

The air is only there to cushion the expansion and prevent over-pressure.....
.......

Not too difficult to understand that the amount of air will dictate the cooling system pressure.

Is simply against the laws of physics. The presence of any amount of air cannot alter the pressure.

[lloydy]

Although I don't agree with the OP's thoughts on the coolant system, what he is saying about air is true. If you heat up a cylinder that is full of cold water, the pressure will rise massively. If that cylinder has a bladder of air in it (at same pressure as surrounding water, and the volume of the bladder is the correct ratio to the water) then the pressure of the cylinder will be stable when it's heated as the expansion is taken up by the air pocket

[itchyfeet]

I think the term expansion tank is misleading
most modern cars have one expansion tank and yes it has air in and a max mark but no valve cap this system is different.

[discipleofsketch]

This could be a factor in my recent overheating issues (http://forum.club8090.co.uk/viewtopic.php?f=37&t=134059" onclick="window.open(this.href);return false;). On my tdi install there is no top-up tank behind the flap. The little hose that would usually feed to the top up tank feeds back into the system as per the golf it came out of, apparently.

When I picked up my van there was a small amount of air in the tank, and I drove it for 1500 miles with no loss of coolant or overheating issues. Before we set of for france I topped the tank right up to the top, and overheating started one hour later. When I start to overheat and pull over the expansion tank lets off a massive load of steam, and I end up with a virtually empty tank, so I let it cool down, bleed and refill (to the top).

Think I might try a test run with some air in the bottle tomorrow!

[AdrianC]

The standard system's unusual, in having the usual one bottle split between two, probably for packaging reasons, so that the filler is catflap-accessible, whilst keeping the actual pressure cap as high as possible relative to the rest of the engine bay for bleeding purposes.

Normally, the one bottle has some air space and a pressure cap to vent excess positive pressure to atmosphere, then suck in air to remove negative pressure.
With VW's original system, the pressurised bottle is intended to be full, and has the pressure cap. The pressure cap vents excess positive pressure to the unpressurised second bottle, and sucks in coolant to remove negative pressure.

Both systems work in fundamentally the same way, the only difference is where the air gap is - inside or outside the pressurised system. Both, obviously, require a fully working pressure cap.

So long as the system works as designed - it really doesn't matter whether it's a Golf one-bottle system in a T25 or in a Golf, or a T25 two-bottle system.

[dave friday]

I thought that the reason for the water being under pressure is to keep the water from boiling [at 1 bar water can get to 121deg c before it boils ]

[AdrianC]

I thought that the reason for the water being under pressure is to keep the water from boiling [at 1 bar water can get to 121deg c before it boils ]

Yep, applies to all pressurised cooling systems. It's why removing the pressure cap from a hot car will result in a boiling fountain.

[dave friday]

Add anti freeze/anti boil= 135deg c boiling point!!!