Why you should have air in your expansion bottle.

[marlinowner]

To quote from the instruction manual that came with the van originally, page 94.

"If the coolant warning light comes on the expansion tank must also be checked and filled to brim if necessary."

[marlinowner]

The instruction manual supplied by VW that came with the van states on page 94 that the expansion tank should be filled to the brim.

However there is a minimum line on the expansion tank (looking at one now) which is at the same level as the mounting holes. Running with an air space in the expansion tank will work and will avoid the risk of over pressurising the system if the dalek cap fails to open or the pipe to the reservoir is blocked. However if everything is working correctly the "proper" way is better because the pressure regulation in the system will be better and air cannot enter the cooling system as long as the level in the reservoir is correct.

[Cruz]

There is no minimum line on my new expansion bottle but there is on my top up bottle.

Exansion tank. in engine bay. always full. no air.......you hope

top up tank. behind flap. keep between min and max

its not rocket science, even for me......these van dont go fasg enough for that

[mark]

the thing that you are missing is that the expansion tank is higher than the rad on the other models and has a max line indicated to allow any air in the system somewhere to go ie above the max line. there is no max line on the t25 because it needs to be at the brim, any air in the system is pushed into the top up tank, the reason for the one way darlek cap is to stop air returning to the sealed system. the expelled coolant returns to the system from the bottom of the top up tank.................which has a max line.

mark

[California Dreamin]

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.

Sometimes.... we can be blinded to 'the facts' by allowing ourselves to be completely 'unmoveable' in our own opinion.
Just be 'open' to the possibility that you could be wrong.....in fact not wrong with most of the 'theory' but incorrect in how you have applied it and the resultant conclusions.

Yes (roughly speaking) gases compress and liquids don't....correct
The 'give in the system' is the pressure release cap and if it is working correctly the system SHOULD pressurize to an acceptable 'design' level, above which, the release valve opens letting excess coolant/pressure into the top-up tank. Whilst I agree that a pocket of air would slow down/cushion this pressurising process I can't understand WHY you have convinced yourself that this is how it should be...when it isn't.
There is also no mention of the low coolant sensor in the centre of the tank recess....presumably the reason (if you say it has one) for a min mark. (allow this sensor to suspend in air and the red flashing warning light will come on (more proof) if it were needed, that this tank should be completely full with coolant.
Expansion bottles eventually succumbing to stress cracks...INDEED! this is a well known problem of using poor quality 'aftermarket' tanks...I would hazard a guess that yours haven't been genuine VW?
Interesting that people quote that pressurized cooling systems were designed to increase the boiling point..Of course this is true but the more important reasons have been omitted: so I will add a couple.
The raised boiling point means the same coolant can absorb more heat energy.
This in turn enables designers to make cooling systems more compact.
Making them compact means that the coolant warms up more quickly ie fast warm up cycle, because as we know, engines run very inefficiently when cold/or not running at their optimum temp (between 85 - 90C).

I notice that you were reluctant to enter into 'the debate' as you put it ...'air/no air in the expansion bottle' . What debate? In the case of this particular engine/vehicle, there is NO debate as VW state quite clearly that the tank should be filled TO THE BRIM.

Martin

[itchyfeet]

I have to say vw designed this system, does anybody think they are clever enough to teach the engineers at vw a thing or two about cooling systems?