Expansion tank works opposite to theory

[rebuilder86]

ok 99 VG transit 2.5 TDI 100PS. (not that it matters)
I have jsut replaced water pump and fan clutch and snapped the expansion tank outlet while tryig to undo it, so i ended up repairing the broken bit and tapped a new thread into the tank and sealed it up using brass fittings. all good now.
However, now that ive focused on it, it has got me obsessed with the stupidity of expansion tanks.
Why do manufactureres want air in the system? These tnaks are in many holdens fords, heaps ofmodern cars.
Every other new vehcile u buy with a standard cooling system with a recovery tank ( one which takes overpressure expelled coolant, stores it, and syphons it back when cold) is filled from factory, to the absolute brim of the radiator, so that there is no air to compress and cause pressure problems holding the coolant at pressure to stop it boiling.
So why the hell do we have these expansion tanks???? I can find no information about it on the net because every single source on the net that may come close to providing an sanswer, has no idea what an expansion tank is, and they all seem to think an expansion tank takes excess coolant and stores it. Even the biggest international parts suppliers, and every single "EXPERT" blog. they are jsut all completely wrong.

Further, my expansion tank does the opposite of what common theory suggests.
When my engine is hot, my tank coolant level is at its lowest. If i open the tank cap while its hot, it bubbles, hisses and the coolant level goss up to max, as if the air pressure in there was pushing it down and forcing the coolant into the hoses and makign them all expand.
Simialrly, if i go to the van in the mornign, when its cold, the coolant level is at its max.

Now everyone will tell u the "MAX" line is for hot!
Why is it so, that the entire world is wrong.

[dumper]

You stil have air in the system

[rebuilder86]

If theres stil air trapped, then it must be impossible to remove it?
Its been 500kms since the repair. Surely by now that air would ha e made its way to the top, that is, to the tank.

[rebuilder86]

Maybe stuck in the heater core?

[dumper]

Try squeezeing the bottom and top hose with engine running also the repair you have done to the tank when you squeeze the hose does water and air come out off it into it .or just maybe start of head gasket going

[Jim Archer]

The pressured sealed expansion tank saves having 2 points to top off the cooling system, plus acts as a high point that ensures that all the time there is coolant present in the tank, it is above the level of the engine cooling galleries/pump. And they act as a de-aerating vessel, after a 3mins, the coolant becomes a air/fluid emulsion due to the micro-bubbles that develop during rapid heat transfer being scrubbed away from their initiation points by coolant flow over the surfaces, a combination of high system pressure, plus easy venting to the tank keeps this to a minimum.

It also solves the problem of a shagged pressure cap vacuum valve that allows fluid out, but pulls air in.

Then we have the useful trait of air being compressable, where fluid isn't.

Jim

[Altransit]

When I were a wee lad, back in the sixties, my dad showed me how to top up his radiator in the days before header tanks. There was always a fill level in the radiator about 3/4 inch below the filler cap, and if you over filled it, leaving no air gap, the car would blow the excess out of the overflow as soon as the engine got hot.

When they started fitting header tanks, it was reckoned that it meant that all of the radiator could safely be used for cooling, and the header tank took care of the expansion air gap. It seems to work OK

[andz327]

The pressured sealed expansion tank saves having 2 points to top off the cooling system, plus acts as a high point that ensures that all the time there is coolant present in the tank, it is above the level of the engine cooling galleries/pump. And they act as a de-aerating vessel, after a 3mins, the coolant becomes a air/fluid emulsion due to the micro-bubbles that develop during rapid heat transfer being scrubbed away from their initiation points by coolant flow over the surfaces, a combination of high system pressure, plus easy venting to the tank keeps this to a minimum.

It also solves the problem of a shagged pressure cap vacuum valve that allows fluid out, but pulls air in.

Then we have the useful trait of air being compressable, where fluid isn't.

Jim

Nicely put Jim Archer

ANDZ

[rebuilder86]

Ok, so this emulsion of air and water is inevitable then? So im always going to see this air being generated and stored throughout the system due to the nature of heat exchange?
Now, this might be a bit left field but its something im certain ive experienced....
Im not convinced that air being compressible makes it a good thing in a cooling system.
I have a little toyota 4k motor in a jungle jeep, and i was having constant overheating issues driving through the mountains where coolant would be expelled out the pressure cap into the overflow, until i found a way to remove every single bit of air from the system.
I did this by warming up the engine with the radiator cap off so the thermostat had some flow, then revved it to 1800 which pushed the air to the top tank and i filled that air in that revving state with coolant which completely removed all air. After that, i never had to add coolant again and its been 2 years since. So in my mind, with my experience air is bad, and i suspect its because the pressure required to stop cavitation at the pump is unable to be achieved when there is room for compression in the system. The system is pressurized by deaign to increase the boiling point and also, to stop cavitation at high rpm.

[rebuilder86]

WThere was always a fill level in the radiator about 3/4 inch below the filler cap

I want to see one of these fil level thing suggesting to fil below the brim. any idea where i can see pic of one on the net?

[Jim Archer]

WThere was always a fill level in the radiator about 3/4 inch below the filler cap

I want to see one of these fil level thing suggesting to fil below the brim. any idea where i can see pic of one on the net?

This would be for systems without a overflow catch tank. My YZ250 dirtbike has a level mark embossed into the fill neck. Back when I used to mess about with old A Series Minis, the trick was to add a overflow tank so's the system will purge all air after a few runs.

The emmulsion of air/coolant is steam from the localised boiling of the coolant at hotspots, it dissolves back into solution in still regions of the system - one of the reasons for the relatively large volume of the end tanks on most radiators. The pressurised header gives any excess somewhere to purge into to cool.

Most automotive waterpumps have very simple 'water mover' impellors that can easily pull any gases out of the coolant in pretty short order once hot, and the pump outflow speeds at high rpms (pump impellor that is) will simply push those bubbles into the coolant jackets.

Also, being usually made with some sort of transparent section in them, they provide a quick visual* indication of coolant level. * not quite so easy with 200K miles showing, and having run cheap coolant for 15 years ......

Jim

P.S. ref the level dropping when hot.

Having read your posts properly, that drop would suggest a hose(s) swelling under pressure.

[rebuilder86]

Wow ok awesome, i sincerely appreciate u taking the time to explain that. I guess the biggest positive is being able to see coolant level.
Bit of a worry bout my hoses then, it wouldnt surprise me because everything on this van is poorly engineered haha.

[Jim Archer]

The only robust thing on a DI engined van is the engine. It'll still be going strong long after the rest of it is being made into new stuff in China.

Jim

[knobby1]

The only robust thing on a DI engined van is the engine. It'll still be going strong long after the rest of it is being made into new stuff in China.

Jim

LOL...

Lord Knobrot