Coolant system question - real life test against theory?

Series cooling systems, and how i understand that they work -

Rad pressurises. At a set pressure, determined by the strength of the spring in the cap, the cap releases its rubber disc, and coolant escapes.

Here's a photo -



and another (ignore the chain) -




So, then the water is escaping...

It should then go down the tube to the expansion tank. See the tiny port on the right in this photo -




I know you can hardly see it - you can see the pipe comming off though.


That tube goes down to the base (not just the top) of the expansion tank.

Here's a photo -





So that's the heating cycle sorted.

Now when it cools....

See the center of the cap in the first photo? That's a valve - its sprung the other way. Here we go again -



So, the idea is that, when the system cools, that little central part springs the other way, and lets volume into the rad.

The IDEA is that the cap seals on that outer metal ring (?) and the fluid is then sucked back up from the expansion tank.


So....

If there was any air at all in the rad to start with, when it expanded, and the cap released, it should flow out, down to the base of the expansion tank, where, of course, it will bubble away. When the system cools, the fluid would be sucked back from the base of the expansion tank.

In this, the system would (sort of) be self bleeding. Assuming the expansion tank was topped up, the rad should never have any air in it at all.


Does yours? When you take of the cap on the rad (not the expansion tank) is it 100% full, no air at all?


See, what bothers me, is mine has air. I cant believe that the metal against metal seal on the outer rim of the cap ever works. Hence my question above as to if yours works.

Thank you for getting this far!

The fluid level on mine is normally about a half inch above the level of the perforated top plate thing inside the radiator so yes, mine has some air in the rad.

Some air in mine as well,you could change to the later type expansion tank which has the pressure cap fitted [plain cap on rad] which is normaly kept about half full.

Mine is full but the outer cap metal seal needs a rubber seal fitting I made mine from a thin piece of rubber possibly damp course material.
Parts list shows a seperate seal and the cap.
Pic of the rad cap I purchased from ebay eventually fitted a seal and now works correctly.

tottot said:

Some air in mine as well,you could change to the later type expansion tank which has the pressure cap fitted [plain cap on rad] which is normaly kept about half full.

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I dont suppose you have links and / or part numbers for both the cap and the expansion tank, do you?

Ive been considering just silver soldering a chunk of brass in the neck of my rad, and using a tdi expansion tank plumbed into the hoses, but your solution seems just as good but for less effort.

You may well be better fitting the smaller metal tank that was fitted to turbo diesel and earlyer 90/110's

The Op nearly did me in. Just what is the problem? Change the radiator cap, they cost about £2.80 plus vat n postage. To further respond the radiator level is what to monitor not the expansion tank.

I thought coolant only transferred to the expansion tank if there was a problem and the coolant in the rad was boiling or very near to it. As the temp of the coolant increases, the pressure in the cooling system increases and this raises the boiling point of the coolant. In the old days there was no expansion tank, just an overflow pipe. If the water boiled in ran out of this pipe and onto the floor. In those days it was common to check and top up the rad very regularly, I still do as a habit.

Col

John S said:

The Op nearly did me in. Just what is the problem? Change the radiator cap, they cost about £2.80 plus vat n postage. To further respond the radiator level is what to monitor not the expansion tank.

Click to expand...

I asked one thing. I asked it twice, just to be sure. I even put the question in bold.

...and yet, you still didnt manage to answer it.

My answer is no, when I take the rad cap off, it's not 100% full. The coolant is still just above the grid plate.

Col

Thank you (my previous response was not actually directed at you though!)

So far as i see it, if there is any air at all, it means that the outer ring of the cap is not sealing. Which is the same as mine.

If the water never gets sucked back into the system, there's not really any point in having the expansion bottle at all - it seems entirely useless.

I would agree with that. I think it is ok for some air to be in the top section of the rad but if some coolant is lost this will reduce the air pressure and that will cause some coolant to be sucked out of the expansion tank, it's a question of hydraulic balance. It's almost impossible to see if the theory works. In almost all the cars I've owned, the level in the rad and the expansion tank never seem to change.

Col

On a rad with an expansion tank there should only be a sealing cap on the top of the rad and the pressure cap should be on the expansion tank. That tank looks to me as if it is just an add on someone has used to catch and keep ejected coolant. It is not a proper expansion tank.

wammers said:

On a rad with an expansion tank there should only be a sealing cap on the top of the rad and the pressure cap should be on the expansion tank. That tank looks to me as if it is just an add on someone has used to catch and keep ejected coolant. It is not a proper expansion tank.

Click to expand...

it is what was fitted to later series 3s,cap is supposed to seal on the inner and outer ,the tank was vented but rad pulled coolant from the bottom of the tank

jamesmartin said:

it is what was fitted to later series 3s,cap is supposed to seal on the inner and outer ,the tank was vented but rad pulled coolant from the bottom of the tank

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Looks a bit of an abortion to me James, how does rad pull coolant from tank into itself past sprung pressure seal? Unless the cap has a one way valve built in?

wammers said:

Looks a bit of an abortion to me James, how does rad pull coolant from tank into itself past sprung pressure seal? Unless the cap has a one way valve built in?

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it should have a weaker one way valve to allow it to pull from the bottom of the tank as it cools
my dad had a couple of new series 3s and i dont remember it been particularly effective once the truck was year or so old,because of cap sealing ,though it did mean you allways had some coolant to top up in the bottle

wammers said:

Looks a bit of an abortion to me James, how does rad pull coolant from tank into itself past sprung pressure seal? Unless the cap has a one way valve built in?

Click to expand...

As per my initial post -

See the center of the cap in the first photo? That's a valve - its sprung the other way.

Click to expand...

The outer bit is sprung against the pressure of an expanding coolant system - the strength of the large spring controls the pressure of the system (or rather the maximum pressure of the system) but the inner circle is sprung the other way - so a cooling system can pull volume from the other side of the cap, though that small center bit.

julianf said:

As per my initial post -



The outer bit is sprung against the pressure of an expanding coolant system - the strength of the large spring controls the pressure of the system (or rather the maximum pressure of the system) but the inner circle is sprung the other way - so a cooling system can pull volume from the other side of the cap, though that small center bit.

Click to expand...

Right got it now. But still seems a bit of an abortion. Much better with a proper expansion tank with a sealed rad top and the pressure cap on the expansion tank. As used on later systems.

That's kind of my feeling. But, i guess, in theory, it would work - I'm just not sure about practice.

(which is the whole point of my thread! : )

Basically, my tdi conversion was (foolishly, in my opinion) set up like a series, even though it had a new rad made for it - id think it would be better to blank off the existing cap entirely, and fit a 300tdi expansion tank high up somewhere (ie a good way above the head).

Its just hassle though, so, if someone was to say "yes i have ##### and my series tank is spot on" then it would be easier than messing about with relocating things.

I have the same type of cap on my Series 3, just the one seal and metal to metal at the top of the neck. It does seem to work though, radiator is always full to the brim. I guess you need to make sure you dont leave to much of an air pocket in the top of the rad otherwise this area acts as your expansion tank. I did notice that this cap http://www.dingocroft.co.uk/acatalog/info-RTC3610.html looks like it may have an additional rubber seal for the top of the neck.