BX19D cooling system - thoughts?

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white exec
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BX19D cooling system - thoughts?

Post by white exec »

from Haynes #1379
from Haynes #1379
from Haynes #1379
from Haynes #1379
.
The BX non-turbo diesel's cooling system is pretty cleverly designed, and incorporates its expansion tank into the radiator, by way of an enlarged side tank, so there is no separate header/expansion tank like the XUD Visa, BX turbo diesel, or the GTi's.

If the coolant level is maintained properly, the head and head gasket seems to survive these days* (despite there not being much warning of over-heating as standard, until too late!), and the system does have one clever feature: there is a hose linking the very top of the water outlet (thermostat housing) to the radiator bottom. This small hose is under pumped flow, and would remove any air trapped in the outlet, and push it down into the radiator, where it will bleed upwards. This is clever, as it auto-bleeds a potential air pocket around the 'stat and temperature sensors, as well as protecting the head from potential hot-spots. Brownie points for that.

What I find odd though, is that the main radiator seems to be designed never to be completely full of coolant.

Early models had a level tube, with (admittedly Haynes) instructions to treat the top of the 'threaded' section as the Max level (when cold), and the bottom of the tube as Minimum.

Although the tube was long ago deleted, SFAIK the level recommendations were not revised (or were they?), and so the cold coolant level could well end up a good way down from the top row of the radiator, which seems less than ideal. I would have thought that radiators should always be full, to maximise cooling.

The amount of expansion of the 7 litres of coolant in the BXD would be around 130cc (in going from 10 to 90°C), so the side tank would certainly cope, but it's interesting that this arrangement was not thought suitable for TD or GTi, which both got separate expansion/header tanks.

* Although they didn't back in the 80s-'90s, when XUD heads were warping regularly, leaving HGs leaking.
Chris

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Re: BX19D cooling system - thoughts?

Post by Kaapelimies »

The radiator is always full. The expansion tank is separated from the matrix end, it only has a hole in the bottom to connect. How it works, must have something to do with physics, as the expansion tank doesn't have to be full..
BX 4x4 estate, several BX Van's and some more normal estates in Finland.

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white exec
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Re: BX19D cooling system - thoughts?

Post by white exec »

Would like to know more about that.
Does anyone have any drawings, or has ever cut one open?
If there's a hole linking the two, the level would be common, surely?
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Re: BX19D cooling system - thoughts?

Post by Kaapelimies »

You can check that yourself by opening the bleed valve from the other end of the radiator. The water will come out. The bleeding of the system when emptied and refilled must be done by the factory procedure, It's described in Haynes.
BX 4x4 estate, several BX Van's and some more normal estates in Finland.

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white exec
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x 54

Re: BX19D cooling system - thoughts?

Post by white exec »

19D radiator.jpg
Have done that, and always do follow the Citroen filling and bleeding procedure when replacing coolant. (It's even more involved on XM.)

When filling the system from empty, the levels in the expansion tank and the main radiator core appear to rise together, and only when just about full does any liquid appear at the top edge bleed point. That suggests that expansion tank and matrix are joined (not unexpected) and that they share a common level.

What I don't understand is how a much-reduced level in the expansion tank would not result in a reduced level in the radiator core.
Chris

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Re: BX19D cooling system - thoughts?

Post by Kaapelimies »

How would the air get in there, since there's always water in the expansion tank? I'm sure there would be no vacuum, as then it would suck the water from the expansion tank and ...well, fill the matrix.
BX 4x4 estate, several BX Van's and some more normal estates in Finland.

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white exec
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x 54

Re: BX19D cooling system - thoughts?

Post by white exec »

OK. That's making sense.
If the linking hole is at the bottom, and then the system is filled - so no more air emerges from any of the bleed points, and then they are all closed - the level in the radiator matrix should not fall - except for any dissolved air coming out of solution, or a HG leak.
The level in the header tank section is thus not critical, so long as it's adequate to prevent escape from the matrix.
It also explains why the low-level (float) warning switch is placed so low in the expansion tank, as it has nothing directly to do with the matrix level, only a warning that it could be jeopardised.

You've explained something important, so big thanks for that!
Chris