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MG MGF Technical - Expansion tank MGF

Hi Guys ...
Just had HGF and new head gasket fitted by local mg specialist.
To avoid becoming paranoid about loosing water and it happening again I have fitted the new style expansion tank with level sensor.
However, when I start the car, water circulates into the expansion tank from the small top hose ... it doesn't fill the expansion tank up ... I guess its being taken out into the rest of the cooling system through the larger hose in the base at the same rate its coming in.

Is this what should happen ...

The reason I ask is that a collegue of mine also has an F and his does not circulate on start up.

Maybe there is a document that explains how the cooling system works under various conditions.

Would appreciate your expert opinion.

Richard X178 CPA 1.8i.
RL Lane

Mine works like you describe.
Mike Howlett

<<water circulates into the expansion tank from the small top hose ... it doesn't fill the expansion tank up ... I guess its being taken out into the rest of the cooling system through the larger hose in the base at the same rate its coming in>>

Exactly what should happen, the pump works all the time the engine is running and constantly circulates the coolant.
Graham Martin

Many thanks for the response ...I guess with the thermostat closed (when the engine is cold) water takes the route of shortest resistance which gives some flow through the expansion tank.

Richard ...
RL Lane

Be interesting know what's wrong with your colleague's system.......
Graham Martin

"Any excess of coolant created by heat expansion is returned to the expansion tank via the bleed line. A jiggle valve fitted at the start of the bleed line, prevents pressure build-up in the expansion tank by regulating the amount of returning coolant."

surely this is nonsense...why does expanding coolant need the bleed line at all? surely it could expand back the way it came in (if that makes sense) & anyway pressure does build up in the expansion tank (because its connected to a system which pressurises), i fail to see what the jiggle valve will do to assist here

Is circulation through the header tank an unintended consequence of something - i see no reason it "should" happen and at least 1 reason it shouldn't!

The jiggle valve is located in a high point in the bypass hose that runs back to the thermostat control bulb and water pump. This high point is in a zone where miniscule air bubbles in the coolant can gather and then be vented along with the small volume of coolant being directed to the expansion tank via the jiggle valve. This is a design charateristic of the MGF's cooling system.


A J Duthie

If that is its role why does the manual not say so ( I have a vision of rover engineers chuckling to themselves as they wrote the manual - surely the pressure thing is bol***ks)
it is the role of the Jiggle valve behind the inlet manifold stub & when working properly its open for air, closed for coolant...
Is circulating coolant through the expansion a sign that the other jiggle was not designed properly/ not working properly?

also I would expect miniscule bubbles to be swept along the bypass flow and gather in the engine (for venting by the former valve)

I think open and closed are too black and white for this conversation.

The jiggle vale is "more restrictive" to coolant.

I think that the majority of the flow does not come from the jiggle valve on the cambelt end of the engine, but from the bleed 'turret' at the other end. Having coolant flow in this circuit is far preferable to allowing anything in the way of bubbles into the radiator because when they get there there is no way out for them without spanners.
Will Munns

Is there a suggestion here that the only time water should flow into the expansion tank is from a hot engine ... and the tank is somewhere for hot water to expand into and therefore not flow constantly ...?

Just as an observation, the water level rises from cold by about 10mm after a good run, then falls back over night when the engine is cold ... I am hoping that after a few 100 miles of post HGF driving any small pockets of air will disipate and the level will stablise.

RL Lane

Water expands and contracts with temp, so the change in levels is entirely reasonable.

You are right, that is the suggestion, but by no means the truth.

Heating liquid causes it to be less able to hold gasses. In an engine the head is the heating point and the water can collect gases when it has cooled within the header tank. It is important to collect the gasses boiled off during normal running, not just the air bubbles in the system from refills.

* for examples of this see your kettle before it starts boiling there is plenty of gas let out.
Will Munns

So to my original question ...
Should water be flowing through the expansion tank when the engine is cold?

"drivers check your engines" just a play on words.
RL Lane

yes, this is totally normal
Will Munns

This thread was discussed between 12/03/2007 and 15/03/2007

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