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MG Midget and Sprite Technical - Expansion tank filling up

I have just been on a week long tour of the Cotswolds in the rain and the midget behaved impecabbly,apart from acquiring a flat tyre !
However the expansion tank keeps filling right to the brim and occasionally expelling water when it gets too hot.I have changed the cap,have a new radiator,heater matrix and all hoses.My thinking is that there is an airlock in the heater that when it expands is forcing the water into the expansion tank.I have tried filling the system from the heater top pipe.I know that /I might be barking up the wrong tree and 'any thoughts/solutions would be welcomed


M Teal

Sure you have the right cap? There are two lengths. If you have fitted the short one then it won't hold the pressure it is supposed to. And IIRC there is some confusion with mixed up part numbers with certain suppliers so you cannot rely on them to supply the right one.

The other possibility would be the beginnings of a head gasket failure, pressurising the cooling system. This would show up by gas bubbles blowing out and visible in the expansion tank if you check it with the cap off and the engine running.

Glad you enjoyed your holiday!
Guy Weller

What Guy said..

Head gasket failure into cooling system, or as we have found recently, on our Úlan , caps not sealing. There are quite a few caps, all slightly different lengths. Even some long 20mm ones did not seal very well, make sure whatever cap compresses the spring a little. Also make sure the two surfaces are flat and clean...
d cusworth


what year of car?

did you check the heater tap was open when refilling?

what temp reading on the gauge when engine warmed up and car stationary
Nigel Atkins

doh, expansion tank so probably a X-flow rad with expansion tank so standard size 15lbs pressure cap

as said check seal and that it's sealing
Nigel Atkins

The operation is that as the liquid expends from heat, it goes into the tank, and when it cools it is sucked back.
If there is any vacuum leak (which often does NOT show a coolant leak), the fluid will remain in the tank as it cools. Then the process repeats, so it is a "slow pump", moving fluid to the tank but never sucking it back. So the tank overflows, but the system proper is ever lower on coolant.

Vacuum leaks are usually at hose connections, the heater tap packing, or pinholes in the rad/heater.
Another contributing cause is the check valves built into the cap(s) being stuck. They should be very free to vacuum. Commonly, small leaks result in the use of sealers, which stop the coolant leak but not the vacuum leak, and also gum up the cap valves.

FR Millmore

It is a X flow radiator and when warm runs at N.It has a kenlow fan fitted and no engine fan.I fitted a new head gasket about 800 miles ago=payen and changed the studs but I hjave not retorqued down the head so that might be the problem ?

M Teal

I would:

Check the length of the sprung length of the cap, against the length to the internal lip of the expansion tank. It obviously needs to be longer in order to exert the required pressure on the seal

Then run the car up to temp with the rad cap off, and then look for bubbling.

As it had a new HG 800 miles ago, it wouldn't do any harm to retorque it now anyway. Then reset the tappets.
Guy Weller

Just a point or 2

Firstly if the car has been touring all week without too much issue then there is not too much amiss

You must remember the expansion tank does what it says on the tin, it holds the water that has EXPANDED in the cooling system. Thus by its very design it is supposed to fill during engine operation and as described by FRM it then empties a bit when engine cools.

If the tank is filled too much then it will blow water out when hot, but this ought to find its own level unless it is refilled again by too much.

Is it blowing water out on each journey even though you are not refilling?

I have seen the same as FRM where by a tiny leak can cause the water not to be sucked back into the system as it cools which then can introduce an airlock and cause water spillage.
Robert (Bob) Midget Turbo

Thanks to you all.I will have a tinker at the weekend and let you know how I get on


M Teal

This thread was discussed on 28/06/2012

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