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MG Midget and Sprite Technical - engine cooling in 1971 Midget

The coolant is going from the radiator in my midget 1275 after a run and filling the expansion tank and under pressure. The pressure can last for several days. On removing the pressure cap, air bubbles come out. Anybody know what the problem might be? Thanks.
I.D PICKARD

Obviously coolant will pressurise the expansion tank as the engine heats up, but then will be drawn back into the main system as it cools down again. This should equalise pretty well once fully cooled.

If it doesn't, as you describe, it rather suggests a leaking head gasket. Are there any other symptoms of gasket failure - white smoke perhaps? Run the engine with the expansion cap off and look inside for a continuous bubbling of air.

Can you do a compression test?

Guy
Guy

THANKS - I'LL DO A COMPRESSION TEST AND RUN THE ENGINE WITH THE CAP OFF AS YOU SUGGEST, BUT I HAVEN'T SEEN ANY SMOKE, AND THE CYLINDER HEAD GASKET HAS BEEN REPLACED FAIRLY RECENTLY. NO SIGN OF WATER IN THE OIL OR OIL IN THE WATER.
I.D PICKARD

It may be nothing to worry about. I would expect the pressure to equalise again over a period of days, which is why I think it may be HGF. The worse diagnosis could be a cracked head or bore, but better not even go there at this stage!

You wouldn't necessarily get oil in water or water in oil, if the leak was just into the water jacket. Another clue is that after a few days standing one or more plugs might begin to show some rust deposits on the tips.
Guy

Do you have the crossflow or vertical flow radiator fitted?
Dave O'Neill 2

The header tank should pressurise so that is not unusual. Equally if you were to remove the cap whilst under pressure and running then you also would expect to hear bubbling etc.

But

Certainly after a day I would expect any residual pressure ought to have escaped so your suggestion that it exists for more time is exceptional. This if it is a fault I doubt would be caused by headgasket as that would give a route for excessive pressure to escape once the engine has stopped and cooled I would have thought?

I am wondering if the system is actually perfect and you are actually hearing a bit of suction when you remove the cap after a few days?

What level do you have in the expansion tank when you start?
Bob Turbo Midget England

You said the head gasket was replaced recently....

Did you retorque the head after the 1st hot run in /100 miles or so ?

If not.... might want to get on that right away

Prop
Prop

If there is still pressure in the system a day or so after then i would sugest the head gasket is sound.
Is there a loss of coolant ? Does the engine overheat on a run ?
If the head gasket was blown then this would be a route for any pressure in the system to escape.

If it all runs ok and does not overheat then i would suggest it is ok. The expansion tank will fill as the engine runs and pressure will build in there. This is normal.

Dont run the engine for to long with the cap off the expansion tank as this could lead to overheating as the water needs to be under pressure to cool effectively.

Make sure you have anti freeze in the cooling system as this not only stops water freezing but also helps to cool the engine more efficiently.
S Langston

This thread was discussed on 23/05/2011

MG Midget and Sprite Technical index

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