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MG MGF Technical - Expansion tank cap

Could someone explain in what sense the coolant expansion screw cap can fail? And how you can tell it has failed? Does it stop screwing up tightly or is it something to do with the metal button inside the cap? Can it fail gradually - so that you might not realise there's a problem with it?

Thanks in advance

John
John

If the coolant is boiling in the bottle it has gone - they are so cheap I would change it if you think it is playing up, some service techs change them every year anyway.
Tony

Thanks, Tony

Mine hasn't gone - it was, as you say, changed at service. I just wanted to know what to watch out for.

I presume from what you say that it's best to keep an eye on the expansion tank water not only before starting out, but on completing a decent journey too.

John
John

John, the tank cap is designed to leak if the pressure gets to high in the tank, the idea being that if the cooling fails it is better to lose coolant from thye cap than busting the head gasket out of the side of the engine or fracturing the radiator or heater matrix.

Conversly the cap is designed to keep some pressu
Will Munns

re in to stop the coolant from boiling in the head (high pressure means a higher boiling point)

A cap can fail in either direction, but more lightly fails (or perhaps is only noticed failing) when it lets too much pressure out and the coolant boils and overflows.

Of course if it fails and keeps to much pressure in when another part of the system fails and you get a HGF would you bother looking for a 3rd failed component that seems to be fine?

Of course not, you'll assume you had an HGF because all F's have HGF, if you had a good mechanic they might notice the cooling fan was not coming on... but the cap?
Will Munns

Will, thanks for the explanation of what the cap is designed to do.

What I'm trying to figure out is what to check for (in a stationary car before or after a journey) to tell whether the cap is beginning to fail (which I think is why mine was changed). Can you tell from the cap itself that it needs changing?

I suppose if you see steam while driving (or loss of coolant later) you could suspect the cap, but it seems from what you say that the cap could fail and you might not know until its too late and some damage has been done.

John
John

yup, the test for a faulty cap is to put it on a pressure rig and pump it up till it hisses, there are some tales about checking if the rivet is off centre, or the cap has a white dot, I don't know how reliable the rivet is as a tell tale and the white dot is ancient history.

typically you wouldn't see steam unless things had already gone wrong, whether the cap was working or not.
Will Munns

This thread was discussed between 11/04/2007 and 12/04/2007

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