Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Water boiling in coolant tank

A few weeks ago the fan failed on my 2003 mgtf.
I've been told that this damaged the head gasket and a local garage used a block-sealer that apparently seems to have fixed the head gasket. I've flushed the whole system about 8 times to get out all the remaining oil and there isn't any mayo in the oil at all.

The water in the coolant tank is now totally clear, but when the car gets up to normal temp the water in the coolant tank boils and bubbles and doesn't seem to be circulating.

Any ideas on what could be causing the water to boil?

J Davies

Combustion gases passing the headgasket into the coolant.
Block sealer must not be used in modern engines as it does just that blocks the coolant passages.
You need a gas anylizer probe in the coolant tank to determine if their is any gas escaping.
Geoff F.
Geoff Farthing

If combustion gases are getting past the fire rings of the head gasket then it will be pressurising the cooling circuit and making it difficult for the pump to work properly, and putting large amounts of gas into the coolant. While there are cases where a sealant has blocked up a hole between the oil and coolant passages, once the seal on a fire ring is broken there's really no option but to unbolt and replace the gasket.

If combustion gases are getting into the coolant, a sniffer test will confirm - it detects hydrocarbons in the coolant, and they could only come from petrol combusting. Try a local MOT station, most have the means to perform that test.

Thanks for your reply. Do you think that the gasket needs replacing again? I'm not sure what sealant was used to seal the damaged head gasket. If the head gasket is changed will the problem be fixed?
J Davies

The engine is particularly sensitive to overheating, so some careful checking of the cylinder liners & cylinder head needs to be done before bolting back together with a new head gasket, and in most cases the head will need to be lightly skimmed to ensure it is flat. If the liner heights are good, no cracks or looseness, and the head isn't badly out of shape, then the likelihood is that a new gasket will put it back to 100%. But (isn't there always one...) the garage that put the sealant in didn't actually unbolt or replace anything, all they did was put a kind of glorified RadWeld into the coolant that, as Geoff says, blocks holes. Unfortunately, it can also block holes that your engine *really* doesn't want blocked, so I'm afraid there's a fair chance that you could have the gasket replaced only to find that there are more problems ahead due to that sealant. Perhaps rather than entrust repairs to a 'local' garage, the smart money is on taking it to a specialist. The cost may be a little higher, but getting away from the 'let's stick that in and see what happens' variety of garage is a Good Thing.

If you were to ask for recommendations of a specialist local to you on the SWMGF forum - - then the legend that is Dave Morris and his merry band will I'm sure be very happy to oblige.

If exhaust gases are bleeding past the fire rings, then the cylinder head itself becomes suspect. Chances are that at the very least, it'll need a skim. Worst case scenario, you'll need to replace the cylinder head.

If skimming the head, I'd take the time first to peen the land area on the head with an old valve to work harden/ obliterate casting voids in the alloy first, or you may find that the gasket replacement effects only a temporary cure...

See Tim's video on YouTube on how this is done:
Rob Bell

You might like to read the "trapped air" thread I started in April over on the "general" board. I'm afraid I've no idea whether some of the issues that were discussed are relevant to your problem or not. But as encouragement, I should say that after finally sorting out the problem, I drove my MG down to the Pyrenees and back without any troubles. Good luck
J Rudofsky

This thread was discussed between 02/06/2010 and 09/06/2010

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.