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MG MGF Technical - Coolant tank bubbling

I currently own a 2002 TF120 with 75,000klms. The head gasket has been replaced for the second time in January this year along with the head shaved. crack tested and refitted with new head bolts. All was fine for 7000klms and now I have noticed that the coolant level is delining on a daily basis. I have been over everything externally looking for a leak from the tank, pipework and besides a bit of moisture around the waterpump area there seems to be leak free. The engine oil is clean with no water contamination. Today I have noticed with the motor running at normal temperature, there is bubbling inside the plastic coolant tank which makes me think that the water circuit is being pressurised by leakage from a possible HG leak ie from combustion chamber to cooling circuit. Am I on the right track or is this normal to be bubbling when the engine is running? Also I am getting a strong hot coolant when the car is stopped. Please help.
S J Dellaca

When the pressure in the system drops so does the boiling point. The first thing that I would change is the header tank cap - these are pretty cheap. If your old one has gone then the system will not keep the correct pressure and the fluid could be boiling with some coolant leaking out of the cap. That might be what you are smelling. If you can't buy a new one right away, a simple test is to wrap some paper kitchen towel round the neck of the tank and see if it's wet after a run.
David Clelland

It is normall for the water to circulate through the tank with the engine running is this what your seeing?
Dave

Dave and Dave, thanks for this information. The bubbling I am seeing could be the coolant circulating through the tank. I understood that the tank was only for expansion and contraction purposes. I will try the bag around the cap in the meanwhile, and purchase a new cap tomorrow. Although it does not seem to be bleeding coolant out of the tank it may be overpressurising the coolant system and forcing out the water pump seals. I am willing to try anything.
S J Dellaca

I have today tried the new tank cap and now all seems well. I have taken the car for a 200klm run including some 70klms of freeway at 110kph and the rest at normal town running. Stopped the car several times and let it cool down and there was no coolant loss. Thanks Dave and Dave for your guiding thoughts as the further I worked my way through the archive threads, the more I feared the worst. Thanks.
S J Dellaca

Glad it seems to be sorted. Just shows you that you should always start off with the simplest and cheapest solutions :-)
David Clelland

This thread was discussed between 14/08/2008 and 17/08/2008

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