The coolant tank on my 2.3l duratec (Ford Transit) just burst while it was at the garage for an MOT. The mechanic said it could have been caused by a blown head gasket, but didn't explain any further. Why would a blown head gasket cause the coolant tank to burst? I'm assuming an increase in pressure, but how is the gasket linked to coolant pressure?
When an engine goes through it's four cycles, one of those cycles is combustion. Combustion creates a lot of pressure, which forces the piston down in the cylinder. Normally, the head gasket helps contain the combustion by creating a seal between the head and the block. If the head gasket is blown and is allowing the combustion to enter into the coolant passages, the cooling system can become charged beyond its normal limits. When this happens, something has to give ... it could very well be an expansion tank which takes the brunt of it. Normally, the radiator cap would allow excess pressure to bleed off, but if the pressurization happened quick enough, it surely could cause it to fail.
Personally, before you go getting the head gasket replaced, you need to check to ensure that's the issue. You can do a check in the cooling system to see if exhaust gasses are present. There should be absolutely no hydrocarbons present in the cooling system. If there is, you have combustion gasses entering the cooling system, which means you have a head gasket leak. In most cases you'll also find there is coolant getting "burned" and exiting the tailpipe. (Mind you, coolant doesn't burn, but you'll know it by the distinct sweet odor, in most cases, exiting the tailpipe.)