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My Polo runs fine but loses water very slowly (about half the expansion tank every 100 miles). I have checked the oil its fine, no leaks from hoses etc. When its cold and I unscrew the expabsion cap, when the seal breaks a rush of water surges as the tank is under pressure evn if its not been used since the day before. Any ideas thought the cap may be faulty as it should equalise when cold.

Thanks

sounds like your head gasket has seen better days get it checked with a chemical tester most good garages will have one
a chemical tester is like a test tube that goes on to you header tank you fill it with a blue chemical use the plunger with the kit to preasurise the system if the chemical turns from blue to yellow it means u got a leak through the head gasket between water cannel and one of the 4 cylinders

If the head gasket was leaking then surely the system wouldnt remain under pressure for over 24 hours

belive it or not but it does mine does the same and with workin on volkswagens every day of the week i just carnt be arsed to do mine

OK thanks for that, is it usually just the gasket or do the heads crack or warp. Is it an easy job to do.

My Mk3 CL engine used to do this, and a few people told me head gasket. It turned out to be a faulty seal in the expansion tank cap, and after replacing it all was fine - worth a try at first I guess?

you might be lucky and find its just the gasket i know the surface either side of the gasket (surface on the head and on the block) do carrode mainly on the block usally on the bottom left corner (when facing the engine) the job is a peace of p*** haines manual helps loads