to cut a long story short I experienced this problem with my Audi A3 2004 140bhp 2.0 tdi as soon as I bought it privately in November 2010.It has 110K mileage on the clock.
First I experienced that every now and then the coolant warning light will come up and in fact on checking coolant level would be low.so I have to fill the coolant level every 100 miles on average I travel.So I started reading it on this forum and found the porous
head problem in these Audis and so I believed that was the cause of the low coolant levels.or some cracks in head or cylinder block.But i never found any water mixing in the oil or any physical leak on the coolant system.Also the car wouldn't start in winter months not even rotate the engine with starter motor.I had to replace the starter motor 3 times and also the battery once.It was as the engine was stuck but sometimes after trying few times starter motor would start the engine with no problems at all.
Someone advised me to use a liquid called "steel " I forgot the exact name but I bought that from ebay and poured it in the radiator.It didn't cure the problem.
Then in summer days the problem of low coolant level and steamed up windows gone miraculously and I drove about 2500 miles in summer months and not had to fill the radiator even one.I thought may be the liquid in the radiator fixed the problem but took couple of moths to actually fix the problem.But unfortunately as today again the coolant level warning came up and I had to fill the radiator up with coolant.Also I noticed that windows were starting to steam up in the last week or so.I always blamed it on the chilly weather.
I am so stressed with this problem as I cant find help anywhere.Took the car to an independent garage previously who couldn't tell me the cause and said he fixed the problem but he didn't.
Please help me with your experience and knowledge and guide me in the right direction.Many thanks to any comments to post