Coolant Loss

[Big Will]

Searched with Google for a website which could give me a complete diagram of the cooling system for my VW Passat 1.9 TDI.
Can't seem to get information as I need urgently as my car needs frequent top ups of water in the header tank (around 1.5 litre every 200 miles) Engine runs perfectly. Does not appear to have any head gasket problem - - - heater blows warm air if requested. When low water sensors warn me that no water is left in header tank (every 150 -200 miles) I top up. Header tank is empty, when I begin to unscrew cap, but pressure in system half fills header tank when cap is fully unscrewed. ? ? ? I am unsure where the previous top up water has gone. ? ? ? Has it been blown out discharge outlet at the base of the header tank? and if so why? Or could it be that it has somehow entered the combustion chamber of the engine and been discharged out the exhaust? ? ?
Can you give me an idea as to what all the various components atached to the pipework/hoses of the cooling system actually do? Could it be that one or more of these components are faulty? Why is there so much pressure within the cooling system - which is released when I unscrew header tank cap?
Given that this car is over nine years old, it is uneconomic to suggest further tests at Volkswagen garage at £62.00 per hour + VAT.
I only seek advice. This will determine whether it can be repaired economically, or scrapped.
By the way: the cooling system on this car has been pressure tested three times, and passed every time.

[cazyp]

You can get the coolant tested (or buy the tester) at a garage for combustion gasses. It will tell you if the HG has gone. Check the oil cap foe Mayo like deposits (water in the oil.)

BUT my guess is you have the usual radiator corrosion problem. The lower corner(s) of the rad are likely to be rotted through and leaking and if so you'll need to replace the radiator. Check this first.

(What I should have said was 'this is a scrapper - i'll give you £100 for it :biglaugh: :D )

[hongkongstuey]

take the car for a drive, get it to 90 on gauge, get it home, fill header to the brim, keep topping up till it stops going down and look for bubbles, keep it running for about 10-15 mins, thats the test they did on mine and my head gasket had gone, i was losing water at the same rate, water was getting into the cylinders, had washed pistons 2+3 clean, temp was normal, power was normal, no leaking but when i undid the header cap it blew a lot of air out, if cap wasnt taken off, it would be very pressurised and blow it out the drain at the bottom of the header.

i agree with getting coolant tested also, mine had tiny specs of oil in it round the outside of the header.

mine cost 350 including skim of head, new thermostat, new water pump and labour.

[martin1810]

Simple test. Find an old plastic bottle and put the overflow pipe from the header tank into it. Go for a drive. If it fills up at all the head gasket is letting gas blow into the water system and pressurising it so pushing some water out.