Coolant loss from overflow tank

[peter_g]

Hi,

I've been reading all the various VW forums all over the world to try and find help with this problem!

My passat is a 99 TDI 110hp now with 273,000km. It is loosing coolant from the overflow tank outlet when the engine is properly warm.

Here is what has been done so far:
- Camshaft belt, waterpump and thermostat changed.
- Coolant flushed and replaced with 2:3 mix of new G12++
- Pressure test on on cylinders, no leakage into coolant
- Radiator cap replaced

Other info:
- No leakage from any pipes, radiator or heater, the only place is the overflow tank outlet pipe
- Coolant remains clean, no sign of any oil contamination
- No sign of any coolant mixing with the oil
- When engine is started from cold, pressure builds slowly in cooling system as it warms (If head gasket was leaking, pressure would build up rapidly)
- Pressure in cooling system will remain overnight when engine is turned off
- Heater works normally
- I drive once a week to Helsinki, 230km one way, and during this trip I loose around .5 litre of coolant
- Outside temperature has varied between -5 and -20, so radiator remains fairly cool (light engine load, steady running at 100-120km/h with no major hills)
- Temperature gauge remains steady at 90 degrees

So does anybody have any ideas what further checks I can make? The car has already been looked at by a VW repair shop and they haven't found anything wrong.

Thanks in advance!

[peter_g]

Hello again,

Forgot to add I have scanned with VAG-COM and there are no fault codes...

[martin1810]

The head gasket has gone or head has crack. The combustion gases are leaking into the water ways under pressure and causing pressure in the cooling system. Feel the water hoses, when warm. They will feel more solid because of the pressure. A garage can use a block tester gas kit to test for exhaust gases in your cooant.

[snapdragon]

If you can get one at ECP cheaply, try a new reservoir in case the seal with the cap isn't very good at higher pressures. Also, as others have said, you can get a garage to take a sample of coolant and add a few drops of some stuff which goes blue if there is exhaust dissolved in the coolant others allow gases escaping from the reservoir to bubble through a sort of shisha that turns yellow in the presence of CO. On some cars it is possible to see exhaust bubbles returning in the flow back to the reservoir if you leave the cap off, try shining a light through it with the engine running and the cap off. Of course this is only relevant if you have a blown gasket to the cylinder and not the outside of the block/head.

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[peter_g]

Hi,
Thanks for your responses!

The VW garage says that the pressure test done on the cylinders via the glowplug hole has eliminated the possibility of head gasket leakage. Is this correct, would it be possible that leakage is only occurring when the engine is hot and under load?

For the overflow tank cap I've tried 3 caps, the original, swapping the identical part from my wife's Golf and a brand new one, and all gave the same result. There is a metal insert in the tank which the cap seals against and it appears smooth and gives a good seal.

For the meantime I've rigged up a piece of hose connected to the overflow tank outlet that leads to a bottle underneath the tank, the idea is to catch the fluid coming out the overflow and see if this quantity matches the quantity that is going missing.

Would it be possible that the radiator is blocked and the pressure from the water pump is forcing the coolant past the pressure value in the cap?

All your responses & comments most appreciated!

[martin1810]

The compression test done by the garage doesn't tell you anything about a possible leak. A cracked head or tiny gasket fault would not show up but enough gas could escape to put pressure in the water system. Get them to do a test with block testing fluid. Don't forget. Feel the hoses.

[peter_g]

Hi,

The saga continues - no evidence of exhaust gas has been found in the coolant. The pressure in the hoses doesn't increase rapidly when the engine is started from cold - although there is pressure still there even from the day before.
Another point is that if I leave the reservoir at around 2cm below the minimum level, then no coolant is being lost.
Replacing the head gasket has been quoted at min 850 euro, going upwards from there if more than just swapping the gasket is required.
Obviously I'd like conclusive proof that it is the head gasket before spending this sort of money!

Any suggestions or own experiences most welcome! Thanks!

[cazyp]

Are there any blocked engine breather holes?

[martin1810]

It is possible that the radiator core is partly blocked. All your tests suggest it's not the head even though the symptoms suggest it is. If the radiator is partly blocked the water pump is pushing water through narrow pipes and the pressure rises, this could push coolant out. I would swap the radiator as this is a cheap job compared to the head.

[adamss24]

Pretty shure you have a porous/cracked head wich will over-pressure the coolant... I had the same simptoms on a car i have sold to a friend, rebuilt the engine, new genuine gaskets, right coolant, etc... 8 months down the line he calles me that there are issues with the car. He had a slow leak from a seal behind the block, filled with cold water, then he took it to a garage for the mot who replaced the seal and topped up with wrong(g11) coolant, then 2 weeks later the oil started mixing with water. He blamed the head gasket i fitted and when i took the head off it had corroded/cracked between 2nd and third cylinders next to the water jacket... Be very carefull with the coolant you fill the engine with as its easy to crack the head when pouring cold water on a hot engine. The friends car would only overheat when given the beans and if driven at a steady 60 Mph would cool down. In your case, the crack may be between the water jaket and thus will expand when hot pressurising the system.