Coolant loss. Head Gasket leaking?

[Pottsy]

Hi I apologise in advance for long question. I have an Audi A4 1.9 TDi, for a while it has been loosing coolant. The cooling system holds pressure (for a couple of weeks if the car is not driven). I would expect the water to expand and pressurise the cooling system when it gets hot and to return to atmospheric pressure when it cools down. The system does indeed pressurise, but does not return to ambient pressure when cool. It also pressurises so much that water is forced out of the pressure relief cap when the car is driven hard. My guess is that my cylinder head gasket is leaking and the pressures involved in the cylinders force gasses into the cooling system, but because the pressure in the cooling system is limited by the relief valve, this pressure is not enough to force water back into the cylinder(s) after the engine is stopped. A ''nationwide'' chain of garages has twice tested my car with some gas analyser and say that there is no way my head gasket is leaking because the car runs ok. They say it's an airlock causing the water to be forced out, but if this were the case, when the engine was cool, the coolant would be below ambient pressure as some of the water has gone and it's not, it's under pressure. Is it possible for a diesel to run ok with a leaking head gasket or is it possible there is another cause for this water loss. My reason for asking this is my car is now out of warranty and I need to act quickly if they have misdiagnosed the problem.

[BigDogsWang]

If your head gaskett is leaking between a water jacket and a cylinder, the engine would 'suck' the water into the cylinder during the induction cycle. This would result in a cloud of white smoke coming from the exhaust. It could well be an airlock, and would be worth draining and re-filling the whole system.

[pauln90]

These symptoms used to point to a thermostat jamming in the closed position.Don't know what the set up is with an Audi though.

Also check that the radiator or heater matrix hasn't got a blockage.

[Pottsy]

Hi, Thanks for your answers. The main symptom is that the coolant system is still under pressure after the engine is cold. If it was an airlock or faulty thermostat, when some of the water escapes, the coolant should be below ambient pressure when cold, but it's not. The coolant is under pressure, even when cold and even when the pressure has been released prior to driving when topping the coolant up.

[AllYourBase]

I had a similar problem with my Bora TDI (same engine). I replaced the water pump about 3 times before the dealer eventually changed the head gasket (at their expense, following the water pump fiasco) which did resolve the problem, despite their lack of belief.