My A4 is losing coolant and I'm not sure why. I've searched this and the other forums exhaustively and read plenty of similar stories, but none are quite the same and it seems that people seldom update their threads once they've got sorted. So, I'm asking for help!
Car is a '98 1.9TDi quattro Sport saloon with 185k miles, AFN engine. I'm doing between 350 and 450 miles a week and I'm having to top up the coolant once a fortnight or so; it's usually one or two centimetres below the 'min' level so the loss isn't great but it's consistent. It appears to be venting from the bottom of the expansion tank as there are telltale splash marks below. When the engine has cooled there is still pressure in the tank and when I release it the coolant level rises slightly. The car is running perfectly normally, there's no white smoke on startup and there are no signs of contamination, i.e. no sludge in the tank and no mayo on the filler cap or dip stick. The radiator itself was replaced last August after it began leaking through corrosion.
The independent I use took a look at the car and this is what they found (taken from the invoice):
As well as the thermostat they replaced the tank cap which made no difference either. I'm not convinced they really know what the problem is so I thought I should run these symptoms past the experts on here before I shell out on a head gasket replacement.CHECK FOR COOLANT LEAK, NO EXTERNAL LEAKS FOUND BUT COOLANT BLOWN OUT OF HEADER TANK AFTER ROADTEST. BOTTOM RADIATOR HOSE COLD, SUSPECT THERMOSTAT. REPLACE THERMOSTAT & RUN UP TO TEMP, OK. ROADTEST AGAIN COOLING SYSTEM STILL PRESSURISING UP UNDER LOAD. SUSPECT HEADGASKET/INTERNAL LEAK.
Just to confuse matters further, shortly after I started losing coolant I started finding it difficult to demist my windows. After researching this I discovered that a leaking heater matrix is the likely culprit, which could itself be brought on by pressure and/or air locks in the cooling system. My carpets are bone dry and there's no sign of there ever having been any water in the plenum chamber so I don't think the drain holes are blocked.
I'm not particularly mechanically minded when it comes to the business end of things but I'm willing to learn, so any opinions and/or suggestions from the experts on here would be most welcome. I assume I need to do a compression test next?
Thanks in advance.