Audi A6 2002 1.9TDI 96KW 284000 miles/455000 kilometers

Hello all! My car has been leaking coolant for years now, and might have been doing this for 100.000 miles now (160.000 kilometers), I just always topped it off with water or a mix with some antifreeze.

I would really like to fix this finally, I recently lost heat as well and it starts to overheat when I need power. It still is reasonable when I drive calmly on the road, but still not full heat and still with me needing to top the coolant off after every ride. Also, hills seems to hurt my cart immediately.

The time has come to change the timing belt, and water pump (just passed 60.000 miles/90.000 kilometers, hope it holds a little longer). I might found a cheaper mechanic that might do the head gasket for a reasonable price, but I really want to make sure it is the head gasket, since I don’t want to miss and keep fixing the car. I was planning to maybe just get the belt done and keep driving, or maybe throw in some coolant flush and coolant seal to clean the car and win some time for a while, but I found a reasonably priced mechanic for the head gasket (I would not be able to pay the second best priced mechanic, and I’ve been looking around for years).

So I was wondering if some people might be able to pinpoint the problem or at least rule out what it is not?

So far, I’ve been suspecting the following:
-head gasket -radiator -water pump -cap -hoses -EGR -cylinder -thermostat

I haven’t noticed any white smoke in the exhaust, don’t have oil in my coolant as far as I can tell, no coolant in oil, the system seems to over pressurize, but other than that I really haven’t got a clue, Audi tried a pressure test but they said it was inconclusive, though dad might have noticed some smoke coming from around the engine when starting the car yesterday, and I sometimes hear water “flushing” when shifting into lower gear or accelerating rapidly. The car has high mileage, and the leak has been there for a very long time.

Many thanks in advance!