A4 2.0 tdi b7 2008.... I have noticed when putting my heaters on, I get hot/Warm air for a few minutes then it turns cool, my temp is normal at 90. My scan pointed to heater circulation flap.......it was indeed knackered so I replaced that. Still the same so I replaced the thermostat thinking it might be stuck open and made no difference.....the top hose gets hot and the bottom is cold..... hoses on the bulkhead to the matrix, bottom is hot and top is half hot and when pried back there's a trickle of coolant. ...... expansion vessel cool to mildly warm with not a trickle in the return.....replaced timing belt 4 years ago and new SKF pump.....always used fresh coolant and its very clean....My gut feeling is the water pump, as the water doesn't circulate, but then i think is it the heater matrix!! Im stumped really, as I would expect a good pump like skf to last longer than 4 years. And if it's the matrix, then why isn't the rest of the engine circulating, (or does it have to circulate through the matrix to make a circuit) I would appreciate some experienced help thanks.