Bought my '98 A4 2.8 last April. Had a very mild winter here so I didn't notice the heater problem until January. It seems to have all functionality except it blows cool to luke-warm air. Took it to Dealer and was told I needed a new heater core ($1800). Having checked the procedure for replacing it I wasn't surprised. But instead of having the work done I thought I'd do my own diagnosis. I disconnected both hoses from core and installed two clear hoses. One to a bucket and the other to a garden hose. I turned on the water (low pressure) and lo and behold it flushed out the antifreeze then flowed clean water. I let 5 gallons pass. I reconnected the car hoses leaving the outlet hose out to the bleeder hole and refilled the expansion tank. I bled the engine bleeder valves too. I have re-bled the core but see no air bubbles before antifreeze flow.To this day I cannot get warm air to blow. The engine temp is normal so I know the thermostat is OK. Anything else I should check? I'm stumped.
(BTW, I decided the Dealer was scamming me so I won't return.)