Hi, I have a 2002 A4 with a 3.0. Car runs great no issues at all but I got no heat. I assumed my heater core was clogged but I just back flushed it and that solved nothing. The inlet hose to the heater core gets hot the return hose is cold. So thinking the heater core was clogged made sense. When I back flushed it with a hose the water ran through it no issues. I thought I had it solved. I reconnected the hose and again, inlet hot, outlet cold. The blower blows nothing but cold air. Any idea? I am freezing my buns off! LOL

 

Most likely cause is it's air locked. With the engine running, slide the heater hose with the small white circle on it off the nipple until it clears the end of the nipple. Wait until a solid stream of coolant comes out, then put the hose back on. It also helps to then rev the engine and to hold it at 2,500-3,000 rpm, varying the rpm (after the hose is clamped back on of course) Give it some quick "blips" of the throttle. Make sure your coolant bottle is full. If it was air locked, you should get heat.

The question is, how did it get air locked? Check carefully for coolant leaks. Look especially for dried pinkish traces more than an outright leak. Small leaks can seal up when the engine warms up, but as it cools and the coolant contracts, air can get sucked into the system. Of course, a headgasket starting to go will do it too but usually there's other symptoms besides just no heat (but not always).

 

Hi,
I did what you said and managed to get some air out but really not change. Here is what I have learned. There are times when there is no coolant coming out the return side nipple even with the hose fully off (just a small dribble). Then there are time I have pressure on the outlet side and then have of course heat! The radiator hose seems to get soft then times it seem to be under pressure, Now it is cold here right now about 34 deg. so that may have an impact I do not know. Let me also stress the car other than this runs GREAT and reaches temp on the gauge every time to right dead center in the gauge.
I am at a loss? Could a thermostat be causing this all? My feeling is if it was a water pump the car would over heat which it's not at all.

 

I think you still have air in the system - I had the same issues. The instructions you were given are correct however, if you still have no or little flow on the outlet, try squeezing all your coolant hoses with your hand (while you have the heater outlet hose open), this will force the air to move to the heater hoses. Then reconnect and drive at high revs, then open the outlet hose again and again bleed more air - I had to do this a number of times and it eventually cleared.
Also, you say you flushed the heater core? You need to make sure the flow was the same with a garden hose in both directions i.e. garden hose in inlet then garden in hose in outlet - if its slightly blocked it can cause no heat. Keep flushing in both directions and you will loosen the debris that IS in there - honest.
Here are the details of my history on this problem fyi: I originally had no heat, so I had Audi replace the heater core (but I kept the original core). All was good for 15 months but then new core blocked, so guess what? I cleaned out my old core on the bench and got loads of debris out of it. I refitted the original core, it did take me a while to get all the air out however, I did all the steps above and my heater is awesome. These cars especially my 1.8T are a pain to bleed...