No heat inside cabin A4 B7 2.0 tdi 2005

Not much flow out of that pipe is there? Is it the return pipe from the heater or the one going to the heater?

Yes, the matrix should be hot all the time. You could remove the pipe going into the matrix and if the flow of coolant is lot more then the matrix is bunged up and needs replacing. No good trying to blast the blockage out, I've found it never works.

Airlocked possibly - that top hose into the heater has a very small hole in it. You pull the hose back so that it's still on the pipe, but the hole is just clear of the end. Engine running and heater on hot. The other thing to check is the top pipe running over the engine - assuming your's is the same as mine (2.0 petrol). There's a bleed nipple there (hex headed).

Ahh - no coolant rail over the top of that engine. When I was watching the job being done, when the pipe with the air-bleed hole was pulled back, there was some air bubbling out and then coolant flowed. I don't recall it being much more than a little bit. Its a very small hole after all.

If all is well with the system, then there should be a continuous flow of coolant - assuming it's up to temp and the 'stat is open that is. Haynes notes that when the engine is cold, coolant passes round the block and head, then through the heater, and returned via the block to the coolant pump.When up to temp, then the 'stat opens and cold coolant passes from the rad via the bottom hose and hot coolant from the engine flows thro the top hose to the rad.

Incidentally, while all this refilling / bleeding mentioned above is being done, the expansion tank should be raised (Haynes mention 100 mm) - it's only held in place by one screw and can be easily lifted out of its slot. This just to give a head of coolant to allow gravity to do what it does best.

So - and no doubt others more expert than I will comment - if you've no heat in the car, then clearly coolant isn't getting to the heater matrix. Assuming all the control flaps etc are working - which you say has been checked.

At this stage, I think I'd be trying to find a decent garage who knew what they were doing. I'd have said that a good mechanic should be able to tell you whether the pump IS working ok, and should be able to quickly check for air-locks in the system. I can only add that in my own case, it turned out that the problems I was dealing with included having to change both the expansion tank and the cap. The caps have a sealing ring which is known to fail, and the tanks are also known to fail. Fresh coolant has a nasty habit of finding any weakness in the system! IF there are problems there, then the system can't work optimally as it's not holding pressure.

I have had this problem (no hot air and going cold on a run etc..) ...I have changed thermostat and temp sensor to no avail.....I removed both rubber hoses on bulkhead for matrix and back flushed etc and the water flowed through.. I found a post that said it was the matrix radiator (heater core ) inside the car, although the water will flow through it doesn't run into the little pipes in the matrix and bypasses these!..and its blocked inside!. I took the matrix out (easy job) and boiled the kettle and flushed it through with boiling hot water and radiator flush and shook the hell out of it ..( left soak for half hour) ..I then returned the matrix and when warm pulled the top rubber hose back around an inch on the bulkhead until the little pin hole squirts all air out ..run the car tonight and it is boiling hot again!!! Yes boiling hot again!!... To take the matrix out took around an hour ..and you can find how to do this on YouTube ..type Audi a4 b7 2.0 TDI matrix radiator removal.. There is an Irish lad on there showing you what to do!!..basically there is the shelf under the steering column comes off (2 bolt scres)and then a plastic cover in driver foot well (left) and then u see the matrix with the 2 pipes ..undo the 2 Allen bolts and wiggle the matrix and pipes until they come out but be really careful as they are alluminium and damage easy..DO NOT use a screwdriver to to pry it out!!! ..once pipes disconnected the matrix will slide into the drivers foot well... Tight fit though!.. I cable tied the clutch pedal to the steering column to keep out the way...I really hope this helps lots and Lots of people and save spending thousands on things other parts ..regards

Changed the matrix heater core today for a brand new one from europarts (80 quid) fitted it myself and heater blowing hot like new ....appears a common fault them clogging up ..