A4 B7 2.0L TDi Cooling system queries

[bobfrankish]

Hi Everyone.
Please bear with me, I am trying to learn as much as I can about the cooling system on this car, scouring YouTube, all the forums etc,. but not really getting anywhere. Quick recap, car has 116K, has been superbly reliable for last 2 years. I have receipt for water pump and timing belt change at 77K I think. Suddenly began to overheat on motorway and boiled some coolant out. Topped up coolant, car is perfect, temp. gauge normal on short trips and left idling for any amount of time, but there is now no heat in cabin, and bottom radiator hose is always cold. Yesterday I flushed out the heater core, replaced the thermostat, flushed out as much of the old coolant as possible. I filled up with new coolant, did 2 pressure tests, engine cold and hot, no pressure drop at all after leaving 30 mins., no signs of external coolant loss anywhere. I have bled the system 3 times using the heater hose hole and revving to 2000 for 3 mins. method. I took the car for a long run, closely monitoring the temp. gauge., I let it creep up to 3/4, stopped the car, turned off the engine and checked. Bottom heater hose was stone cold, as was the radiator itself. There was still no heat at all in the cabin. It appears that there is little or no water flow through the radiator. Any help or ideas would be much appreciated. What would anyone do or check next? Thanks for reading.

[gareth23]

Sounds like an air lock somewhere, can you drain and refill the system using a vacuum coolant filler?

Also is it a genuine stat that you've used? The one I fitted to my B7 was a Euro car parts one (cant remember the brand) and it was garbage as it opened far too early and was very slow to close so car never warmed up . Changed it for a genuine one and that was perfect.

[bobfrankish]

Hi Gareth, thanks for the reply. The stat I fitted is also an ECP one (Circoli) but I used to work for ECP and they are generally very good stats, almost OEM quality. also the fact that the symptom is identical before and after fitment, I'm not going to worry about the stat at the moment. Air lock? I thought the same, but you have to think, car is perfect for 2 years + then one day it overheats only when driven on the motorway. I was wondering about a blocked radiator, but in over 50 years of tinkering with old cars, some with the filthiest cooling systems ever, never been flushed etc., I have never had to change a blocked radiator. Air locks don't happen by themselves. I am leaning towards something that has failed, and the only thing I can think of is the water pump, regardless of the fact it has been changed. What I could do with knowing is whether there is a way to confirm it without going to the trouble of changing it?

[gareth23]

No probs

If you're happy that the stat is good then it does suggest the pump, years ago a mate had a BMW with exactly the same symptoms and in his case the pump impeller had fallen apart. I'm not sure how you can check it without a lot of work.