fat al

Updating because there was a new reply. On my car the heater core was full of some sort of silty scale. Flushed it with fresh water, then radiator cleaner and it worked much better. I might have had to do this more than once, I think the problem recurred (it was a few months ago and I don't remember perfectly). Heater output is comfortable, though possibly not as hot as it should be - so it's possible the core isn't fully clean and needs replacing to work perfectly. It's unclear what caused (or is causing) the scale, presumably something before my ownership of the car. The whole system has been flushed and refilled with the right coolant since I bought the car so hopefully there isn't more scale developing.

Thanks for the advice. There is not a valve with the position/function in this car. With the 2.0 TDI it is apparently only part of the system if there is an auxiliary diesel heater fitted. The change of pump did not fix the problem. Can't help feeling that the mechanic should have seen that the old one was functioning fine from the diagnostics - which (having got the car back and been able to check for myself) it turns out do show data for it... Next step will be to flush the heater core and, if that does not work, to drain all the coolant, flush the whole system and replace the heater core. Feeling the hoses shows there is good hot supply to the heater core but a significantly cooler return which makes me suspect poor coolant flow through the heater core.

I don't think they are totally confident that the pump is definitely the issue, but they think it is the most likely candidate. They've offered to replace it for cost of parts and only charge labour if the repair doesn't solve the issue which seems to show willing - but it's still nearly £200 for the part. Opinions differ about whether there is a coolant divert valve. It seems some TDIs have them, others don't, and they are not all in the same place. I'd expect it to have thrown an error in scans if the pump was not working right. And for the car to be totally cold - it's running with some warmth, but not as much as it should have... I'll update if I/they find a solution.

Thanks. Mine is out of warranty; it is with an independent mechanic who reckons the electric water pump is misbehaving. Nothing showing in scans though...

I have a similar issue with a 2.0 tdi and suspect the solenoid or the electric coolant pump may be to blame. What's the physical location of the solenoid?