Slow to get up to temperature and luke warm heater

[Colesy]

These two might be related but probably aren't.

Currently after first starting and taking the S2 on a 10 mile run the temp gauge doesn't get above 2 notches below 90. So circa 70C. The obvious suspect would be the thermostat. The one fitted is OEM and ~2 years old and has been tested as working fine. Is there anything else that could cause this cool running? One thing to note is I have a custom alloy rad which is slightly larger than stock and have deleted the aux rad.

Also the heater is not getting hot, even when the car does get up to 90C. The hoses that go to the heater matrix get hot and the system has been bled, drained, cleaned and bled and bled again, so I'm pretty it's not a bleeding issue! The dial seems to be working although it might be a little loose. There is a definite change in temp from freezing to luke warm... Is this likely to be a flap issue? Or is there anything else that's easy to check before removing the dash!

[SteveH]

The low temp on the dash could be due to the sensor in the water manifold. Non OEM sensors seem to show low on the gauge. You can check the values the ECU is seeing in VAG Com from memory. The low cabin temps have to be either an airlock in the heater matrix or perhaps the cables aren't correctly attached to the airbox from the adjusting knobs. There are some tiny plastic clips that hold the ends onto the airbox and they can easily snap. Try wrapping the temp sensor in some insulation. If all the OE plastics/radiator aren't in place, you could be getting some cooling to the sensor itself from air flowing through the engine bay. Perhaps this is an unlikely cause though.

[martinjs]

Mine is exactly the same, I did the matrix and recovered the flaps 18 months ago, it was fine then, but gradually been getting worse.
Thermostat is OEM and about 3 years old.

[newsh]

Run Vagcom to see the temperature used by the ECU measured by the sensor at the back of the cylinder head.
If you’ve never re-covered your heater box flaps they will need doing.
Try a new thermostat, I’ve found these don’t seem to last long these days, my coupe is on it’s 3rd one during my ownership.

Some people have seen poor coolant circulation due to the impeller coming off the water pump. I’m not sure how this affects the heater though and would suspect coolant temp to increase.

Thermostat and flaps refurb usually sorts it. Of course the matrix could be blocked but you’d change that if you had the box out to do the flaps.