Heater doesn't work - intermittent issues

[white_shark]

Hello folks, I think I have reached the limit of my (mis)understanding of how the cooling system works in my A4 avant 35 TFSI and I'm gonna outline the symptoms I'm currently having. The car is a 19 reg bought about a month ago from the dealership with only 8k on the clock.

When I start up the car, the engine warms up within 3-5 minutes of driving or idling and the temp indicator stays put at exactly 90 degrees. No fluctuation dependent on revs/speed/outside temperature, so overall it seems to be working fine. However, the heater might or might not work.

In the morning, when the car is totally cold, heating usually starts working beginning at ~45 degrees of engine temperature and keeps doing so until I shut the engine or drive for too long. Starting up after a short break would most likely make heater blow only cold air.

However, what helped me multiple times is revving up the car to 3000-4000 on the first gear on a slightly inclined road (facing the incline). Then within 10-20 seconds the heat would come back and stay until I either stop the engine or drive for long enough. This doesn't seem to visibly change the coolant level in the expansion tank though!

Lastly, after revving up and 'fixing' the heater, there's a slight coolant smell in the cabin, which I won't mistake for anything else. Especially after accidentally blowing up the heater core ~15 years ago in a W201, which resulted in clouds of evaporated antifreeze going right into the cabin. 

I believe the issue is with the coolant flow as when the heating doesn't work, the incoming heater core hose (after the valve) is colder than the radiator upper hose. When the heating works, they are both about the same temp, or at least the heater matrix one is distinctly warm. All the dials and AC electronics seem to be working fine because when the heater works, it works perfectly. I haven't done any alterations to the cooling system or car whatsoever and I believe the previous owner or the dealer haven't too. They had barely serviced anything in the car at all as the mileage is low.

The only thing I did was topping up the coolant by ~0.5L once as I saw the level dropping to the MIN mark on a cold engine when it became colder outside.

Any help/suggestions/personal experience is highly appreciated! Thank you.

Edited January 19, 2023 by white_shark