[Gnits]

I have started noticing a diesel smell inside the cabin when the car is sitting still or idling. Never had this before but I am noticing that now.

Might be just your Webasto auxiliary heater kicking in because of the weather getting colder. Or door seals not sealing properly anymore, allowing fumes entering the cabin. Or the exhaust pipes - or more likely: the exhaust manifold - having cracked somewhere, and now leaking. Or DPF cracked, and there's now unfiltered exhaust passing by it. Or, or, or...

You can't really know until you at least visually inspect the car from the bottom while running. If there's nothing obvious, a pressurized smoke test might yield some clues.

 

[Gnits]

I had this issue for years now, and because it never surfaced in warmer weather, and is loosely coupled to activating the A/C, it's definitely related to temperatures. I do not always smell exhaust, but if I do, it's either right after the car has been started after sitting for a longer period of time, or right after I turn the heating on.

Because of that I'm theorizing, that it might be because of a tiny crack or gasket leak somewhere along the exhaust or auxiliary heater exhaust systems, which hover seals itself, through thermal expansion, as soon as its surroundings reach operating temperature. And in the summer it's not actually there to begin with, because the ambient temperature is high enough to not allow for the crack/leak to actually open/appear.


One interesting this is, however, that since I've added Liqui Moly Super Diesel Additive to the tank a week ago, I didn't smell the exhaust a single time - not even under circumstances, when I otherwise did. I guess this is because this additive makes the fuel combustion more efficient/perfect, and thus reducing the fumes and/or the strength of the smell of the raw exhaust.

 

[Gnits]

Could the dpf be clogged causing increased back pressure?

Even if your DPF would be clogged up and pushing back exhaust, that would only choke the engine, but couldn't exit the system - unless you had a leak somewhere. And if you have a leak in the system somewhere, then exhaust will exit there, even if your DPF is not clogged up. Also, a clogged up DPF would be most likely detected by the ECU and trigger a CEL.