Halliwell2003

Hi guys Am new here and tbh don't really have much knowledge of cars however I don't mind a tinker, any ways absolutely love the 320d especially compared to the is220d I had previously however I seem to be having a couple of issues which are niggling me and may not even be issues but I figured you guys are the experts. Firstly a couple of weeks back I noticed a smell of something in the cabin when stationary (ie at lights) and popped bonnet when I got home and found that a pipe on the left hand side had comes loose and after reconnecting all was fine. (This may not be related to my actual issue but best to give as much infor as possible) Then today driving home from work in heavy traffic at low speeds the economy seemed to drop dramatically to like 20/16mpg on the instant read out (no matter how little I put my foot down). Once traffic had cleared and I got up to speedthe readout went normal back up to 50+mpg. This may even be normal but I never recall it acting that way previously. Cheers in advance for any help or guidance on what to check for or test