Lachlan
En-Route
My 150 is so leaky that even with all the vents closed, the cold winter air is still blowing past as a good clip, so fumes are not as big an issue. And unlike those with nice big engines, the little O-200 does not put out an excess of cabin heat. Full heat is just enough to be comfortable with a sweater. Better than an open cockpit for sure.
Fix your door and window seals. My 150M will make you sweat in a t-shirt if I use full cabin heat. New mufflers and good shrouds.