I’ve just purchased a 300se 1991. I can’t figure out why the car smell likes oil/fuel/coolant inside the cabin when the heater is on. I can’t figure out what the smell is. The ac doesn’t work as it doesn’t have any Freon installed in it yet. The smell gives you a slight headache too.

It’s got a new radiator, water pump and thermostat installed. Installed a new EGR valve, fuel EHA, ac compressor, alternator, all new hoses, complete new braking system. Car has a fuel smell outside the car when you start, I’m thinking it’s just old fuel that’s been sitting in the tank forever. The car has no other issues now besides the Freon and the smells mentioned above. The car runs great. Engine is quiet and smooth.

Replaced just about everything. Runs perfectly now. The smell is the only issue.

Side view

Front view

I'm surprised nobody else has chimed in, but since you can't find a source for the coolant leak, it's almost definitely the heater core leaking. It smells sweet? You can clamp off the hoses going to it to prevent it from leaking before you replace it or to test whether it's the heater core or not.