400E terrible fuel economy

[Palmz]

Hi,

I bought a 1992 400E just over a week ago and have been in love with it. There are some problems with the car though.

For some reason the car won't get better mileage than 15 mpg on the open road and probably even worse in town. The exhaust also has a strongly petrol like smell to it. Rich mixture?
It has plenty of power and the idle seems reasonably smooth

I checked the spark plugs for soot yesterday, but could not find anything wrong with them. Could this indicate a bad oxygen sensor?

The car has 138k miles and I use 98E5 gasoline.

Thanks!