The #1 reason for poor mileage on the 1990 300CE would be a non-functional O2 sensor.

Either the fuel mixture is out of adjustment OR the O2 sensor is NOT in control of the mixture.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator