Mercedes: 1988 300E gas mileage, decent gas mileage, poor gas mileage

Question
I bought a 1988 300E from its original owner in 1991. I have concientiously maintained the car. The car used to get decent gas mileage probably averaging about 20 mpg. Now I am only getting about 12 mpg on the highway. Can you tell me why I am now getting such poor gas mileage?

Thank you--Amy

Answer
Best guess:
A cracked fuel hose.
Cold Start Valve leaking
E.H.A. Valve leaking
Fuel Inj Holder O-Ring leaking
Fuel Injector leaking
Fuel Injector Seal leaking
Water Temp. Sensor (for computer) bad
Possible bad valve guides = valves not sealing.