Fuel distributor failure can result in suddenly increase fuel consumption. I've seen this several times, it cannot be fixed by adjustment. Appears to be a result of corrosion of the spring diaphram in the fuel distributor, shows up as incorrect differential pressure.

Another possibility is sticking or stuck piston in the mass air flow sensor, so that the fuel pressure regulation system is "stuck" at high flow (the amount of fuel is determined by the position of the "flap" in the air flow meter, transmitted to the fuel distributor via a plunger that operates a "controlled leak" betweek the upper and lower FD chambers). If the plunger is sticking, it will run unneccessary fuel down the intake on cruise, same as if you had the throttle open further. Can be fixed by replacing the o-rings and cleaning the plunger, or replacing it if scored.

The only other posibility is a failed O2 sensor or computer, allowing very rich running, but this is unlikely due to the nature of the feedback system -- no current flow to the electrohydraulid actuator EHA results in basic mechanical mixture setting without electronic feedback control.

You need to get the car to a shop that can perform the correct diagnostics, anything else is just guessing.

You should get 22mpg at least on the highway.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!