C 250 TURBO DIESEL LACK OF POWER "98 auto"

Since you're desperate I'll venture a hypothetical - and that's all it is for the USA doesn't get these engines.

Modern diesels are using many of the devices found on gas engines - mass air sensor, crank position sensor etc. for control inputs. One of them has become intermittent.

The shop finds no repair codes?

Is the engine's operating temperature in the "normal" range or low?

Poor power and excessive fuel use suggests very much retarded ignition (injection timing here). (Crank position sensor fault.)

Or it might be that the computer senses a "cold" engine and so runs it in a "warm-up" mode (if that's what it's supposed to do, I don't know).

If this engine has an intake air flap to control the air volume like a gas engine that could be restricted, choking the motor - ah more speculation, its just endless!

I'd better stop or you'll be even more desperate!

Mine is the same. I have allways put it down the ECU being in the warm-up cycle. Anyway 2500rpm in 5th on this model is above legal limit in UK. Once warm performance is more than adequate (goes like a 'a bat out of hell'). What do you call heavy on fuel ? I get 37.5 mpg around town 45 mpg on a long run. Mine is an Estate so a bit heavier than the sedan or saloon model.