Poor fuel mileage (MPG) 99 E300 TD

[timwstephens]

My 1999 E300 TD sedan, 178,000 miles, has started getting around 27 mpg on the highway. It was getting 34mpg. There are no codes, no fuel leaks runs great. no brake/bearing issues. idles well, seems to have plenty of power. Doesn't seem to be running rich, it doesn't smoke at all unless I kick it into passing gear and even then, it's just a whisp. isn't using oil. I keep the tires up on it. I've considered it could be the AC pump, it does seem to run all the time. It doesn't seem to drag the engine at all though...seems to turn fairly free. Running the AC at idle doesn't seem to affect it at all. I tried getting diesel at a different store. Changed the air filter. Been doing this at least 4 months. I'm just really at a loss to ID a cause. Should I just expect this at 178k? Any ideas will be greatly appreciated !!! - Tim (North Carolina)