Poor fuel mileage (MPG) 99 E300 TD
 

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)

Live data?
 

Have you had a live data scan done? Maybe your MAF is heading south?
Did you say that you AC is always being driven? When off, the clutch remains engaged? How are your brakes? Check for brake drag?
Just a few ideas to check
Lowflyingbird

Alignment maybe.

Thanks for the suggestions so far. The A/C clutch does kick off when you turn the a/c off. I considered the MAF sensor, but I was thinking it would cause some other performance issue as well. However, I looked at it today and I did clean the screen which was a little dirty. Tires seems to be wearing very evenly - car doesn't pull left or right so I don't think it's an alignment. No brake issues...all wheels spin freely.....appreciate more suggestions! Thanks - Tim

Can't say what would "suddenly" cause a drop in mileage. My car has 178,000 miles and until recently, I was getting a max of 29 MPG highway and 25 combined.

I just swapped the injector nozzles, adjusted the timing and re-torqued the delivery valve holders and it just purrs now. Plus it's getting 32+ MPG which thrills me no end!

Thanks Evan, I think it possibly could be the injectors...and I probably should have said I noticed it suddenly I can't say the mileage just dropped one day.... - Tim