Poor MPGs for 99 E300

[bmisf]

Hi -

I recently purchased a 99 E300, approx. 80,000 miles, running quite well overall.

However, I've been really shocked at the poor fuel consumption I'm having, and am wondering if you all might have recommendations for what to look into to improve this.

My last tank, which was mixed highway driving (65-75 mph) and city, with A/C on perhaps 50% of the time, came out to 22.8 mpg. This seems really out of line to me.

The best I've ever seen was 33 mpg while driving at night, no A/C, 75 mph, all highway. Tires are ContiTouring Eco Plus M+S, and a bit worn but still within normal tread guidelines and inflated as per the placards.

I don't really see any smoke, and acceleration and power seem very good. I put fuel additive into the latest tank and will see if it makes a difference...but perhaps you all can tell me if this is out of line (especially the latest, mixed driving tank), and what to look into if it is. Since I've seen others posting 35-38 mpg for highway driving and lots of people claiming 30 mpg for mixed driving, the 22.8 mpg in particular seems a bit alarming.

Thanks.

[W140 S600]

Hi,

Im running a 98 E300, and so far ive been getting great mileage. Last trip i got about 32 mpg WITH the ac blasting doing a solid 75-80. I would definately do an air filter change...diesels are very sensitive to clean airfilters. I boosted my mileage up by doing that. An oil change and fuel filter change wouldnt hurt either. And if you do a lot of city driving, put the tranny in "winter mode," which starts the car in second rather than first, and keep the revs below 2000, let the other drivers wait. And when you do get on the highway, a good old italian tune up cant hurt.

[bmisf]

Thanks - just did an oil and oil filter change (Mobil 1 0w40), and will take a look at the air filter.

I'll try the "winter" mode.

Hmm...what's an "italian tune up"?

[W140 S600]

Even if your air filter doesnt look that dirty, id change it anyway, my looked pretty good but after changing it, the mileage went up...as for the italian tune up, let your car warm up properly, and floor it, make sure it red lines...to blow any built-up carbon out...suprizingly, your car may smoke a little...good luck

[gsxr]

Check all filters, or just replace them if they are of unknown age. Check for fuel leaks as well. Wouldn't hurt to check for dragging brakes - jack up each tire, the wheel should spin very easily with almost no drag. Increase the tire pressure to 32-34psi if you can, the low MB recommended pressures are more for comfort than MPG. But yes, 22.8 seems low. Make sure you are doing the math properly, you'd be amazed at the number of people who don't do this correctly:

1) Fill up completely
2) Reset trip odometer
3) Drive until the reserve light comes on
4) Fill up completely
5) Divide miles traveled (trip odo) by number of gallons used in step #4
6) Repeat starting at step #2

You get much less accurate numbers if you fill up before the tank is near empty.

'Italian tuneup' means to drive it like you stole it - lots of full-throttle runs to the redline, uphill if possible. This is not hard on the engine, actually it's GOOD for it, as it burns out carbon deposits etc and often makes things run better afterwards. You should get at least one or two WOT runs to 60-70mph every tank to help keep things carbon-free. If you see lots of black smoke (at first), it's working as intended, and the smoke should get less and less as you drive it harder.

[bmisf]

Originally Posted by gsxr

Check all filters, or just replace them if they are of unknown age. Check for fuel leaks as well. Wouldn't hurt to check for dragging brakes - jack up each tire, the wheel should spin very easily with almost no drag. Increase the tire pressure to 32-34psi if you can, the low MB recommended pressures are more for comfort than MPG. But yes, 22.8 seems low. Make sure you are doing the math properly, you'd be amazed at the number of people who don't do this correctly:

1) Fill up completely
2) Reset trip odometer
3) Drive until the reserve light comes on
4) Fill up completely
5) Divide miles traveled (trip odo) by number of gallons used in step #4
6) Repeat starting at step #2 :

Heh - I'll definitely do the Italian Tune-Up.

And that's exactly how I calculate MPGs...there will be small variations because I don't top off tanks past when the pump clicks off the first time, but I'm confident my numbers are pretty accurate overall.

I'll go through all the filters and clean things up. I'm getting new tires in a few weeks, so that will be a good chance to check brakes and other things in the drivetrain from underneath (the brakes and rotors were replaced less than two thousand miles before I bought the car, but you never know...)

The last thing to check (and which sounds like it would be the most trouble) is something in the injectors or injector controller. Ugh.

Thanks for the suggestions.

[gsxr]

I always go past the first click when filling the tank. Usually the second or third is about right. Some pumps click off WAY too early the first time.

BTW, next oil change, I'd switch to the new Moibl-1 5W-40 (which is actually Delvac-1). I'm not real crazy about the 0W-40 weight, even though MB approves it (for MPG reasons, not longevity or protection!)