OM617.952 Poor Mileage...... Causes?
 

As the title says, my friend's 300D is getting in the low 20's (60 MPG 350 mile cruise netted him 22 MPG). I've made sure no brake calipers were dragging, fuel filters changed and the odometer is right on.

Does anyone have further suggestions?

Best,

Did this come on suddenly, a week or two back?

If so it could conceivably be just crappy winter diesel. My fuel economy just took a huge dive on the last two tanks.

Same story last winter.

I am not sure what speed the Car needs to travel at to get the best Milage but escesive speed eats up fuel as well as a lot of City Driving.

Valves out of adjustment
Injectors worn out and with low opening pressures
Valve timing off due to Timing Chain/Gear Wear/Stretch
Fuel Injection Pump timing off
Low Compression
Driver has a heavy Foot on the accelerator
Bunch of heavy stuff inside of the Car or Trunk
Under inflated Tires
Biodiesl or Diesel/Biodiesel blend will get less miles pre-gallon.
Using the Air Conditioning or other Electical Accessories
Driving in an area that has Hills.

I have found that 'real world' mileage figures for a 617 are between 22 and 27mpg.

I get 2-3 mpg better mileage when using name brand diesel, the best of all, thus far, has been Shell....

I've also experienced a 2-3 mpg drop in fuel economy due to winter diesel.

However, there was a time a couple years ago when my 300D got around 22 mpg, maybe even a little less. Always baffled me, and since it was my first MB diesel I didn't have anything to compare it to. Then, in an effort to eliminate an oil leak by the turbo drain tube I removed and rebuilt the turbo. Immediately began to get 27mpg steadily. Also more power. Oil had been leaking past the seals in the turbo, turning into carbon and restricting its ability to spin. You can check for side to side play in the turbo shaft to determine if it's worn. I used a kit from **************.

Injection timing is late.

Well guy, it is all "hearsay"! It's your friends car. How do you know how he drove? How do you know he correctly measured the amount of fuel used and the miles driven. It's all off the cuff and is a worthless question. Like Diesel Doc says, typical normal mileage on a 300D starts at about 22 mpg. To get into that 25/28 mpg you have to be a real achiever and everything has to be done really right. They are a heavy car and not really good mileage makers in the current era of high fuel costs. Back in 77 to 1985, they looked a lot better for mileage.

Things from what I have read on the Forums Prior Owners don't seem to do, have the Valves Adjusted, have the IP Timing adjusted or have the Injectors removed and serviced or rebuilt.

Some Prior Owners may have occasionally done a Diesl Purge but I doubt if most of them have it done.

Doing all of the above stuff could increase a Cars Milage.

[QUOTE=Merica;3047010]I've also experienced a 2-3 mpg drop in fuel economy due to winter diesel.

However, there was a time a couple years ago when my 300D got around 22 mpg, maybe even a little less. Always baffled me, and since it was my first MB diesel I didn't have anything to compare it to. Then, in an effort to eliminate an oil leak by the turbo drain tube I removed and rebuilt the turbo. Immediately began to get 27mpg steadily. Also more power. Oil had been leaking past the seals in the turbo, turning into carbon and restricting its ability to spin. You can check for side to side play in the turbo shaft to determine if it's worn. I used a kit from **************.[/QUOTE

Checking actual boost pressure developed is something not discussed much if at all from a mpg perspective. Usually only suggested for poor performance if anything. A dragging turbo does make sense to me as a possible culprit.

Normally the turbo engines running well give better fuel milage than the natual aspired engines. So measuring boost prressure at operating levels under load should be part and parcel of lower milage claims than average.

I fill my own tank on my '84 300d and check mileage every time. Always use dieselkleen additive and always get 24/25 mpg. Tanked up 1tank without from an odd station and got only 21!! Winter diesel or poor quality fuel seems to be an everpresent problem.

Same here. 22 to 27 for me. If I drive a lot of stop-and-go traffic (rush hour traffic), MPG drops. Same with speeds over 65mph.

My average, with AC, is around 23 to 24 MPG. Without AC, I can do 25 to 26 MPG on average.

Early September I changed the air filter and did the valve adjustment and added startron ( still need to do filters) Saw a big change in performance and mileage. Then about 2 weeks ago you could tell that the evil winter fuel change over had occurred. As for mileage I have been driving my wife in to work lately and it is mainly highway. If I keep my speed below 60 I see a huge difference in how far I go on a full tank. I did a quick check on my last few tanks and i am getting 26 mpg. My trunk does have a full load of various tools ( still moving things to storage ) This is on my 85 300SD which just rolled over 211,000. miles ( need to check into timing chain replacement )

Fuel filters that are not problamatic yet but still more restrictive than they should be can pull the fuel milage down a little as well. No scientific proof but a few owners over the years did mention they noticed the fuel milage was somewhat better after a fuel filter change. Probably where the owners that are pretty anal at keepng an almost constant check on their fuel milage reported this.

At first I thought this was not too logical but decided the partial obstruction was probably lowering the fuel pressure in the base of the injection pump. Perhaps changing the equality of the power balance across the engine.

I also believe that a little more air pressure in tires than sugggested hurts ride quality a little but really reduces rolling resistance. This may also be a little dependant on the tire stifness from a design and manufacturing perspective.