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I have a 84 300DT 123 that now gets 24mpg, used to be 19mpg. I've had the valves adjusted by a import mechanic. I cleaned the banjo bolt and line for the ALDA system, new fuel filters (2) and a new air filter. I have also run fuel injector cleaner through it, the kind you put in the fuel tank.

Shouldn't I be getting around 35mpg?
Any suggestions I haven't tried?
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
No, you won't get ~35, not even close. Maybe 27 at best. 24 is not bad.

Check your tire pressure, I keep mine at the maximum recommended by the tire manufacturer in back and lowered as per MB in the front.

Usually the winter fuel blends don't have as much energy as the rest of the year. Bio doesn't have as much energy per gallon either.

Keep up the routine maintenance and drive slowly.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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You're welcome, driving slower, to a point of course, will produce better numbers.
 

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1984 300D
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5,070 Posts
It would be nice to know the Mileage on the Engine/Car?

What sized Tires are on your Car. Different aspect ratios of Tires for the same Rim size have a different diameter.

Some time when the Wheels are off of the ground spin them by hand and see if the Brakes are dragging. If so that will use up some energy also.

Some people have said that using Synthetic Oil gives a slight MPG increase and easier starting when there is cold temps.

If there is over 100K on the Engine and you think the Injectors have never been pulled out and checked they could be due.
Depending on the condition of the old Injectors after Injectors are rebuilt not all but most say they feel more power.
The better they atomize the Fuel the better the Fuel burns.

Your ALDA has an adjustment screw. With that it may be possible to adjust it so that it decreases the Fuel you get when your Turbo is boosting.

This was for gas Cars but back in the 1970s they said you could get one more mile per gallon by getting rid of 400 pounds of vehicle weight.

If you ever get around to doing your own Valve Adjustment you can get some idea of your Timing Chain/Gears Strech/Wear by lining up the timing mark behind the Camshaft Timing Gear and the Camshaft Bearing Tower and taking a look at what degree mark the Pointer is pointing to on the Crankshaft Damper.

And of course Fuel Injection Pump Timing. For this I would check the Timing Chain stuff first; since the Fuel Injection Pump is driven by the same Chain.
If the Timing Chain needs any correction with an offset Woodruff key it would be better to do that first and then time the Fuel Injection Pump.
 

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Daily Drive 82 240D 4 speed
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291 Posts
if you're getting 24 at 70+mph your doing pretty good. because of the relatively short gearing on these cars higher speeds can make pretty noticeable differences in fuel consumption.

If it's mostly highway travel, find a set of slightly bigger wheels like ones off the s class models (126). bigger wheels effectively give you taller gearing, which is better for highway cruising. swapping out the differential from an 85 300D could help because of the taller gearing equipped that year, or better yet one from a 420 gas w126. It'll be slower and less economical off the line, but if your 90% highway travel and you can do the work yourself, it would be worth it.

the 35mpg claim you get from craigslist or ebay is bologna. yes it's a diesel which has more potential energy per gallon than gas, but it's also 4,500lbs has a slush box for a transmission, and aerodynamics much like that of a hummers. it just doesn't happen.
 

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1985 300D Turbo
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349 Posts
I can Get between 26 and 27 while doing 65 mph on the interstate. Going faster and mpg really drops off a cliff. I also "inflate properly" as in 36 psi.
 
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