These are older engines that deserve a tuneup. Many have not been done for almost forever. Basic tuneup includes valve clearance check, amount of timing chain stretch present as it affects injector pump and cam timing and the injector timing itself.
If the above items do not increase milage then perhaps an injector check is in order.
Even if it does not land up increasing milage you have the peace of mind your engine is not going to self destruct. The archives have many good guides to follow as well. Plus deisel giants helpful suggestions. Plus the help available on site. Hundreds if not thousands of these cars owners have done it before.

Oh yeah, I forgot about the valve. Try adjusting them. That might help.

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Definitely an anomaly with your mileage... my 240 D mileage has ranged from 28 -32 mpg since 1982. It is usually on the lower range when traveling the hills, city streets and during the winter. Open Highway mileage normally around 30 (or perhaps a little lower in the winter).

Suggest changing the air filter more frequently and adjusting the valves and injector timing. Also consider checking and if necessary, changing the timing chain + tensioner + guide rails.

Perhaps you have a leaky fuel line or return hose??

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Bama1


2008 SLK 280, Firemist Red - "Hurricane"
2001 F150 Lariat 4x4, Black on Black - "Badboy"
1982 240D 4 speed survivor -"Pearl" - Donated to Vietnam Veterans
1962 Oldsmobile Dynamic 88 -"Gertrude"

1954 model original owner - ~2.5M
Gray softtop/solid exterior/modified chassis

YOu have not mentioned what your driving is like. If it's city driving only then 20 MPG isn't terribly off. I'd say if you barely took it over 2400 RPMs ever then there would be something wrong. But I bring my car up to 4500 RPM every time I drive it and I get 20 mpg/city. This is with valve adjustment and turbo adjusted to 13 psi in 4th gear.

Will jump to 25 on the highway.

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1985 CA 300D Turbo , 213K mi

This will be a true test of the highway mpg. Up till now, I've had her since early December of last year but never drove a long trip on a working odometer. I had the odometer fixed. I currently get 21 mpg after having a maintenence tune up, which sucks b/c it was 19 mpg in December. So this is nowhere near what the other 300 D owners report. Although mine is a turbo which is a factor but it doesn't seem to matter if I run dino diesel or biodiesel. The mpg is the same. I do not drive it like I stole it but I don't drive like an old lady either.
Would anyone be interested if I keep a diary of my SF to LA trip on our diesel forum?
I know that the 240's are supposed to have better mpg than the 300 D so I am puzzled as well aside from the major tune-up that others have already suggested. I am in your situation so I am paying attn to what ideas come up. I want to improve my mpg too. I doubt that the PS Cetane Boost that I add to the tank is helping much, maybe improvement 1 mpg.
Frankie

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Frankie
It never ends!
1985 300D Turbo 181K Anthracite grey, "SOPHIA"
1984 300 SD Turbo(sold)
2004 Subaru Forester XT,Cayenne red.

Well, don't forget mine is a 123 240D, manual transmission, so it should get much more fuel mileage than your 300 turbo. Also, air conditioning does not work, so I assume it should get even more mileage because of that.
My driving is mostly city streets, but no hills (LA is flat), but I never drive during high-traffic hours... My driving style, as I mentioned before, is "little old man...", just because I want to be easy on the car and I am very conscious about saving on fuel as much as possible. I am very surprised that other 240D owners report much lower fuel consumption (perhaps my 123 consumes more than their 124s?).
I have no idea how many RPMs (I don't have that gauge on my panel), but as I said, I drive SLOWLY.
You guys have mentioned several possible causes... I'll start with the less expensive fixes first: change the air filter again (I doubt this is a contributing factor, as after installing a new one last year it made very little difference as far as mpg). I'll change the fuel filter, which I have never done and might not have been done in several years (can that alone really make a substantial difference in terms of mpg?)
Then there is the slow odometer thing, but I doubt it might be that... I did some driving last night and checked my route on mapquest, and the mileage given on the website matched all the way what the odo reported. And no, Kerry Edwards, it does not stick at all when the last digit is moving from '9' to '0', I checked. It runs smoothly all the way...
Leaky fuel line and return hose... I put in new return hose segments last year and I checked yesterday with the engine running (after a long trip) and they don't leak.
I'll check the fuel line underneath the car... but my car is garaged, wouldn't I notice fuel on the floor if it were leaking?
Breaks dragging... that might be a possibility... I need to look into it and learn how to inspect them...
Tire pressure... I always check it at every trip to the gas station and it has been always maintained at the maximal level for my tires.
Lastly, major tuneup/valve adjustment... I'll leave that for last as I understand that it is a major procedure that requires much expertise, and it is likely to be pricey too, right?
My car has a good pull, so am I correct in assuming that the problem is not likely to be one that a tuneup/valve adjustment could fix? Wouldn't an engine out of sync limit the performance of the engine and slow down the vehicle?
There is some malfunctioning I have noticed that perhaps can be helpful to you guys to get an idea about what might be wrong with the car: engine oil comes out from where the return hose (the one that connects from the top of the engine to below the air filter encasement) is attached at the top of the engine. It has been this way since I bought the car two years ago. I tried fixing it a couple of times, but the oil still comes out... To limit the quantity that gets expelled, I have learned to keep the oil level between the low mark on the dipstick and midway between the high and low marks... This way I get just a few drops, daily, on the garage floor.
The other problem, this one very recent, is the banging noise in the back of the car when I make turns (I'll replace the differential fluid, and see what happens)... but it just came up last week so there's likely no connection with the poor mileage I get out of the car.

If the new details I've given here seem to pinpoint where the problem might be, please let me know.

Rino

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

Don't need GPS to check your odometer... just watch the mile markers next time you're on the highway. That's what I do with any new car I buy! My old VW bus was about 10mph fast on the speedo, but odo was dead on. Go figure. Good luck!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)

Rino,

I suspect that the city driving has much more to do with the mpg than anything mechanical because as you mentioned, you do not have the benefit of any sustained highway travel, period. Most LA and Orange County surface streets I have traveled are controlled with lights or stop signs every few blocks, except for PCH and there is also a tremendous "gawking" factor that, all to often, slows traffic for no apparent reason. I think perhaps your expectations may be very optimistic and the reality is you may be actually getting fair to good mileage considering your conditions.

My 240D 4-speed averages are derived from 75% highway at 65 MPH & 25% surface streets at 30 MPH or less...daily round trip of 64 miles.

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Bama1


2008 SLK 280, Firemist Red - "Hurricane"
2001 F150 Lariat 4x4, Black on Black - "Badboy"
1982 240D 4 speed survivor -"Pearl" - Donated to Vietnam Veterans
1962 Oldsmobile Dynamic 88 -"Gertrude"

1954 model original owner - ~2.5M
Gray softtop/solid exterior/modified chassis

It might very well be that... I rarely drive more than 6-10 miles per day (mostly just 3-4 daily miles consisting of several short trips) and my driving is exactly as you described. I've read in one of the new oil threads that driving the car frequently but doing very short trips (unfortunately, exactly what I have been doing) is not very healthy for the engine... Highway travel would be much healthier...

So, perhaps nothing is wrong with the car in this sense and I should not complain about the fuel mileage that I've been getting...

Rino

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

There are ZILLIONS of variables in fuel mileage. The first one that comes to mind when reading the original post is driving conditions. If you are driving stop light to stop light in town with no highway driving, then 20 MPG might not be too far off the mark.

So, conditions as described above are probably the biggest variable coupled with driving style. The next most important consideration is vehicle weight which is fixed, but if you are hauling around a spare engine in your trunk or it is packed with tools or bootleg whiskey it will have a considerable MPG effect.

All that said, if you drive mostly highway miles with an empty trunk, you do need to pay some attention to the car. If this is the case and you are getting 20 MPG I would expect to see black smoke out your tailpipe.

20 MPG seems low based on hundreds of posts read and the fact that I get 27 consistently with my car, even on winterized fuel. I'm not conservative with the pedal either, except that I usually keep her under 70MPH...

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'83 300D Turrrbo 295K miles

Definitely too low. I'm getting 29,25 since November (winterised diesel?) and 31,5 before that. Mix of city and freeway in Phoenix which is even flatter than LA. Also 4spd manual with no ac.

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

I have a 240D with an automatic, driving in a mountain town with slow traffic and hills I get 26 mpg in town. The best I measured was on a trip to San Francisco from the Sierras @ 34 mpg, traffic was very slow but moving all the time. I think a valve adjustment is in order at the very least if you have not done that yet. Second what tire size are you running? I have 185/70 14 on mine. I've seen some with 195 and 205 70 14 which is too tall of tire and your clearly loosing torque by having too tall of tire. I turn you will be holding the vehicle back and adding more throttle to compensate for the loss of torque. You really notice this in the mountains where I live.

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)

I used an 83 240D auto for a number of years as a service vehicle. It really paid for itself with consistant mid to high 20's driven floored alot.
It wound up saving my life in a head on accident giving up it's own. It also came very close to keeping up with traffic.

Later bought a clean 77 240D stick. Did everything to it. Adjust valves, diesel purge, replaced injectors, little chain stretch. Drove like I stole it.
Poor MPG's and slow even for a 240D. The engine was just worn out.
It sits waiting for me to put a replacement engine in it. Been putting it off.
The replacement engine is a late 240D auto.

Also have owned an still own 300D/123's. There can be a very large difference in power/mpg's between a strong one and weak one.

John Gregg

I have P195/65/R14 on... To what extent can that actually affect mpg? And affect odometer/speedometer outputs?

Thanks for pointing it out, I wasn't considering that at all...

Rino

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1979 240D, W123, 105K miles, stick, white w/ tan interior.