I have a 79 300D. It's gets 19-20mpg with mostly around town driving. If I drive some on the freeway, I might see 21. At 100% freeway, I might get a little over 22. The manual as well as many a post say I should get 24-27. I have done the following:

New injectors
New Fuel Filters
Use Redline
Use biocide until I can get the tanked steam cleaned and all due to algae issue.
Adjusted valves,
Checked for fuel leaks, replaced rubber fuel lines
Replaced the return injector lines
Can't find any leaks
Was able to check compression on 3 of the 5 cylinders that I could get the fitting into the glow plug hole. (1,2 and 4 I think) All were over 300psi.
Tire pressure is to spec and tires are new.
Brakes aren't dragging.
The engine has 280K on it but has been very well maintained:religoius oil changes, valve adjustments, etc....

The car doesn't smoke much at all. Only when you are really getting on it say getting on the freeway. Reving it in the driveway you wouldn't know it's a diesel except it's sounds like one and it produces a nice stink nebula around it after a few.
It starts great.
It has good power, goes up hills at 80 with the AC on not even floored which is good for a motor with 280K on it. She'll break 100mph.

All and all, it seems to run great. It just sucks down that diesel like it's cool. I'm not sure what else to do? Any ideas?

Frank

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'82 300SD

I've read that the Bosch rebuilt fuel injectors are very iffy quality.

I just switched to synthetic oil, Shell Rotella T 5W40, actually, and I just hit 400 miles in mixed driving without the low fuel light coming on This thinner oil is about the consistency of water at room temperature, so I get power much sooner than I did on 15W40 dino.

Are you easy with the throttle? I get maybe 20-22 mpg in mixed driving, lucky if I get 25-26 mpg on a long trip. I must have a lead foot, because my mileage is always on the lower end of the range, no matter what I drive. I was down to 20 mpg in a 5 speed 1985 Nissan Sentra.

My best fuel mileage in my 1977 300D was no better than 22-1/2 mpg or so, and that was on a long trip.

Winterized diesel fuel gets poorer mileage because it's formulated to prevent clouding, which means less paraffins and more aromatics.

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic

I have a backwards type situation where my 300TD delivers about 20 mpg on a trip (running hard) and around 26 in town (typically mellow driving). By "town", I mean semi-country driving. Strange.

Hope you discover the solution to poor economy

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

I'd recheck the overflow lines for deterioration. I'm currently getting rotten milage on the Volvo, the work car, and found a bad return line the other day. Too cold to smell the diesel fuel baking off, which is my usual indicator.

Once the engine is hot, the fuel will evaporate from the injectors -- check the hose itself to see if it is wet. If any are, replace the whole set. Cold weather seems to make them deteriorate faster.

The other thing I'd check is the tranny for slipping.

And try going easy on the throttle -- I love blasting off the line in the 300D, but it lowers my milage from low 30s to mid 20s!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Remember that fuel economy drops exponentially with speed. If you get 30mpg at 55, you may be lucky to get 20 mpg at 75. What speeds do you drive at. You don't have to have a lead foot to get poor economy.

Also remember that there are many definitions of city driving (most times our definitions aren't the same as the automakers who sell us cars that are susposed to get 17-25 ect.). In actual city driving, with traffic, I get 18-20mpg. I usually get 25 mpg with mix of city/subburb. 25+ on the highway if I don't speed to much.

If your only reason to own a diesel is for economy, get a VW TDI

Good Luck
ATLD

Let's see, I replaced all the return lines when I did the injectors, no signs of leakage there or around the new filters. I also did the rubber hoses at the tank.

I drive mostly around town in stop and go or windy roads. I do drive fast on the freeway, 75-80 mostly. Oddly enough though, I got my best mileage on 5 coming down from Frisco 80+ the whole way. I probably take off around town on the quick side. Still, though, even when I cruise on the freeway under 75, which I have done, I'm lucky to approach 22mpg. I see numbers here even over 30mpg! That would be great, heck I'd be happy for 24 around town.


Something has changed, my dad drove this car for 15 years and got 24 out of it no matter what driving he did. Shortly after I got it there was a drop. I fixed dragging brakes, fuel leaks, put in injectors.... on and on... I just can't get her back to the former fuel economy.

Frank.

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'82 300SD

Just a guess, maybe the fuel changed to a lower cetane number? Have you checked your mileage with out-of-state fuel? PA diesel costs at least 15 cents a gallon more than NJ diesel; not sure if the formulation is diefferent. But, I live close enough to the border that big truckers may bypass our area to save 15 cents a gallon. Like I said, just a guess.

Frank,

It looks like you have addressed all the "low hanging fruit" in your quest to improve mileage. I think at this point you need to look at the possiblility the valve seals leaking on the intake valves so that a build-up of carbon is restricting the flow of air into the cylinder. My 240D, and the 220D developed this problem. I learned of it on the 220D as the build-up eventually interfered with valve seating and the car became hard to start, but that was after quite a while and a lot of build-up. I am not familiar with a way to get this off without taking the head off and taking it apart. I took the head to a shop and they essentially boiled it in some nasty solution to remove the stuff. When it was returned to me it looked like new.

Another potential issue might be the condition of the injectors and the prechamber. I know you recently changed the injectors, but it might be worthwhile pulling them and having the spray pattern/pressure setting checked, and looking inside the prechamber to see if there is anything amiss.

In my case the valve stem seals cured the problem, and the car ran much better. I have also changed injectors a few times (the old days you could get nozzles for $12 each and change them out your self, but today they are rebuilt injectors and they charge quite a bit more). Bad fuel can damage them, as can putting in the wrong seals.

Finally it may be possible that after that many miles the injection timing is off enough that you are not getting the optimum pop for your load of fuel. I think I read here that chain stretch controls this but I do not recall if the timing is advanced or retarded. Either case probably not good for achieving peak fuel economy.

Good Luck, and hope this helps! Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Have you ever had the injection pump recalibrated at a Diesel injection shop? You might get the injectors tested at the same time. Then check the injection timing when you reinstall the pump.

P E H

I'm afraid I won't be able to try this. I live in San Diego Ca. We have very strict fuel formulation standards here and it's a long ways to Nevada or New Mex. I do buy gas as various stations but nothing seems to make much difference.

Frank.

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'82 300SD

That valve seal issue looks like a real possibility. We've long suspected a valve seal issue in this car. If that's the case, I'll rebuild the motor, I have an extra in the garage that needs rings, I can work on it on the side.

I see the timing issue come up a lot. I'm looking into a dial indicator for my magnetic base to check that with the valve timing. I try to do the injection check as well.

Thanks for all the great info guys.

Frank.

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'82 300SD

Just a thought.........

Would an adjustment to the ALDA help improve mpg?

Here is a thought out of the blue.... Have you checked the odometer to see if it is reading correctly?

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1981 300SD 164K Miles

I don't know. What is the ALDA and what does it do?

Frank

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'82 300SD

I believe it is. I've taken numerous long trips of known distance and it reads similar to other vehicles. Also, the speedo seems to be on as well. I would expect a few % error in any speedo/odometer, but I don't think that could account for 20-25% decrease in mileage I'm experiencing.

Frank.

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'82 300SD