I have the same problem as the guy further down the threads. My 79 300sd runs fine, starts fine, and has good accelleration.
I have only had the car for a few weeks and my first full tank fill up showed 20.8 mpg. I was crushed. I thought I would have to really lead foot it to get below 25.
The air filter is clean and the previous owner had set the valves within a year. The car did a lot of sitting during his ownership. I dont know if this could be a factor but I burned most of the old fuel before my first fill up. He is pretty mechanical so i assume he set the valves properly.
Someone in the other thread mentioned the EGR valve and the possiblity of disabling it to increase mileage. I would like to hear more about this. How is it done? The possible negatives? What does it do anyway? Will this turn me into a soot spewing eco terrorist?
Thanks
Eric z

Bill said :
" One of the best DIY guys on this site has never adjusted his valves, or had it done by a mechanic."

Lets talk about this.... wouldn't that statement pretty much disqualify this guy from being one of the 'BEST' DIY , just by definition... He could still be the Best answer source, Best answer writer, ETC.... but if you DON'T DO IT how can you be one of the BEST DO IT Yourselfers ? LOL ,Greg

I found a Chilton Diesel book today at a used book store (published 1980). It refrences the EPA's combinded city/highway for the SD at 26. The 300D's EPA numbers were 23 city/28 highway.
So I should be getting better MPG.
The needle on this tank seems to be hovering much higher so hopefully it is just an issue of bad fuel from the filling station. Still curious though what the first lines of attack are when going after bad mileage.
eric z

Manufacturer's mileage claims are made when the car is NEW, not 20 or more years old! Mileage WILL usually deteriorate over time. The SD is also a bigger, heavier car than the 300D.

You might want to check all your fuel lines for leaks...have you looked at the injector return lines, the "cigar" hose, prefilter hoses, etc.?

Mike

__________________
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1979 300 SD
350,000 miles
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1982 300D-gone---sold to a buddy
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1985 300TD
270,000 miles
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1994 E320
not my favorite, but the wife wanted it

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My mileage is usually in the low twenties in suburban driving and mid twenties on the highway. If I keep it below 70 most of the time I can get a hair over 26 mpg. These aren't VW TDI's; but remember a gasoline Benz of this era wouldn't get over 16 mpg highway, at least with me driving it For me to get over 20 mpg in anything but a 4 cylinder econobox makes me happy.
Things to check for maximum efficiency? Tire inflation, air and fuel filters, oil viscosity (switch to synth?), dragging brakes, injectors, leaky fuel return lines (I can smell fuel under the hood after shutoff) etc. Try running a bottle of Redline Diesel Fuel Conditioner through your SD to clean out the fuel system. Can't hurt.
Also, you don't say where you are. If you are using winterized #1 diesel (most of the northern US switches to this stuff around the end of November or early December) it has less paraffin content and more aromatics to prevent gelling at colder temperatures, and will deliver poorer fuel economy than #2 diesel.

__________________
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

Saying the cars are old, and/or high milage, is no excuse for fuel consumption that poor. My 1984 has a similar problem (20mpg). I know many people complaining of the same thing, and I would love to figure out what the cause is. I know others who routinely get 26-32mpg on a car of similar years and mileage.

For my car, I have already ruled out the simple things like tire pressure, clean filters, leaking hoses, dragging brakes, fluid levels correct, use injector cleaner regularly, use synthetic Rotella, done Italian tune-ups, etc. Still get 20mpg EVER tank - except that one fluke tank which was 24mpg (over 400 miles that tank!) Runs great otherwise - no smoke, starts instantly, low oil consumption, etc.

I'm wondering if the fuel quality available at the pumps today is the major cause of low MPG...? Or at least through certain gas stations? I wonder how much a stretch timing chain afftects MPG?


Best regards,

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After the usual things are done and you still don't think you are getting the mileage you deserve... then I would start looking at the injectors and precombustion chambers. There are seven small holes for the fuel to spew out of and if carbon or a tip from a glow plug is obstructing any of those holes it will sure affect mileage...Greg