My 300SD OM617 fuel economy is garbage

[stephenc1984]

1985 300SD non Californian 2.88 diff and ************** egr delete kit. The fuel economy is absolute garbage. Highway speed 65-70mpg is 20 and city is 16-18 and has more power to give at that speed. I’ve owned a lot of w123 and w126s over the years and never had anywhere close to those numbers. Always high 25-30mpg range. I’ve ran two cans of diesel purge and seemed to help a little shake at idle. Also, changed all filters including tank screen. I haven’t done compression test but it fires up within the first couple rotations every time. I’m thinking of upgrading injectors. I want the monarks but I would like to buy the entire injector not just the nozzles. Can someone point me in the right direction? I don’t want to have to fiddle fart around with pop testing and shimming. Thanks, Steve

[JHZR2]

It’s dubious that injectors will make a huge difference unless they actually flow more.

I’d check timing, chain stretch, etc. Then worn parts, blowby, compression, etc.

Also check for bad brake hoses and dragging calipers.

[jsb357]

Originally Posted by stephenc1984

985 300SD

I haven’t done compression test

get a test done or DIY

you need to know what you are working with
before spending money buying parts

[barry12]

Buying or rebuilding injectors before testing what one has first. Seems all too common.

[mytimeyet]

My 85 300D w/2.88 gets around 25mpg in all modes it seems. If this car is new to you check the speedo/odo with a gps to see if it is giving you good numbers.

[ykobayashi]

I drive my SD like an old lady and get 25mpg. This thread makes me interested in swapping in a 2.88 diff. My 85 300D California with the 2.88 runs like a completely different car.

Anyhow. When I first bought my 82 SD I had similar mpg. Like 20mpg or so. I looked everywhere for the problem - flat tires, dragging brakes, fuel drips, tuning.

I finally found out my odometer was slipping. I wasn’t clicking off enough miles between fill ups. It was especially bad at highway speeds so taking long trips didn’t really improve mpg which it should have in theory.

Try downloading an odometer app on your phone and take long drive. Mine showed I was reading 20% less miles. This was caused by some worn components in the electromechanical speedo. Took a few years (mostly because I didn’t care once I knew it was just the odometer) to sort out.

[t walgamuth]

Good responses. One more thing is the diff ratio. Check what it says on your housing.

[automch]

I have an 83 300SD that I bought new and now has 629,000 miles on the original engine and trans. I consistently get 25mpg in local driving and 28 - 29mpg on the highway. You need to adjust the valves and while there check timing chain stretch adjust as necessary, then check you injection timing. Replacing the injectors is not the answer.

[ykobayashi]

One more thing. Here is the thread where I shimmed the odometer gears to keep them from slipping. Oddly the speedometer was perfect. The speedo and odometer are on separate gear trains in that gizmo.

I can now get 30mpg when I drive long highway trips and 25mpg mixed driving in a hilly area.

Fixed 1982 300SD odometer slip at high speed

[vstech]

Originally Posted by ykobayashi

One more thing. Here is the thread where I shimmed the odometer gears to keep them from slipping. Oddly the speedometer was perfect. The speedo and odometer are on separate gear trains in that gizmo.

I can now get 30mpg when I drive long highway trips and 25mpg mixed driving in a hilly area.

Fixed 1982 300SD odometer slip at high speed

Yup, odd slipping is the common issue with the SD.

The W123 has a mechanical odometer, and the gears slip on the shaft.

My 85 gets around 30mpg when I keep it under 70… well, it did until the VP bit the dust and split the camshaft in half…

Don’t ignore slow shutdown/weak brake booster symptoms… you’ve been warned…