diagnosing poor fuel economy: 22mpg - PeachParts Mercedes-Benz Forum

Philosopher · #1 08-18-2010, 02:23 PM

Hi All.
I am working my way though a list of fixables on my my '84 300TD wagon, and trying to only have one or two questions per post.

I am a reasonably competent mechanic. However, other from having always wanted one, I don't really know very much about these very fine old diesels.

I am trying to diagnose how to get my fuel consumption up from about 21-22 mpg up to around 35 mpg.

I drive my wagon A LOT -- about 120-150 miles every day, six days a week. I am a roofing contractor who is stuck working pretty far from home for a season. I have a happy 20' ladder strapped down on a nice Yakima roof rack pretty much all the time:

I assume the ladder eats up one or two MPG. I tend to drive some what heavy footed, and that also affects my numbers. But even when on long road trips at a steady 70 mph, (without the ladder) I am still only getting about 24 miles to the gallon.

I have noticed an occasional little puff of smoke on cold starts, and sometimes when slowly driving home at night through Austin traffic, I can see a humble hint of haze in the glow of the tailgating headlights behind me. At a stop light I can sometimes smell the presence of diesel smoke.

In addition to this, I have occasionally experienced the sensation that the engine is being starved for fuel -- it will bog down for a moment, and I will loose acceleration. This will last for two to five seconds, and then will pass. It feels like a fuel restriction that gets cleared....

I have recently replaced and verified the correct routing of my vacuum and fuel lines, also replacing the fuel filters. (I have some slightly confusing extra plumbing from a WVO set up).

And, yes the car is set up with a pre-heated dual-tank WVO conversion. I currently have it disconnected as I sort through some other issues.

Using my GPS I have verified that the speedo and odometer are more less accurate, even though I have seen the odometer trip meter get stuck a few times.... Nevertheless I most often get an indicated (+/-) 310 miles per 14 gallon fill up. About 22 miles per gallon.

The engine in general sounds reasonably smooth, and there is little visible blow-by when I remove the oil cap.

So here are my ideas to try to get the car to a baseline from which I hope to take steps to improve the fuel economy:

1. Drop and flush the fuel tank, replacing the pre-filter screen. I suspect there is up to a gallon of crap in there. I'll service the failing fuel level sensor while at it.
2. Flush the fuel lines and replace the filters again.
3. Remove and rebuild the injectors. I don't know if these are serviceable, but I believe I have access to an ultrasonic cleaner, and I am pretty good at rebuilding motorcycle carbs..... so how hard can it be?
4. Verify timing and valve adjustment.

That's pretty much my plan, even though it probably will be another week or two before I get started. Because I work such long hours right now, I need to be careful to only take on an afternoons' worth of work at a time.

I currently am going about 840 miles a week. At 22mpg (costing about $3.00 per gallon) I am spending about $115 a week on diesel. If I could get the fuel economy up to 30 mpg, it would save me about $140 a month in fuel cost alone. That is quite an incentive to me.

So I am interested in whether or not 35mpg is a reasonable expectation, and I am open to any tips and suggestions to help me get there.

Thanks for the input.
-Steve

kerry · #2 08-18-2010, 02:37 PM

Aint gonna happen. You've been reading too many Craigslist or Ebay ads. Avg is typically about 24mpg for a 123 with a 617. Maybe a little higher if you're only driving highway miles.

Keep running the WVO if you want to get fuel mileage into the infinite range. It will get there because the engine won't be consuming any fuel at all anymore.

andrewjtx · #3 08-18-2010, 02:38 PM

I'm still chasing down my MPG issues. I get about 20mpg, but I have a heavy foot.

Like yours, mine runs smooth and I have great power. No smoke and little blow by. Cranks like a dream, even on a cold morning.

I just haven't had the time to pull the injectors or do a compression test.

Your other items to look at are brakes and bearings. Check to see that all wheels spin with ease. I've determined the brakes aren't dragging, but I'm not sure about my bearings. The driver rear center cap had a slight but of grease on it but with the wheel off, doesn't seem to be leaking on the hub. Strange. I've cleaned it and continue to monitor.

andrewjtx · #4 08-18-2010, 02:39 PM

Yeah, 35mpg will never happen in this car. I'd be THRILLED with 25.

Philosopher · #5 08-18-2010, 02:41 PM

I have heard so many things about these cars and their MPG.... Maybe all I really need is a reality check. I had a buddy with a 300D sedan which he swore got 30+ mpg.

Mine is the OM617.952 --with a theoretical 92 kW (123 hp) @ 4350 RPM;, and 245 N·m (181 lb·ft) @ 2400. I don't know how that affects things.

Thanks!
-steve

andrewjtx · #6 08-18-2010, 02:46 PM

Possible your buddy had a non-turbo? Or Dick Cheney's weather machine behind him, always giving him a tail-wind?

Biodiesel300TD · #7 08-18-2010, 03:18 PM

You'll never get 35mpg. The best I ever got in my wagon was 27mpg. I averaged 24mpg and I kept detailed milage records. At 24mpg you're getting about as good as your going to get for and over all average. You might be able to raise the average a mile or two but thats about it. Make sure your tires are inflated properly and your alignment is correct, your valves are adjusted, rebuild/replace the injectors, and all the filters are fresh. The tires themselves can make a difference as well. Make sure you've go good quality tires.

A couple mods you could to to reduce the drag of the car would be to build rear wheel covers that fill in the wheel well, and remove one or both the side mirrors. Legally you probably have to have a driver's side one, but I'm not sure.

You have to realize too that a sedan weighs slighty less than a wagon so milage can be slightly higher.

Philosopher · #8 08-18-2010, 03:56 PM

Thanks for the quick feed back. After reading and searching the forum for about an hour I see that my 22-ish MPG is not all that bad. Thanks for the info.

peace and grease,
-steve

pawoSD · #9 08-18-2010, 04:04 PM

Running a 300TD with a heavy foot and a huge ladder on top is not going to get good mpg's. I'd be happy with 22 if I was you! I only get 25-26 in my 300SD, and it is more aerodynamic, and I do 95% highway driving. The 617 engine is rugged, not real fuel efficient.

zeke · #10 08-18-2010, 04:23 PM

x8 or however many wagon owners have already chimed in...

35 is a pipe dream even for a 4-sp NA 240D. You have a larger engine, heavier car, ladder, AC?, auto tranny....AND if you decide to run WVO again (or better yet biodiesel, please!), you will lose several BTUs per gallon......

so, if it goes up to 26 on the highway with the AC and ladder off, I think you are peaking.

fruitcakesa · #11 08-18-2010, 05:05 PM

Yep, best I ever saw is 32 mpg on my 81 4 spd and that was light footed
When I step on it, it drops into the 27 range,

t walgamuth · #12 08-18-2010, 05:11 PM

You're lucky to break 20 with that huge ladder up there!

kerry · #13 08-18-2010, 06:18 PM

In my experience there is no noticeable difference in the mileage of turbo and non-turbo models in the factory set up.

pawoSD · #14 08-18-2010, 06:32 PM

Quote:

Originally Posted by kerry

In my experience there is no noticeable difference in the mileage of turbo and non-turbo models in the factory set up.

In theory the turbo should be slightly more efficient as it has a higher oxygen and burn efficiency per stroke due to the increased pressure. It is recapturing some of the exhaust heat and energy to increase the power.

Brandon_SLC · #15 08-18-2010, 11:58 PM

I agree. 22 is about what I would expect. I measured every tank in my 300SD and the range was 19-27 ish. Similar weight but more aerodynamic than a 123 wagon.