Bad, bad mileage... 300D - PeachParts Mercedes-Benz Forum

driad_98 · #1 04-05-2004, 02:23 AM

Hey all.. I'm a little concerned. I must have the low-geared differential out back, but my '80 300d with an aftermarket turbo, freshly adjusted valves and tires with 34psi around just showed me 18.36 mpg on a 95% highway useage of 11 gallons. What gives? There's only ever a little smoke if I give it full throttle at night, and it idles fine, so I'd tend to think the injectors are fine. Is this kinda mileage normal? I tried to keep speeds under 70 for this test.

Is it possible/adviseable to swap differentials? I'm kindof a fuel economy freak, and I want to make sure my car isn't telling me something with bad mileage. This is the first time I've been able to test mileage without distraction, so I guess it's my baseline.

Thanks in advance for your input!

Ash

wolf_walker · #2 04-05-2004, 03:56 AM

Check your odometer or the miles traveled somehow, there known to be wrong.

I get a solid 25mpg in my 83, wish it was more.

It's been my experience that some diesels just get crapy mileage, part of there personality. What was the mileage before the turbo addon?

tjohn · #3 04-05-2004, 08:07 AM

Yes, your mileage should be better, although I have dipped into the teens several times. Usually its just a bad batch of fuel that clears up with another fill up. I personally get inconsistent mileage results unless I measure fill-up to empty.

P.E.Haiges · #4 04-05-2004, 10:59 AM

Driad,

How do you add more fuel when the turbo boost goes up with the added turbo charger? Even with more air in the cylinders, the engine will not have much more power if more fuel isn't added. That is what the Alda does on the OE turbo charged engines.

The line from the intake manifold to the Alda deteriorated on my 300SD and the engine, even though the turbo was creating boost, didn't have any more power than a naturally aspirated engine because the Alda wasn't causing extra fuel to be injected to take advantage of the extra air in the cylinders. Just replacing that leaking line, restored the engine to full turbo power.

P E H

Zoonhollis · #5 04-05-2004, 12:30 PM

I hope driad's engine was not originally a normally-aspirated diesel. Aside from the obvious fuel/ALDA issue mentioned by PEH, a NA diesel is not built to withstand the added stress that a turbocharger yields.

driad_98 · #6 04-05-2004, 12:35 PM

Hey guys.
I got the car with the turbo job already done. I don't know what the mileage was before, and I don't know whether the IP was changed, or 'tuned' to accomodate the addition of the turbo...

That said, I'm wondering if they DID change the IP, or enrich the mix somehow, as that'd account for the poor mileage. Or would it?

Thanks also, for the anecdotal evidence.. I was thinking about changing my injectors, but that seems silly. All systems are WAY nominal, it's quick, it's smooth, it doesn't smoke and it idles.

I guess I shouldn't complain too much, I think I remember someone saying the window sticker claimed 21/22, so I'm not too far off. I'll have to try running a tank at 55 mph, and see what that does for me. Hehe... My truck, however, gets about 18 on the highway, too, and is quieter, faster, and has an airbag.

I'm still figureing out my vehiclular 'deal', and I'm thinking the 'benz may be the city car/bridge runner for when I need to take the dog to the GF's house in SF. Then there's the truck for long roadtrips (AC works in THAT one, and the cruise doesn't surge), and the bike for, well, the bike.

How about changing rear diffs?

Wolf, how fast are you going to get 25? Do you know what diff you have? Thanks again, guys!

Ash

driad_98 · #7 04-05-2004, 12:38 PM

Zoon, I'm told the only differences between the NA and the TC versions of this motor are the oil jets below the pistons. They cool the pistons from below. I figure if I limit the WFO application of throttle, I'll be fine. Are you just talking about added heat when you mention the 'stress' a TC adds?

I figure I'll run what I brung till it becomes a problem. Thanks for the insight!

Ash

wolf_walker · #8 04-05-2004, 01:36 PM

Pretty much all driving for the 25mpg figure. I've seen lower crossing the mountains on I-40, but that's it. I got 30mpg once over a 300 mile stretch, going and comeing.

I'm not sure what diff, but they only good highway gears these things ever had was in the 85 model as I recall.

You sure you don't just have a later turbo motor installed in your car? At any rate I would not worry about the turbo added to a NA motor. It's been done countless times with gas and diesel engines and they seem to last reasonably well. VW diesel guys are especialy known for this, and the VW has the same oil squirters and quite a few more additions on the turbo motor for longevity, but the NA seems to take to a turbo and mildish boost just peachy.

driad_98 · #9 04-05-2004, 01:47 PM

I'm not sure, but I do know it's an aftermarket turbo charger, and that I haven't seen anything that makes me ask "What's that?" that might be an ALDA, whatever those look like.

I'll just continue to love my car, I guess, and to revel in the fact that WE get to burn used vegetable oil instead of (in addition to?) dead dinos. Woo!

Cheers!
Ash

#10 04-05-2004, 01:55 PM

Wolf ,arent you running WVO? Wouldnt that account for your great numbers?...correct me if im wrong

kerry · #11 04-05-2004, 03:46 PM

Triple check your odometer. Odometers are notorious for going bad on these vehicles. The first gear that drives the rest is just pressed on to the shaft. If that gear slips as it often does, your mileage will show up as less.

driad_98 · #12 04-05-2004, 03:55 PM

I'm pretty cogent of mileage adn odometers, I've been kinda obsessive about fuel economy for a long time... My trip meter just quit the other day, but the odo still pretty accurate... In any case, I guess another of the lessons this car is to teach me is to just relax and drive.

It's not like I'm getting 12 mpg, or belching plumes of engine oil. I'm looking into a 2.88 rear diff, though they're telling me $325 for a used unit... sound like alot to yous?

Ash

tjohn · #13 04-05-2004, 07:06 PM

Quote:

Originally posted by driad_98

Wolf, how fast are you going to get 25? Do you know what diff you have? Thanks again, guys!

Ash

IMO that's the great thing about the turbodiesel. It seems like running the accessories and at speeds up to around 75 have little or no mileage penalty.

Not sure if the effect is the same in w/ an aftermarket turbo.

I bet if you get everything sorted out you can at least get into the low twentys. Having said that, I've been unable to get mine much over 24 mpg.

wolf_walker · #14 04-05-2004, 07:15 PM

Nope, no WVO for me, YET. I'm stuck waiting till I can get settled somewhere and set up a straining area and such.

If I stay with the MB diesels it's virtualy a must do though, even at 25mpg, I drive so much it's just too much. If I wanted this kinda mileage I'd drive something with a late model EFI V8 and and overdrive trans, not a twenty year old diesel. I'm used to VW's and 45+ mpg, spoiled rotten I guess..

That 2.88 diff would be good, search on here for a post about "easter egg".

I've always noticed great consistency with reguard to mileage in diesels as well. It is a big plus to be able to not censor your right foot and still get good mileage.