I recently (unintentionally) hijacked another thread about W210 E300 models. Mine was getting decent fuel mileage on the highway -- close to 30mpg on B99. Around town, it was doing pretty poorly, sometimes as low as 15mpg (yes, properly measured!).

I wasn't having any fuel drips on my sealed garage floor, but some of our good experts here tell me that it could have gotten trapped in the belly pan and then blown out once the car was up to speed. Some of it may have burned off or something, too.

Several people pointed out that the braided fuel return line is a weak point. I took the plastic engine cover off today, and the second injector in (from the oil fill cap) was sitting in a pool of fuel. All the others looked pretty dry to me.

Today I bought four feet of replacement line (viton, I believe, with a black braided cord over it). On removing the old line, several sections broke -- it was pretty dry rotted or something. I pulled the remainder out with needle-nose pliers. Lots of fuel leaked while I had the old hose off, until I got the new hose put on. I mopped up the best that I could, then ran the engine for about 30 minutes. No visible leaks, just the fuel I wasn't able to mop up.

So do we think this will solve the mileage problem? I'll be keeping a close eye on this tankful, as I'm mostly going to be in town driving for the next week or two. I still need to put the plastic engine cover back on.

If that one line was my leak, where was all the unburned fuel going? Dripping out from the cover somewhere and burning off? Boiling off? Seems like it would have to lose a lot of fuel to get my mileage down that low.

Anyway, sorry for the long post. I appreciate the tips from you guys, and I'll try to post an update after I drive a hundred miles or so and fill up again. I'm hoping for better results!

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

While I am not certain this could account for your abismal mileage around town it certainly doesn't help having leaky return lines so it was time and money well spent fixing it. Typically if that last one, the one with the plug in it leaks or the plug comes out you can see lots of fuel leaking - more like spraying all over the back of the engine. Since you at least found a leak you know you had some fuel being lost.

I also suggested trying another source for fuel as the quality of bio-d can vary greatly even if the source claims to anhere to industry standards.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

I "think" Marty may have also suggested running some D2 for a couple of tanks and doing a comparison whcih I would agree with.

Mine gets 31 mpg burning D2 and 27-28 on B99. Burning B99, I hear my turbo a LOT more than I do when running D2. In my mind this means I am burning more fuel to achieve the same driving style I have when running D2. My commute (110 mi/day) is pretty consistent day to day and works out to 20/80% city vs highway. My feeling is that if I burned an entire tank of B99 in city driving conditions that I might get as low as 20 mpg on B99 due to a heavier foot.

You may be more enthralled with your turbo and kickdown switch than I and therefore are achieving even less mpg than I???

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

Yeah, I suggested trying a couple of tanks of dino-d and seeing how it goes. I have never used any bio-d in mine so I can't comment about it but know the quality varies quite a lot.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

You must have had a pretty bad smell of unburnt diesel fuel when walking around the car after a short ride in town?

That's how I diagnosed the proble on mine. A clean diesel engine has no smell of unburnt fuel. It it does, it means it's leaking somewhere.

I had the same problem with the fuel return lines. But while working in the fuel line, I did them all around the fuel pump as well (although the o-rings are usually the only source of the problems). My guess is that I had a leaking o-ring on the electric shot-off unit on the IP.

Anyhow, hope things go for the best for you now.

Bye

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BigBen

'98 E300 Turbodiesel 244 000km
RIP '92 300D 2,5 Turbo 632 859,4km due to engine failure

Thanks for the comments. The car seems to be working fine after my return line replacement project yesterday afternoon.

I'll consider the D2 suggestion -- I'm not ignoring it, just doing one thing at a time. Right now the fuel tank is brim-full, so it'll be awhile before I refill. Also, I bought this car and swore to myself that it was only to run B99 -- that was really the whole point. I'm not promising which way I'll go on that just yet. Eventually I hope it will run on WVO -- just haven't gotten to that project yet. I currently buy only commercially-produced ASTM certified B99, no homebrew stuff. None of my friends using the same two suppliers report problems, but I'll keep that in mind.

I don't think this is a function of liking the turbo or kickdown too much. Best I can tell, I've only hit the kickdown once in two months of ownership. When I'm in the cabin, I essentially can't hear the turbo. I can tell (from the butt dyno) that boost rolls on around 2500 rpm. Most of my in-town first-to-second gear shifts seem to come between 2500 and 3000 rpm -- it's a big city with plenty of traffic and lots of stoplights on my short commutes. Second-to-third and beyond tend to come at lower revs. My highway driving tends to average around 70-75mph, or whatever the lower limit is, and the car seems to steadily turn about 2700rpm to maintain that cruising speed. I don't seem to have a fuel mileage problem (or at least a dramatic one!) while doing highway miles -- just during stop-and-go driving.

I know that my monster BMW gas V8 gets driven in a much more sporty manner than this car, produces three times as much power, and will nearly match the diesel for in-town gas mileage (and actually gets 25mpg highway too, now that I think about it...). Something more than a leadfoot has to be wrong here -- I ought to be able to turn the E300 at 4000rpm all day and still get better mileage than the Bimmer.

I'll keep an eye on it this week and see how I'm doing mileage-wise. Thanks again.

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

For what it's worth I gained a little over 5 mpg just off replacing the return lines on my 300d. It may not look like they were leaking that much, but under partial throttle, when lots of fuel is returning to the filter, those lines can be really dripping. I wish I had video'd it.
-nB

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'83 300D Turbo
Current: ???K mi - 19.2mpg -> 17.4mpg -> 22.9mpg ---> ODO Died
bought at: 233.8K mi - 10MPG For $1.00
3.5 cylinders work: 320 320 100 340 280
Got insurance? FarmersReallySucks.Com

Theres more than just fuel leaks to worry about. Tyre pressures, sticking calipers, bad wheel bearings, bad differential, propshaft issues, gearbox wear, torque convertor inefficiences, inefficient combustion......the list goes on

Parrot makes good points. The "hand-spin" test indicates that none of my brakes are dragging. I do have an oversize wheelset (18" rims) from the prior owner, plus wider-than-normal directional rubber. I'm looking to get back into some stock 16" wheels (maybe 17) and some good all-weather radial tires. I always run proper-to-high pressures in my tires, so I figure this change alone might pick up my mileage just a little bit. Inefficient combustion I can't say for sure...but I'm not smoking out the tailpipe or anything like that. No odd vibrations, etc.

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

If you are running 18" rims on this car I can almost guarantee you that your odometer and speedometer are readings low and therefore you are actually covering more distance than the odometer is reading. You need to first compare the circumference of the tires you're running with the stock 215/55/16 to know what the conversion should be. This would explain at least some of the problem.

I found this page helpful. http://www.5speedtransmissions.com/calculators.html

The stock tires have an 80.7" circumference. When you get the circumference of the tires you are running divide it by 80.7 to come up with the correction factor.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

That's interesting. Does that mean if you have tires with a larger outer circumference than stock you will be traveling further than your odometer says?

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Absolutely.

The odo/speedo is set up only to meaure revolutions of the wheels and converts that to distance based upon the stock tires being mounted. Anything different than that will produce errors in both the speedo and odo readings. If you use larger tires with a greater circumference you will travel further on each revolution, but the car doesn't know how far you have travelled only how many times the tires have revolved.

BTW-Anyone with a GPS navigator can easily check the accuracy of their speedometer as most provide this as a function. On my '98 the speedo is accurate to within 1 MPH at all speeds I tested it at.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

The tires on the car are 235/40/18, all the way around (non-staggered). This is indicated as an acceptable "Plus-Two" size on the Discount Tire Direct website. When traveling distances I know very well, the odometer seems to indicate within +/- 1% or so of what I expect it to say. Speedometer also seems to register accurately. So I don't think this is a huge part of my problem.

On the other hand, the weight of the wheels and rotational mass and softness of the Nexen 3000 rubber could all be playing in as factors against good mileage as well. They're nice CLS-style replica wheels, five-spoke design, etc., but they're not to my taste. Gotta replace them and sell this set. I e-mailed the Luke guy on the forum about that more than a week ago and never got a response. May just buy locally since I don't see factory wheels or anything particularly inspiring online anyway.

Thanks for the tips... If I'm way off on this, let me know. It wouldn't be the first time.

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI

Actually those 18" tires are about 1% smaller than the 16" ones so if anything you'd be off the other way, but as you noted it is an acceptable margin. I have seen some with 10-15% larger tires than stock suddenly wonder why their fuel economy drops and then they figure it out, but in your case it looks like the trouble lies elsewhere.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz