Fuel mileage taking a nose dive - PeachParts Mercedes-Benz Forum

The Warden · #1 08-14-2004, 05:42 AM

Hello!

Haven't really been active lately since I've been so busy with the truck. However, I've noticed a somewhat alarming trend in my car's apparent fuel economy.

I've checked the fuel system for leaks, and haven't found any. Also, AFAIK she's in good shape mechanically, although I'm about due for a valve job.

I keep super-anal notes of when I fuel up, and how much fuel's burned and whatnot. Over the last 10 fillups, these have been the calculated miles per gallon (using a standard formula on Microsoft Excel, so there shouldn't be any math errors):

26.129
25.707
30.408
26.802
25.331
26.120
23.599
22.299
21.149
21.287

As you can see, the first 6 fill-ups are pretty good, and are roughly the average mileage for this car. The third reading (the 30mpg one) is somewhat of an anomaly, as it was taken after a pure-freeway tank on a long trip. The other numbers are from around-town driving, and they're all from the same fuel station, so I don't think it's the fuel, unless they got a bad batch or CA changed something. As mentioned, my valves are due for adjusting, but I don't think that that would make the mileage reading drop that far that fast. Same with an odometer failure in the early stages, although I do intend to find a set of speedo check marks on the road (I'm embarking on another trip tomorrow). The car's performance hasn't changed, and neither has my driving style.

The last time I've seen any numbers below 24mpg, I had a known fuel leak...but, again, the fuel system's in good shape (although the injectors may stand a cleaning as well).

Any ideas? The car may just need a long road trip, and I'm giving her that tomorrow, so we'll see how she goes...

Thanks in advance!

vwbuge · #2 08-14-2004, 06:58 AM

Welcome to the club. My 300SD has been getting 22mpg around town for a while now. Even after I switched the emissions. How many miles on your car? I suspect my car is just getting a little old. That sharp of a decline is a little suspicious.

braverichard · #3 08-14-2004, 07:26 AM

My '84 300SD gets 23mpg under harsh driving conditions (hard acceleration, highway speeds around 75mph). If I drive nicely with fuel economy on my mind (gentle, steady acceleration, highway speeds around 55mph) the best I've ever had was 25mpg. My valves are all well adjusted and I have no diesel leaks. I don't believe the car can do better than that. Plus I have pretty low miles on the car!

R Leo · #4 08-14-2004, 11:09 AM

Check to make sure that your thermostat is functioning properly; an open t-stat could cause a sudden drop in economy like you are experiencing. A slipping odometer gear could produce similar calculations as well.

I'm also suspicious of the quality of diesel from various retailers...something as simple as a 5% drop in cetane would be virtually undetectable until you 'ran the numbers.'

FWIW, SWMBO's car fluctuates between 22 and 25mpg all the time. Her 300D's absolute, all-time best mileage of 26.9 was achieved was on Texaco/Shell diesel, at 75mph with a substantial tailwind and temps in the 60°F region. Marlene clocked an approximate 27.5 (with a 10% reduction in calculated milage made for the fast speedo readings after the tranny conversion) once on the way back from OKC. Here mileage is generally in the 22-25 region as well.

73MB280SEL · #5 08-14-2004, 12:24 PM

If it runs and drives like it used to and it isn't smoking, I'd vote for a slipping odometer. I'd think that the two things that would affect your mileage are a timing change or a injector spray pattern change. I think you would notice either of these.

You might also check to see if your brakes are dragging, but they would have to drag alot to cause this loss of mileage.

Sholin (arms deep in blower motor replacment on my 190D 2.2).

rdetoy · #6 08-14-2004, 05:29 PM

Are you buying the same fuel at the same station each time you fill up? When all fuels were SO expensive a few months back I found myself filling up wherever was the cheapest, which was typically ARCO or Union76 where I live. However, I also noticed a rather upsetting decline in fuel economy. So I started testing the effect of different brands of fuel on my car, and discovered that while I paid (only) a few cents additional per gallon for Chevron fuel, that it repaid me with 2-3 miles per gallon MORE than any of the others. My car runs better on Chevron fuel, as well. Food for thought...

Hatterasguy · #7 08-14-2004, 06:31 PM

Try changing your fuel and air filters, when I did mine my mileage went up. My air filter didn't even look that dirty! Also check your brakes maybe one is draging.

Eskimo · #8 08-14-2004, 11:52 PM

Quote:

Originally Posted by The Warden

I keep super-anal notes of when I fuel up, and how much fuel's burned and whatnot.

I don't have a clever idea about your fuel economy mystery, but I'd like to point out what a great habit (in my opinion) this is to develop. I'm not in general a maniac about this sort of stuff, but a few years back I started keeping a small spiral bound notebook in the glovebox of each vehicle, for the purpose of recording fuel fills, oil changes, maintenance, repairs, etc. It only takes a few seconds to record that sort of information, and it can be really, really useful later.

I know, I'm preaching to the choir here, but there might be some in the congregation who haven't gotten this particular sort of religion yet...

-- eskimo

P.E.Haiges · #9 08-15-2004, 02:15 PM

Warden,

You can use the mile markers on interstate highways to check the odometer.

If you check for 100 miles or more, your results will be very accurate.

P E H

The Warden · #10 08-15-2004, 08:59 PM

I may have found a possible culprit. The a/c compressor clutch decided to partially come apart yesterday while on the trip. No damage was done to any other part of the vehicle (the clutch didn't separate completely), but the compressor will no longer work. I'm going to start a separate thread on that in a minute...but it may have been somehow putting an extra drag on the engine before it failed. That said, though, I use the a/c regularly (and am sweating like a pig without it!), so I'm not sure that that's all of it.

I didn't have a chance to do a mile-marker check (too much traffic on the road), but after making this trip (I'm in southern California for a week, at the moment) that I make on a regular basis, the trip odometer had about the same reading that I would expect. I still need to do a proper check, so I'm not discounting the odometer, but I wouldn't call that the first suspect.

I've got 255K miles on the car...but, again, given that the drop was that fast, I don't think that this is due to the engine wearing or anything to that effect. Also, I've been getting fuel from the same station (except when on the road) for quite a while. It's a Rotten Robbie, and happens to be the cheapest fuel in the area...but I've been told (by, among others, fuel tanker drivers, so I have good reason to believe this) that Rotten Robbie adds a fuel additive to their diesel tanks to increase lubricity. So, while inexpensive, I wouldn't call their diesel fuel bad (although I wouldn't even joke about buying gasoline from them).

My thermostat's fairly new; was replaced last December. Unless the temperature gauge's lying to me, it's working properly.

Air filter's a possibility, but again, why would it change this drastically this quickly? Fuel filters shouldn't be affecting fuel economy a much as they would affect the power level.

I'd love to hear any more ideas...thank you all

And I'll post back with results of testing the odometer as soon as I get a chance to do so.

#11 08-16-2004, 01:36 AM

I've noticed that ambient temperature has alot to do with the fuel economy on my 300CD. In the winter it gets horrible mileage, but it also has alot to do with winterized fuel, in the spring and fall it seems to get the best mileage, and it the summer it doesn't get as good of mileage, but better than it does in the winter.

I have also noticed tire pressure can make a huge difference in fuel economy, so I try and run my tires at around 35 psi, and it helps alot...

#12 08-16-2004, 10:32 AM

Have been tracking the mileage very carefully with my 81SD, since I started driving it 112 miles/day. The fuel economy ran between 23.5 and 25.3 mpg last winter, and increased to 25.5-27.2 this summer. During the unseasonably warm days we have over here, it actually gets to 85F. Incredible.
That is all driven at 60-65 with the cruise contol. I think the diesel just runs better hotter.

The Warden · #13 08-27-2004, 07:07 PM

Belated update:

Looks like the a/c compressor was for-sure the culprit.

More fuel mileage numbers:
21.287 (last one that I had in the earlier list)
21.697 (the a/c compressor clutch died about a half mile before I refueled and got that reading)
26.703 (first fueling after a run with the a/c not running)
31.206
25.065
25.908 (today)

I'm thinking that my compressor has suffered from Black Death syndrome, so it looks like I get to pay a shop $$$$ to clean out the system

Maybe I can get a parallel-flow condenser on there while I'm at it?