Exceptional gas milage or a fluke? - PeachParts Mercedes-Benz Forum

unkl300d · #1 10-06-2002, 11:43 PM

Today, I filled my 300D up, 101.3 miles after the last fill up.

It took 2.027 gallons until it was overfilled.

Do the math.

You see , I recently had the injectors changed and I was taking the MPG to compare with past performance.

After 207 miles I filled it up and calculated 16.41 MPG.
Looked under hood and saw that the end cap (injector by pass hose) was leaking.
Got that fixed, filled up and ran another 125.3 miles. Filled it up again and calculated 17.1 MPG.
Still below my past averages.

Between this last fill up and today's I observed the above
data.
My odometer concurs with the trip odometer.
My notes concur also.
The tank was filled to the brim, concurred by the fuel gauge above the full mark.
Even if I were able to squeeze in another gallon, that would be
33.7 MPG.

The driving scenario was mmostly freeway combined with some city driving.

Too good to be true.
But I share the anecdote.

I will continue to observe the future fuel usage.

BillGallagher · #2 10-07-2002, 01:22 AM

The best way to check MPG is use the mile markers on the highway for several miles to check your odomo and head to the diesel station. When you refill, have the fuel level three inches from the top. Record amount of gas.
Go to highway markers again, and then go to a different station, fill up again.
Use you calculator on the your computer to calculate MPG. The figures should be the same. Post results on forums..

Bill

the_good_fellow · #3 10-07-2002, 01:34 AM

My 300SD is getting 30+ MRG in normal 60/40 hwy/city driving Lately. I don't know why. I used to fill up every 7-8 days, now every 10-11 days

Here are something I've done:

Back in Jan I installed a home made Boost Controller, and disconnected the EGR. I got about 23/24 MPG.

In March I adjusted the valves(most of them were very tight) and Oil changed. I got 24/25 MPG.

At the end of April I installed a K&N AF and modified the blow by oil collector to prevent too much oil going to the AF. Reconnect A/C wire(has it disconnected forwinter) and the A/C Comp locked up. Since then I haven't driven it that much(mostly city in the evening)but I have noticed each tank I got about 27 MPG.

In AUG I finally got a new A/C comp installed so i use it more often. And every tank I got 500+ miles with 17+ gal fill up.

my last two fill ups:
17.35 G and I got 535 miles ~ 30.84
20.20 G and I got 624 miles ~ 30.89

My boost is set at 14psi and It's 15" 205/65 tires before all these.
what's the mileage error with over size tires??? can someone help figure this out?

janko · #4 10-07-2002, 06:52 AM

http://www.miata.net/garage/tirecalc.html

Cannon · #5 10-07-2002, 12:02 PM

My last 5 tankfuls have given me an average of 28.5 mpg. That's city and highway. I've yet to go on an all highway trip. Top speed for me is 65 mph.

stephenson · #6 10-07-2002, 01:49 PM

I have had two 300Ds and a 350 SDL ... very little difference between highway (usually 75-80 mph = 26 MPG) and mix of city/highway commuting miles (=24-25 MPG) ... I am sure I could get 30 MPG, but would need to maintain 55 on the highway to do it.

As far as getting above 32 or so in a 123 or 126 ... nah. Likely to be some foaming that is causing premature fill indications.

Do the math over about ten tanks and see what it is ... should average out any errors.

#7 10-07-2002, 02:21 PM

I agree with the above post. My 300SD does 24mpg with mostly city driving and a/c on, 26mpg on highway in cool weather. And there is a definite drop off in fuel efficiency when driving over 80mph for long periods of time, so 55mpg would probably get 27, maybe 28mpg. And I mostly fill up at the same station, always fill until I can see the diesel so it is about the same place, and the numbers are really consistent. So, no way does any stock 300SD get over 30mpg.

Robert W. Roe · #8 10-07-2002, 10:49 PM

Man, I want one of these 30 mpg diesels!
The two gallon 50 mpg figure is a most likely a statistical fluke.
Been there done that.
The car mags average out fuel consumption over a full year.
I started keeping an Excel spreadsheet tracking my fuel purchases and was averaging just over 20 mpg.
I haven't kept up with my data entry, having just bought a house, and trying to sell my old house. Maybe after I have some $$$ I might try to tune up the old SD, or get a newer car of some type, maybe a TD wagon.

unkl300d · #9 10-08-2002, 12:30 AM

I suspected back pressure causing a 'false fill up'.
It was not foam because I waited and pushed the rear up and down to shake the fuel down.
Nontheless, I suspect something fishy.

The odometer is fine and the fuel gauge seems fine, but I still think it is a fluke.
Yes, I'll see over time what happens.

I can get 25MPG on all freeway driving as well.
So, if I would have been able to squeeze in two more gallons
beyond the 'apparent' fill up, then it would have panned out at 25MPG.

We shall see.............

Robert W. Roe · #10 10-08-2002, 12:39 AM

Oh, with respect to the tire size question, I remember calculating this once before for 205-65/15 vs 195-70/14 and the 15's were about 3% larger, so 300 miles indicated is really 309 or so. Or 20 mpg is really 20.6 mpg.

angst · #11 10-08-2002, 02:16 PM

Quote__________At the end of April I installed a K&N AF and modified the blow by oil collector to prevent too much oil going to the AF. Reconnect A/C wire(has it disconnected forwinter) and the A/C Comp locked up. Since then I haven't driven it that much(mostly city in the evening)but I have noticed each tank I got about 27 MPG_______

Describe your oil blow by tube mod.

I did an intake mod that some might find interesting. I have a K+N but I did not have the car long enough before doing this mod to get a mileage baseline. The bigger intake did help the pickup up at low speeds.

http://kayakoutfitting.com/mercedes/intake.html

Eric z

#12 10-08-2002, 03:28 PM

I always get 22mpg with mine, no matter what I do.

stephenson · #13 10-08-2002, 03:54 PM

Just FYI, but seems like I recall that free flow air filters will make a difference ONLY at full throttle ... maybe it was Pat Goss? So, if you drive at full throttle only occasionally, it likely is, again, on the margin (admittedly, we drive at full throttle more often in diesel cars than in gas power cars).

CJ · #14 10-08-2002, 10:16 PM

I agree, the free flow filters will not really make a noticable difference on a MBZ diesel engine.

the_good_fellow · #15 10-09-2002, 01:22 AM

I agree with the mpg(23/28 city/hwy) most of you're getting. That's what I used to get since I bought it six years ago. It's still a puzzle to me at this point. It's the 1st time I got over 600 miles a tank. I'll keep you guys update on next fill up.

BTW 300D gives me 25/34 (K&N doesn't help much, may be some mod.) and 300 SDL gets 23/27

Eric,

To reduce the blow-by you will need to take the air filter cover off. remove the lid of the oil collector/separator where the breather hose connected to. If my memory serve the tube is about 1~2" deep and it's blocked by a little fin at the end which prevents the oil from free fall. it may splash all over the place too if you rev eng. I removed the fin and connect a high temperature oil hose to the tube and secured it with a clamp. the other end of the hose is barely touching the drain hole. that way oil can drip directly into the drain hole and the vapourize oil can escape. I took the elbow hose off this evening and there is no more oil near the turbo inlet.

oil in compressed air does reduce the amount of oxygen molecules intake which reduces the efficiency of the combustion.