1984 300CD poor mpg

[ivandrocco]

Fresh fluids all around, as in changed oil and atf last week. New monark injectors, new return lines, no fuel leaks under the hood. Good tire pressure, valves adjusted less than 5,000 miles ago.

Just got a GPS, and burned 5.88 gallons of diesel driving 100 straight Highway miles at 70-80mph. Thats 17mpg. What gives!?!

I'm checking the valves tuesday, and will check timing then too. What bases am I missing?

[Biodiesel300TD]

6 gallons of fuel isn't really a good test of mileage. Partly because the the fuel station attendant may not have filled you up to the same point. My Jetta will sometimes look like it get >50mpg, and other times it get 35mpg. Most of it is due to how much the attendant fills up the tank. Sometime they try to squeeze in as much as possible and sometimes they don't. I wouldn't worry about your mileage just yet. Run a few tank, and check the mileage each tank.

Also, a side note, winter diesel usually yields lower mileage, though it should only change a few miles.

[ivandrocco]

pumped it myself believe it or not, so i know it was more full after the first fill up than the last. i had to use a big nozzle the first time so it sits further out.

[Biodiesel300TD]

There are too many variables to measure mileage over 6 gallons of fuel. Things like road slope and wind can drastically change mileage. Run a few tanks and see what the results are. Then I'd look at tuning if needed.

If you find that your mileage still sucks after a few tanks, I'd look at brakes dragging and alignment. Don't forget about the parking brake. My wagon's parking brake was dragging and bringing down my mileage.

[LUVMBDiesels]

Exactly.
You need to take the average fuel economy over a series of full tanks to get an accurate idea of what you are doing.
I always get a receipt at the station then punch the numbers into a spreadsheet. I get the average for all the vehicles

the SDL averaged 27+ the Jeep ZJ 22 the 'Burban 15 and so on... I keep a running total of fuel used, price paid, and miles driven so I can see the cost per mile difference between vehicles.
I also keep track of oil and filter changes on the same sheet so I know when I need to get under the cars again...

[rscurtis]

You're not going to get any kind of mileage from a car that was geared for 55MPH while you're driving it 70-80.

[ivandrocco]

thanks for the input guys. I like the spreadsheet idea, i'll start keeping a log. The problem is I don't always have the cash to fill up, and until now I couldn't get an accurate mileage reading without the GPS.

I know 6 gallons isn't enough to get an "accurate reading", but 17mpg solid highway miles just don't seem right- even going 80mph the whole way. I also filled up on top of a half a tank of B5 and threw in some leftover ATF.

A lot of people on this forum say 30mpg is a myth. However...

I talked to John Allen, of JF Allen in Shelburne, MA last week. He's got a really good reputation on this as well as other Benz forums. He says that his 1983 300CD gets 30mpg driving HARD, aka doing donuts and topping out between 110-120mph on the highway. If I understood correctly, the only mods were an older manifold without an EGR, a boost controller, and a well maintained timing chain and valves. He said it even has the original injectors.

Any thoughts?

[Jim H]

Originally Posted by ivandrocco

...17mpg. What gives!?!

What WERE you getting at the same speeds before???

[vstech]

the CD is a tad shorter, and thus less mass to get moving. it's also got a rakier profile that may or may not improve drag... IDK, but I'd bet the average CD out there gets similar to a D or SD which would put it at 24ish. I doubt 80mph runs on an un-maintained or mildly out of tune 84 would hit 30. 17mpg sounds low, but as noted, the figure is hard to quantify. are you basing it on running out of fuel at that distance or did you put in 6 gallons, and drive until your fuel gauge showed the same point? top off to top off? it's REALLY hard to tell when the tank is full, so you could have easily had plenty of room at the top of the tank full of air bubbles. (you did say you used a large nozzel on the first fill up) and a small nozzel on the second? so your first FAST fill didn't top off the tank due to foam, and the second fill with the car nozzel let you top up properly, and it took more fuel to fill.
simple!

IMO, 17 is too low and 30 is too high. As an example, I just measured 25 mpg in my 300D with a fresh motor averaging 65 on highways.

[Zeppo2K2]

First, most of us pump our own diesel, except in OR where it has to be pumped by an attendant.

Start by filling your car, hopefully with a pump you can return to. Record miles from odometer. Then run your car, recording every gallon you add, even if not a fill. When you can, return to your original pump, fill again. Read miles and do the calculation. The greater distance you drive for your calculation, the more accurate the results. The original fill does not go into the calculation.

With this method, I get 30mpg on my '71 220D, manual, no air, no p/s. (I usually get 20-22mpg on the '93 Cummins Dodge with 4WD not engaged, camper off, trailer off, and 330K miles on the clock.) I drive a mix, 20% city, 80% freeway at about 70mph. These old M-Bs are wonderful cars, for their day. But if I wanted the best mileage in the US, I'd get a modern Jetta.

BTW, from what little I know about cars, I doubt 30mpg at 120 mph, even with a Jetta. But if really true, I think I'll go to the showroom, I've never seen one up close.

Ron

[vstech]

Originally Posted by Craig

IMO, 17 is too low and 30 is too high. As an example, I just measured 25 mpg in my 300D with a fresh motor averaging 65 on highways.

how you liking that new metric motor craig?

[ivandrocco]

I've been getting poor mileage all along.

Most estimates i've taken come out around 22mpg. I usually check on longer trips where I fill up before and after, and note how much I burned. Because my odometer is shoddy I usually check distances with Google Maps. Not the most accurate measure, which is one of the reasons I sprang for the gps.

vstech-

the big nozzle I used was so big that it caught on the lip visible under the gas cap. When all was said and done, the tank was so full i could see diesel, nearly overflowing. The second fill up was with a regular nozzle. I'm almost positive it was more full after the first top off, unless your foam theory is correct.

[whunter]

Originally Posted by ivandrocco

thanks for the input guys. I like the spreadsheet idea, i'll start keeping a log. The problem is I don't always have the cash to fill up, and until now I couldn't get an accurate mileage reading without the GPS.

I know 6 gallons isn't enough to get an "accurate reading", but 17mpg solid highway miles just don't seem right- even going 80mph the whole way. I also filled up on top of a half a tank of B5 and threw in some leftover ATF.

A lot of people on this forum say 30mpg is a myth. However...

I talked to John Allen, of JF Allen in Shelburne, MA last week. He's got a really good reputation on this as well as other Benz forums. He says that his 1983 300CD gets 30mpg driving HARD, aka doing donuts and topping out between 110-120mph on the highway. If I understood correctly, the only mods were an older manifold without an EGR, a boost controller, and a well maintained timing chain and valves. He said it even has the original injectors.

Any thoughts?

That is bad information.

I just spoke to John Allen on the phone.

Average driving = 30 MPG, KKK turbo, tweaked injection pump.

J F Allen Co
3 Peckville Road
Shelburne, MA 01370-9643
(413) 625-9256

[ivandrocco]

whunter,

he told me about that car. i thought that was a sedan he was modding, not the coupe.

Maybe i just got the details mixed up, he has a lot of cars. I also spoke at length with one of his mechanics, so I didn't mean to put words in anyone's mouth.