Fuel Economy - PeachParts Mercedes-Benz Forum

zsmith29 · #1 03-16-2008, 07:20 PM

I am getting poor mileage in my 84 300D. I have heard and read about the importance of a clean banjo bolt. Is the bolt that I need to keep clean the one that the fuel filter spins on to?

Also, what else can I do to make sure I am getting all that I can get outta my MB.

imagesinthewind · #2 03-16-2008, 07:36 PM

What are you getting now?
Could be you're thinking that they get better mileage than
they really can.

What I do:
*Coast when I can.
*Take my foot off the gas long before I need to brake. Allow the car to slow itself.
*Don't floor it from a stop. I know it's already slow, but those behind you can wait the extra 5 seconds it takes to go 25 mph without flooring it.
*Stay at speed limit. No more 70-75 mph. Leave 5 minutes early so you're not in a hurry to get places.
*Fill the tank at the half way point instead of allowing it to go to empty.
*Use an injector cleaner when needed.
*Do a purge to get the gunk out that's there now.
*Check for any fuel leaks in the lines and around the injectors.
*Check tire pressure and keep it at optimum.

And don't expect the car to get 30mpg.

GRIESL · #3 03-16-2008, 07:54 PM

Quote:

Originally Posted by zsmith29

I am getting poor mileage in my 84 300D. I have heard and read about the importance of a clean banjo bolt. Is the bolt that I need to keep clean the one that the fuel filter spins on to?

Also, what else can I do to make sure I am getting all that I can get outta my MB.

The banjo bolt is the fitting in the back of the intake that connects via a vacuum hose to the ALDA on the injection pump. I'm just guessing here, but I don't think a clogged up banjo bolt is going to cause poor fuel economy, since it assists in causing the IP to turn up the fuel under load. Seems to me that you'd just be driving around as if the car were naturally aspirated.

I would look at other causes. What is your fuel economy? Especially with winter fuel, I don't think you should expect any more than 26 or 27 mpg.

Skippy · #4 03-16-2008, 09:26 PM

Yes. How bad are you getting? Keep in mind that mileage varies greatly depending on speed and driving conditions. City mileage as low as 20 would not be out of the question if you have a heavy foot. OTOH, if you have a light foot and keep your cruise speed down and do mostly highway, you should be able to touch 30 mpg.

Things that can help:

1. Clean fuel filters, and tank screen
2. Valve adjustment
3. Properly functioning thermostat
4. Injectors in good condition
5. Proper injection timing
6. All the stuff Ginny just said

zsmith29 · #5 03-17-2008, 12:09 AM

Ok, Sounds good. I don't know why I thought that the banjo bolt was critical to fuel delivery

Ara T. · #6 03-17-2008, 12:13 AM

The 300D Turbos are easy to drive economically in town, they will happy shift at 2000-2400 rpm in each gear so you dont have to rev them very much. If you find the performance lacking in that rev-range and feel you must rev it a lot to get it going then something is wrong.

That said, don't expect much more than 22 or so mpg in town. They are big heavy cars with 3.0 liter engines and do ok for what they are and what they were meant to be.

zsmith29 · #7 03-17-2008, 12:22 AM

I need to get my vac issue delt with b/c I have hard shifts that rev some b/f shifting. I do not exactly know my mileage but I think it is around 23 on the interstate.

t walgamuth · #8 03-17-2008, 06:25 AM

I never could get as good as 20 in town with mine. And low to mid twenties highway was what I found. High twenties for a turbo motor was never in my experience.....to get that kind of mileage you need to drive 65 highway or less or have a stick conversion.

Tom W

#9 03-17-2008, 06:27 AM

Quote:

Originally Posted by zsmith29

I am getting poor mileage in my 84 300D.

Good or bad economy is mostly subjective. What is "poor mileage" to you? Anything between 22-28mpg is normal.

Dana B. · #10 03-17-2008, 06:53 AM

Quote:

Originally Posted by imagesinthewind

*Take my foot off the gas long before I need to brake. Allow the car to slow itself.

*Fill the tank at the half way point instead of allowing it to go to empty.

I'm curious about the above two points.

The first suggests that when stopping, engine fuel flow with the foot off the pedal is affected by the rate of decelleration. This doesn't make sense. The engine should get the same mgp whether you're coasting to a stop, or have locked up the brakes.

As to whether a half-full tank gets better mpgs, it would seem that the reduced weight from less fuel would increase mileage, hence it would be better to fill up at empty.

Ara T. · #11 03-17-2008, 07:06 AM

If you take your foot off the gas the RPM go down do they not? As opposed to barreling along at 2000 RPM the RPM is now 1000 and dropping.

JimFreeh · #12 03-17-2008, 07:30 AM

Quote:

Originally Posted by Dana B.

I'm curious about the above two points.

The first suggests that when stopping, engine fuel flow with the foot off the pedal is affected by the rate of decelleration. This doesn't make sense. The engine should get the same mgp whether you're coasting to a stop, or have locked up the brakes.

If you use the brakes, you're scrubbing off speed that cost you additional fuel to maintain during the period when you *could* have started your decelleration by taking your foot of the accelerator.

Perhaps not a huge MPG booster, but it all adds up, and this habit saves the brakes, and there is less wear and tear on the car.

As I tell my wife, "No matter which pedal you push, it cost money each time you push it.".

I'm a confirmed believer in coasting whenever possible, and my commuting MPG in my 83 300D is a consistent 26-27mpg.

Jim

zsmith29 · #13 03-17-2008, 09:41 AM

Ok I suppose I have just heard these numbers like 28 and 29 and I just assumed that I should be getting those to b/c it is the same make car.

Ok, I'm gonna throw a new one in there. I recently changed my rear brake pads and I noticed that when I sit on a slight hill and the tranny is in N the car does not roll down the hill like it used to. Is there anyway that I did something wrong when I replaced the rear brakes and they are adding friction to the rotors?

t walgamuth · #14 03-17-2008, 09:48 AM

If the're are dragging they should generate heat. Check for that.

Tom W

imagesinthewind · #15 03-17-2008, 11:13 AM

Quote:

Originally Posted by Dana B.

I'm curious about the above two points.

The first suggests that when stopping, engine fuel flow with the foot off the pedal is affected by the rate of decelleration. This doesn't make sense. The engine should get the same mgp whether you're coasting to a stop, or have locked up the brakes.

As to whether a half-full tank gets better mpgs, it would seem that the reduced weight from less fuel would increase mileage, hence it would be better to fill up at empty.

It could be just me and my car but I get 250+ miles on the top half of the tank. If I wait and fill up at empty the total miles are usually in the 480 range. When I fill at half tank I'm putting in around 9 gallons, getting better than 27 mpg. When I wait and fill from the reserve light I get around 24mpg for the tank. Don't know why. But I now never let the tank get below half.