Fuel Economy 101 - Mercedes-Benz Forum
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post #1 of 6 (permalink) Old 02-04-2004, 09:24 AM Thread Starter
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Fuel Economy 101

Many members have written about bad fuel economy and I am one of them I would like to target this issue and resolve it once and for all. I have gathered bits and pieces from other posts and the net to compose this thread. I would like to attempt to compile a check list with the help of all of you that everyone can contribute to, and use to help resolve each of our particular fuel economy related issues. I am looking forward to participation from everyone who has some insight. Also, maybe we can all post our gas mileage to compare. This is a general compilation and can apply to vehicles other than Mercedes.
Thank You


The fuel system provides fuel storage in the vehicle, filters out particulates, delivers fuel to the intake air system, and mixes the air and gasoline in the correct ratio to provide the most efficient combustion (i.e., the highest fuel efficiency) in the engine.

Making sure your flow of gas is not blocked is vital to getting the most mileage for your money. Optimal use of your gasoline is helped by fighting the accumulation of performance robbing deposits in the fuel system. Today's modern engines are more susceptible to deposit build-up than ever before. Deposits have been found to start forming in as few as 1,500 miles. Studies have shown that deposits can decrease your fuel efficiency by as much as 11 percent.

The list:

1. One main factor for decreasing fuel economy is a partially blocked exhaust . As catalytic converter undergoes wear and tear, they may collapse internally, leading to restricted exhaust from the engines.

2. Dirty air filters can also cause your engine to run at less than peak efficiency.

3. Malfunctioning brakes can significantly increase fuel consumption because the vehicle must work harder to overcome the resistance.

4. Check for misaligned tires , which will drag instead of rolling freely, leading to increased fuel consumption and causing problems with the car's handling.

5. New spark plugs alone can increase fuel economy by 3%.

6. The Positive Crankcase Ventilation or PCV valve regulates the flow of unburned gases from the crankcase back into the combustion chamber, to be re-burned. The valve helps maintain correct fuel economy and idling, and prevents oil and gases from accumulating in the air filter housing.

7. Fuel injectors are manufactured to operate and last for 150,000-plus miles. Because of their significance on emissions, some vehicle manufacturers warranty injectors for five years or 50,000 miles. However, over a period of time, harmful deposits can build up around an injector nozzle. Deposits can also build up inside the injector or clog the injector filter basket and reduce the amount of fuel being delivered. When the fuel delivery decreases, the injector pulse width will increase, creating additional heat in the injector. A leading cause of this is short-drive cycles. Short-drive cycles with repeated temperature change create fuel diffusion. The lighter gases evaporate and the heavier particles of the fuel settle at the tip of the injector. Engine heat then bakes the heavier particles, making them hard deposits. The deposits can clog an injector, reducing the volume of fuel delivered or distort the spray pattern. When the fuel delivery is out of spec, driveability problems exist because the powertrain control module (PCM) is unable to maintain the proper overall air/fuel ratio. Some injectors may be commanded by the PCM to go richer or leaner depending on if the problem injectors are clogged or leaking.

8. The thermostat plays an important part in obtaining good fuel economy. Temperature sensors on the engine tell the fuel injection computer to inject more fuel when the engine is cold. A faulty thermostat will prevent the engine from reaching normal operating temperature so extra fuel is always being injected. If the temperature gauge is reading low or the car heater is not putting out hot air, have the thermostat checked immediately. Your fuel economy will increase.

9. Change Your Fuel Filter . Your fuel filter removes contaminants including rust and corrosion from the fuel before it enters the fuel injectors or carburetor.

10. Oxygen sensors "may" need to be replaced. The codes are 41 and 42.You can check your Oxygen sensors with a (DVOM) Digital Volt Ohm Meter.
Common symptoms of a worn out sensor include excessive fuel consumption, high emissions, engine surging or hesitation, or premature failure of the catalytic converter. When examining the sensor, a shiny deposit on the sensor's heat shield or any gummy deposits indicate it's time to replace the sensor and determine the root cause of those deposits.

Inoperative or faulty sensor signals can result in the ECU causing engine backfire (due to a rich mixture), rough idle quality and lean condition misfire. Also, bear in mind that a “faulty� oxygen sensor data signal may actually be caused by malfunctioning component(s) of other systems such as the EGR (Exhaust Gas Recirculation) system. Also included are plugged fuel filters, vacuum leaks, inadequate (too high or too low) fuel pressure, fuel injectors leaking gas or air, leakage on the exhaust system or other related parts. Do not automatically assume that the oxygen sensor itself is the root cause of the sensor code. Remember also, a degraded oxygen sensor does not always set a code. If the sensor is replaced, a future sensor code or engine damage may be triggered again if the root cause is not corrected.

11. Exhaust Gas Recirculating (EGR) system .
Here is a good link: http://www.autoshop101.com/forms/h61.pdf

12. Throttle Body inspection and cleaning.



Some of the members have recommended getting some
BG 44K Fuel System Cleaner.



I already bought some Liqui Moly Jectron, Liqui Moly Ventil Sauber and Liqui Moly Pro-Line Fuel System Cleaner, because it is available locally and the BG44k is not.
If the Liqui Moly doesn't work, then I will try the BG 44K.

The liqui-moly site is :

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post #2 of 6 (permalink) Old 02-04-2004, 11:15 AM
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RE: Fuel Economy 101

Philip: You forgot one of the most important element.
Proper driving techniques can easily decrease fuel consumption by 15% to 20%.
Avoir jack rabbit accelerations
Avoid full throttle
Coast down to a stop.
Do not let the car idle for more than 3 minutes.
Let the car slow down a bit while going uphills.
Get your left foot off the brake pedal.
Slow down
Get your weight down, get rid of that unnecessary junk in the car or in the trunk.
Keep all window glasses closed at highway speed.
A few years ago, we had an "economy run" where we would loop a 85mi drive using our regular driving technique. MPG was then calculated: Mine was 28.4mpg.
We were then asked to do the same loop, but this time, using our best knowned techniques to save fuel.
This time, my mpg went up to 33.4. Same car, same road, same distance, same weather conditions, same load.

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post #3 of 6 (permalink) Old 02-04-2004, 07:24 PM
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RE: Fuel Economy 101


Thank you very much for taking the time to put this together. Very interesting reading.


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post #4 of 6 (permalink) Old 02-04-2004, 10:12 PM
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RE: Fuel Economy 101

increasing your tires diameter can also aid in fuel economy if you do a lot of freeway driving.

as can increasing the tire pressure. however this can lead to premature/ irregular wear if done overzealously.

and while drag race starts are certainly not the way to go, getting up to cruising speed as quickly as possible is a good way to increase your economy.
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post #5 of 6 (permalink) Old 09-26-2004, 02:57 PM
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RE: Fuel Economy 101

I agree with neanderthal. Now I have 215/60 R 15 tires on my car, and I used to get noticably better mileage with the old 205/65 R 15 tires I had before. (205/65R15, when mounted on 15" rims <38.1mm> is both about 10mm narrower and about 8.5mm larger in diameter, thus having less resistance and traveling a further distance for the same number of revolutions, which translates into more fuel economy in longer distances, though with a little more sluggish acceleration.) Now I'm seriously considering dumping the 215/60 Falken Ziex ZE-512's and refitting 205/65 Hankook Ventus HRII H405. The Falkens are not very comfortable for my taste, though they are so sticky that I only had the ABS cutting in once or twice during my ownership of the car!

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post #6 of 6 (permalink) Old 09-26-2004, 06:49 PM
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RE: Fuel Economy 101

Good information, good thread. I'm not going to make this sticky at the top too avoid congesting too many threads at the top, but I highly suggest everyone searching this thread before they go making a post about having bad fuel mileage in the future. Fantastic read. I think this should serve as a good check point for everyone, do a check up soon on your car and run down this list.


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