I am a recent owner of a 2002 C240. I have to say that I love it. there is nothing like driving a Mercedes Benz. I do have 1 complaint. My car burns gas like you would not believe. I read through the manual and it says ONLY use 93 Octane Gas(the good stuff). No problem, but when I use the high stuff, I find myself filling up 3 days later. Would it really hurt my car if I used the "middle" grade gas? Where I live the average price for 93 is $2.28/gal. The middle grade is averaging $2.09/gal.
Will the middle grade hurt my car? Is it some Mercedes Benz law that states that you can only use 93 octane in your car?

SOMEONE PLEASE GIVE ME SOME INSIGHT.

Signed,
I'm off to the gas station again

If you do a search on " octane ", etc., you'll find enough reading to keep you busy for 3 weeks.
The best part is, while you're reading you're saving a whole lot of gas.

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2007 C 230 Sport.

Most "premium fuel" cars built since the mid 1990s have a detonation (or knock) sensor(s) that will automatically adjust the timing to the fuel's octane rating. In some cases this may result in a detectable loss in performance, particulary low end torque. Most owner's don't notice a loss of performance with lower octane fuel, but it would be more likely on a boosted than a naturally aspirated engine.

If you want to experiment, here's how. When the tank is near empty, pump in about 5 gallons of regular unleaded. TURN OFF THE STEREO and listen for detonation. If none occurs, install another five gallons when it gets near empty again and continue the test. If you do not detect detonation, you can continue to use regular unleaded. If you do detect significant detonation, you can fill up the tank with premium. (No one who has ever done this experiement at my recommendation on a car less than ten years old has ever reported detonation.)

There is some variation among manufacturers, but most have multiple timing maps stored in the engine control electronics. The default map is set up for the recommended fuel, but if significant detonation is detected, the system will switch to a less aggressive timing map, and the knock sensor will continue to monitor for detonation and adjust timing as required.

As as example, the '05 Corvette LS2 engine has a 10.9:1 compression ratio. The fuel requirement is: "91 octane recommended but not required". Most manufactureres have gotten away from the "severe engine damage" warnings if you use less than the recommended octane because the engines are protected by knock sensors.

On pre-knock sensor engines, such as M102/103 you can usually use less than "premium" fuel, but you may experience some transient low rev detonation, especially in warm to hot weather or at high engine temperatures. It's possible to "drive around" the detonation by shifting at higher revs to avoid high engine load at lower revs or the timing map can be altered to something less aggressive by swapping EZL trim resistor. Detonation is typically more likely at low revs than high revs.

Duke

Thanks Duke! I will try it and see what happens. I know they say a Mercedes is a rich mans car, but DAMN! Filling it up should not put you in the poor house.

Thanks again.

Ha these posts always pop up as the price of fuel goes up.

Sure you can run 89 but your mileage will drop off. Newer cars computers will retard the timing and it won't knock. The manual says 93 for a reason.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Oh, come on, it's a C-class. What about all the bozos and bimbos who are driving Suburbans, Yukons, Denalis, Excursions, Expeditions,... and all the Japanese clones.

And you think YOU'RE spending a lot for fuel???

Of course, as far as I'm concerned, all the above mentioned "b&bs" are getting their just deserts.

Maybe this runnup in fuel prices will finally get people to buy rational vehicles, but the ones with the gas guzzlers are going to find that their trade in value is zilch. Tough sh.t - serves 'em right for driving "stupid ugly vehicles" that consume inordinate quantities of a limited resource. Of course, we're all going to pay the price for their greed.

Duke

Sadly, the only really thrifty cars Benz makes are the diesels.

If you are getting 25 mpg, you are doing well. If you are NOT, however, I would check for

Bad coils/wires/plugs (you still have to replace even platinum plugs eventually)

dragging brakes (high hard brake pedal, jerky stops all the time?)

bad alignment (usually with excessive tire wear)

Leaking fuel system (pressure hoses)

bad O2 sensor, car running rich all the time (ditto for temp sensors, or bad wires on them!)

The C-class goes like scared rabbits, you really cannot expect econo car milage, but the do usually get fairly decent milage.

Use the high octane -- if the engine and ignition systems are working correctly, you will get both better performance AND better milage -- the ignition will be severely retarded all the time if it knocks on anything else, and while the engine won't be damaged, it will be sluggish and use MORE fuel.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Unfortunately the C240 is kinda underpowered as far as fuel efficiency is concerned. I find that when I get them as loaner that I need to give it more gas to be where I am used to with the C230 I have. I get horrid gas mileage in my 190E because it is power limited for getting up and going. The C240 I find has to be almost floored to get what I get with half pedal and a 6 speed tranny out of my C230. I mean the car has enough power for me and all but it gets horrid mileage compared to my C230. Of course either car of mine gets filled normally only once a week. But when I had a C240 Loaner for a week I noticed I had to fill it once and I got it with a full tank. And the C240 is not exactly rated for the best fuel economy in the C line-up. As for putting lower octane gas in, you will cause the ECU to retard the timing due to Knock. And Knock no matter how quickly it is detected and thrawted is not a good thing for your engine. Also keep in mind that there will be a power loss due to retarding the timing as well as worse fuel economy. So it is up to you.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

study hard, learn to do basic repairs and maintenance yourself... the gas should/will be a non factor.

If you spend 40 dollars a week for gas.... the difference between mid grade and premium is usually 4 bucks ...

bag a lunch, make your own coffee... you can make up that difference...

17 gals. translate to 40 US. dollars here in north FLA.average 12 MPG on my daily driver SL 500, city driving 8 miles a day.