Hi !

I just had a surprise today, and I really don't like what I found out. My car was running in limp mode all day because I was playing with my tuning box- I didn't know that because of my driving style. I noticed that I was doing amazing fuel comsumption in the city , but that wasn't all . I went on the highway , reset the computer and I was getting 4.1 l/100 km ( about 55-60 mpg US) with 130 kmh speed ( limiter). This never happened to me at this speed, I was getting about 5.5-6 l/100 km ( 39-42 mpg US) as an average before this moment.

It all seemed very strange to me so I decided to go for higher speed on my return ( the same road ). So I floored it and the speedometer stopped at 160 kmh - that's when I realized that I was going in limp mode... ( I am a very careful driver, I always change gears under 2500 rpm - the limp mode limiter stops acceleration on 3000 rpm so I didn't notice all day long ).

The surprise didn't stop there, I went about 10 km with this speed and I got an AMAZING ( sorry for the caps) 4.5 l /100 km ! WOW ! That's 55.2 mpg US at 100 mph with an old diesel engine from 2005 running in limp mode.

I then removed my tuning box after this happened and the car returned to it's normal "hungry mode" as before... what a shame.

My question is this : why doesn't Mercedes make the engine run as economically in normal mode? Why does limp mode ( emergency mode ) make the car run much more better ? I almost want to make the care run in emergency mode, it saves me 25% more fuel than what I was using before.

My car is a C200 CDI facelift from 2005 with a 122 hp diesel engine. I wait for your opinions. Is this just a freak accident that hapenned only to me ? Could the tuning box have anything to do with it ?

 

The only way you can use less fuel is to put less in which restricts performance & gives driveability issues. This is especially so with a diesel where performance is a direct relationship with the amount of fuel injected as long as air is available. Limp mode does this. Short term incidental fuel consumption readings can also be misleading.

Benz could make the engine use a lot less fuel but then it would be a dog & you would not want to drive it.

 

Good explanation, thanks.

But even in limp mode the car runs better than my diesel VW Passat. Mercedes cars are the best, I guess they try to refine the engines too much in terms of ride quality and luxury feeling, ignoring the huge potential for extra fuel efficiency.

 

With a diesel, all things remaining equal, more fuel in = more power out until you can't get enough air & they will start to produce smoke (overfuel - unburnt hydrocarbons)

If they made it more fuel efficient then power output would drop correspondingly & the vehicle would develop flat spots at certain rev ranges.