2005 C230K Erratic Gas Mileage... - Benzworld.org

renisis · 07-10-2008, 09:24 AM

I was wondering if any one had suggestions for my issue:

I have a 2005 C230K Sedan and I usually get very good gas mileage out of it (25 - 27 mixed & 31-32 straight highway). Lately I have been sporadically getting 19 - 22mpg (~21.4 over two tanks of gas). I haven't changed anything (ie: driving style, route to work, gas grade, etc) and the MB dealership near me says there is nothing wrong because they aren't getting any computer codes.

Do you guys have any suggestions? BTW this is the same dealer that has the car for the 4th time to get rid of the same squeaky seat noise.

Thanks in advance...
-Matt

renisis · 07-10-2008, 09:32 AM

Another thing to note is that I will still see good gas mile occasionally.

It really sounds like a computer / sensor problem to me but MB keeps insisting that they can't do anything unless there is a code.

FFemt5287 · 07-10-2008, 09:34 AM

i have the same car and consistantly see over 30mpg (as high as 39) per the computer and hand calculated at every fillup with 93 octane fuel. thats mostly around town driving (rural, 45-55mph)

have you bought heavier shoes?

renisis · 07-10-2008, 09:54 AM

Nope... Everything is stock and the tires have been the same since I got the car (some flavor of Dunlop)...

I was impressed at the mpg when I first got the car, but now I am wondering what the hell happened.

-Matt

2002c230 · 07-10-2008, 03:57 PM

How many miles does the vehicle have? You might want to clean the MAF sensor. It will get dirty over a relatively short period of time, and that can cause a rich burn. Might be the cause of your bad gas mileage. If it's just dirty there will be no CEL, or pending codes, so just because the dealership says there are no codes definitely doesn't mean everything is alright... If that doesn't seem to help try a different gas station to fill up at. It's possible they filled the resevior with regular unleaded instead of premium, or just got a shipment of bad gas. Also, check your tire pressure if you have not done that already. When is the last time you had your tires balanced and/or rotated?

MelloC230 · 07-10-2008, 04:16 PM

I would have to agree with 2002C230 on the tire pressure and gas station comments. Due to weather change tire pressure will be affected which will cause a variation in overall mileage. You cannot rule out the gas station theory. Just because it's listed as XX octane on the pump does not mean that is what's being pumped into your tank. Gas is never consistant from one gas station to the next. Try to stick with the same petrol station if possible and try to stay away from the Pot Lot gas station. Sure their prices are low but there is a reason for it

renisis · 07-10-2008, 05:57 PM

I can't say the gas station theory isn't a problem but I already did check the tire pressure and since the tires are only a few months old I wasn't even thinking about rotation/balance.

The car has 35000 miles on it (there were 32500 when I bought it).

The strange thing about the problem is that it doesn't happen predictably (ie: due to gas). I went to my family's house for 4th of July and going there the computer read 20mpg and on the way back (I was taking it extra easy on the car both ways) I got 28 mpg.

So I asked the dealership to take a look at the MAF sensor this afternoon. Hopefully tomorrow they'll give me a car back that had a dirty sensor but we'll see.

BTW thanks for the help. I appreciate it.

I did have another quick question: My wipers have the rain sensor but will go off randomly while I'm driving when it's not raining. The dealership told me they are suppose to be off unless its raining but I thought that was BS. I know how the sensor works (the curse of being an engineer) but I didn't want to keep pushing the issue if I am truly mistaken. Any thoughts?

Thanks again,
Matt

MelloC230 · 07-11-2008, 09:11 AM

Keep us posted on the MAF issue as my C230 has about the same mileage as yours. I do notice though mileage can also be based on traffic conditions. On the weekends I get better gas mileage due to lack of traffic but during the weekdays when I commute to work I hit periodic pockets of stop and go traffic which does affect my overall mileage on the highway.

mediatwin2 · 07-11-2008, 10:16 AM

I picked up my wife yesterday from work as usual. the lights had changed timing due to construction. My computer dropped from 25MPG to 16MPG, just be sitting at 6 consecutive red lights in 85 degree weather....even the littlest changes can affect MPG. If the windows are open, you can watch the drag pull the MPG numbers down, Heavy shoes screw up the pedal feel and suddenly you are changing your habit without noticing.

Oh and the Rain sensor thing......my SA told me that leaving it on when dry is no problem, you just have to put up with the schizophrenic activation, seems one of those "personality" things. I have an expansion seam on a bridge I drive over, that is guaranteed to send them wiping.....

RodrigoMex · 07-11-2008, 02:18 PM

I would suggest you check the parasitic weight you carry on your trunk (get rid of it), and inflate your tires with a couple extra psi's on your tires as indicated in the manual (inflate always with pure nitrogen). That way you'll get just a little bit harsher ride, but better dynamics, less pot hole tire damage risk and increased gas mileage.

The above is independant and always a nice extra besides ensuring clean gas and air filters, properly cleaned MAF, and proper working spark plugs, but if you are getting erratic fuel milages I don't think this basics apply to your case.