Hey guys,

I looked around a bit but I haven't been able to find the correct answers. Essentially what I need to know is what maintenance measures do you guys take to ensure peak fuel economy. I do normal oil changes and recently I have replaced spark plugs and air filters on the car. What else should I look at? My economy has been falling drastically lately. Plus with it getting colder this time of year it will be getting a little less and less.

 

Assuming general maintenance was performed correctly, my next guess would be adjusting the tire pressure.

With temps getting lower most folks are probably not adjusting their tire PSI accordingly, which would be to add more air. Make sure your tires are set to the proper PSI in the morning prior to taking it out for a drive, since proper reading must be done cold.

Low PSI tires yield lower fuel economy.

 

I believe my tires are correct. If they are not they are not too far off because the car hasn't gave the light saying pressures are off. I haven't calibrated the TPMS in awhile on the car.

 

Personally I'd rather not assume since checking is the quickest and is the lowest hanging fruit so to speak. You can also set the TPMS warning to whatever pressure you want, so just because it's not on, that doesn't mean you're not running slightly under inflated.

 

I will check that and see just to make sure. I was leaning to something more mechanical causing the decrease in efficiency.

 

How many miles in the car ? Are you running correct oil ? Try adding a bottle of Techron to clean injectors. How much is your economy dropping ? After a few mile drive check each wheel for excessive heat - unlikely but could be wheel bearing or sticking brake caliper - also scan odb port for pending codes to rule out fuel/air mix rtc.

 

Has your driven route or driving habits changed?.

New tyre brand?

Changed fuel supplier or lesser quality of fuel.?.

Changed address ie higher altitude different fuel air mixture.

Empty car of any unnecessary load ie golf clubs, & tow ball tongue.

Children or wife driving car !

Good luck & congratulations on noticing the change & for taking action.

 

115K or so Miles on the car. Oil should be correct, I had the same oil serviced in the car since Ive owned it. Its roughly 19 MPG currently. About a year ago it was around 22 MPG.

There are not any codes that I am aware of. I did have a fuel evap code a couple months ago but it went away and I presumed it to be the gas cap wasn't tightened all the way.

I don't notice any excessive heat or the car doing much else different.

 

I checked the Tire PSI. I set the rears to 33psi and fronts to 29psi. The backs were around 28psi or so. I am going to see if that helps any with the MPG I am getting.

As far as cleaning fuel injectors. Do you guys physically take them out of the engine or do you just use some type of cleaner such as Techron through the fuel system/tank?

 

29F 33R sounds low for a C350 - did you check the fuel filler door for the correct PSI? My C350 (has AMG rims) says 36F 39R.

 

I checked on the stickers thats on the frame of the car when you open the drivers side door. I think the one thats in the fuel door is the one that says if you are going in excess of a certain mph.

 

What rims are you running? If you have the AMG rims these pressures are too low.

 

It is the AMG sport wheels. Not the ones that the C63 has.

 

Okay 29F 33R is definitely wrong for those rims. Those are the settings for the standard 17s. You don't have the year listed on your profile, but I have had 2 W204s with the 18" AMG sport rims (a 2011 and this 2014) and they said different things, but both were much more than you have used. My 2011 said run at 33F 39R and my 2014 says 36F 39R.

 

Pressures specified on the fuel door placard are the ones you should be using.

 

Mine is a 2012 C350 (RWD). Minimum tire pressure listed is 36F/39 Rears, with +3 for high speed driving. For 72k Miles, I've been running +3 at 39F/42R with close to even tire wear (more on this later).

For comparison, this picture was taken during my morning commute 3 days ago.

 

I'm interested to hear your take. I run the 36F/39R as discussed and as called for on the fuel flap door on my 350 RWD as well with the same rims as you. I could bump it up, I just don't want to get uneven wear or have a brittle ride. There was a detailed post a while back on how running lower pressures would cause the rough ride in these cars, though.

 

I believe it's the other way around. Lower tire pressures will give you a more compliant ride, while higher pressures will give you a rougher one (but better efficiency). Regardless, I'm actually a stickler for air pressure mainly for safety reasons, since the balance of comfort vs mileage can vary from person to person.

EXAMPLE:
There is a reason why Mercedes recommends +3 for "high speed driving" and it's not a coincidence that it matches Tire Racks recommendation to the tee.

http://www.tirerack.com/tires/tirete...jsp?techid=301

I do not have the stock tires any longer, but I do recall it being a W-rated tire (factory ordered). Mercedes in stock form also has an electronic speed limiter rated to 130 mph, which if you look at the link above, will show you that if you intend to test the limiter, that your tires should be increased to +3 PSI (which in our case is 39/42). For those that got H-rated tires it's even worse, as they should be running +5 if they intend to go that fast, even though their tires will feel like hockey pucks on an average commute (and yes, you can blame the W204's that were shipped with H-rated tires to be the primary cause for the limiter to be set so low, since the limiter can only be as high as the lowest tire/wheel combination).

Now...if you run Y-rated tires like I do now (Micheline Pilot Super Sports), you can technically run lower pressures since Y-rated tires have higher heat ratings.

http://www.tirerack.com/tires/tirete...jsp?techid=302

HOWEVER...Since I have a tuned W204 with an unlocked ECU, I prefer to stick to the +3 PSI even for Y-rated tires so I can safely go 149 if I choose to and after having gone through 3 sets of tires in the rears, +3 is the most I'll go for daily driving (since +3 shows a very slight sign of over inflation when I took my last set beyond the wear bars, but you get the MPG results like in the post above).