Charolastra

I recently took my 2007 C2S in for my 60k servicing (at 55k, just to be safe). In addition to the standard 60k maintenance, I also replaced the spark plugs, ignition coils, and battery, and did a full BG 44K engine/fuel addition. ~$2k later, I was on my merry way. Some observations on pre/post differences:

1. Not sure what was the cause for it, but my car now starts like a dream! Very fast turnover to ignition; awesome!

2. Car accelerates a lot smoother/harder now. It pulls like a dream.

3. My miles per tank have SIGNIFICANTLY decreased, from ~18.2 mpgs per tank to ~15.0 mpgs per tank. Driving habits have not changed. In the 2 fillups following the service, I am averaging ~210 miles per tank, which seems low compared to my standard ~240 miles/tank.

With regards to #3 above, can anyone weigh in and provide some insight into why this may be the case? Not sure if the BG 44k would really affect this, or if somewhere during the installation of the new ignition coils + plugs something happened to make the engine start running rich. Any ideas on how to fix this, or will this go away after all the new parts have "broken in"?

Macster

My WAG is your driving habit/style has changed. It doesn't take much to see a big change in fuel consumption. It is far easier to worsen gas mileage than improve it too.

One or two hard acceleration runs will do it that is obvious.

But also just buzzing a perky engine up another K of rpms every once in awhile will do it too.

The there is the variable of how full you fill the tank too. Turn the fuel nozzle handle upside down and let the auto shut off work. Based on my experience this gives the most consistent fill up.

Last but not least be sure tire pressures are ok. Under inflated tires will affect fuel consumption.

Charolastra

Macster: Appreciate the response, but as stated in my OP, driving habit/style has not changed. Neither has fuel filling methodology, or tire pressures (TPMS is working well, and I have gone so far as to manually check to confirm that TMPS is accurate).

In other words, with all things being equal (save for the updated parts/tuning following my 60k service), why would my MPG decrease?

Rocco07

Assuming the BG treatment was effective at cleaning up the injectors, perhaps your computer hasn't reset the injector timing to back off the fuel. Is the car running rich?

USMC_DS1

Are you using the same fuel source/gas station before and after the 60K service?

Charolastra

All: thanks for your replies! Appreciate your insight!

Rocco: I can't feel that it's running rich, but it is my assumption that it is, given the ~15% fuel economy drop. How do I know when the computer resets injector timing?

USMC: Yes. Same gas station/fuel source.

Rocco07

Like everything else, it may take a few miles for the system to react to the O2 sensor's output. If the O2 sensor detects too much fuel in the mixture, the computer should adjust accordingly. But I'm not sure how long that reaction takes, or if it's done in increments until the right air/fuel ratio is achieved.
Good news is, you have no fault codes.

Side note: My father changed a filthy air cleaner in his car after complaining about poor gas mileage. After replacement, the mileage improved but the car gave a CEL. We removed/reinstalled the BATT cable and the light did not reappear.
Over time, the car makes slight adjustements to systems. Drastically changing things, even for the better can upset sensitive monitoring systems and they again need time to readjust after considering new data.

CarGuy21

Maybe it's time for new O2 sensors and a EMS computer reset.

fastm3

I just recently replaced all my coils and spark plugs. My observations:

1) City fuel economy went from about 15 to 16-17 mpg

2) Highway fuel economy went from about 22 mpg to 25 mpg

3) Car was somewhat lethargic on startup. I thought it might be the battery cable. Now, like your car, the startup is much faster.

4) I couldn't say the acceleration is better or the engine runs smoother. It ran fine before, and I'd say it's about the same, at least in normal driving conditions.

CORSASCHNELL

As you know it is impossible to get the exact MPG for a similar drive due to many variants such as temps, humidity, mass of the car, condition of the car, etc. My guess is that the car is rejuvenated with service, drive is enjoying the car even more, pushing harder ergo getting 3 MPG less.

I wouldn't worry about it and enjoy the refreshed car.

Macster

Well, that your driving style has changed, or your fuel consumption measuring is not right, are the best explanations.

That big of an increase in fuel consumption just doesn't happen.

The engine is very precisely fueled. There is some adjustment but at around +/- 10% adjustment the check engine light should be on.

It is not. Thus the DME does not appear to be adding excess fuel, but is fueling the engine just fine which brings me back to your driving style or how well you are measuring fuel consumption.

It is hard to believe a switch to another brand of gasoline could have the fuel consumption increase so drastically. Ditto a seasonal change in gasoline blends. Or getting a tank or two of stale (very very stale) gasoline.

Long shot time: You might consider especially if during the service the fuel filter was changed a fuel leak. This may not happen -- if it happens at all -- while the car is being used, but afterwards when the engine is shut off and you are away from the car.

Visiting the car a few minutes -- say at 5, 15 then 30 minutes -- after the engine was shut off and sniffing for any gasoline smell might be worth the trouble. You should not smell gasoline at all.

Charolastra

I have good news everybody: I figured out the problem!

While I was detailing/vacuuming my interior, I noticed that I had one extra quart of motor oil lodged under my passenger's side seat. I had brought my own motor oil to the dealer to have them use my oil instead of theirs, given that they tend to add a ~40% markup to good 'ol Mobil 1. They must've not seen the extra quart during their oil change process, and (somewhat negligently) only added 8 quarts instead of the usual 9 quarts. I corroborated this with my check oil computer, which showed my oil level one bar north of the minimum requirements. Upon adding this extra quart, my mileage instantly improved, and I am now back to my pre-inspection mpgs, if not slightly better!

Problem solved!

Long story short: my engine was one quart low following my 60 major service, resulting in a ~15% mpg drop. Upon adding this quart, my mpgs have gone back to normal! Thanks everybody for your input!

Wolfk

Oh boy,

JW911

You've got to be kidding?

Maybe if you add a couple more quarts you can average 30mpg?

daberlin

Running 1 qt low isn't going to cause a 3 mpg reduction in fuel economy. This is primarily due to driving habits.