Higher fuel consumption/possible wear issue

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Hi All

All help gratefully received on this......I have a 55 reg CLS350. It went for an Assyst Plus minor service and also due at the service were spark plugs and an oil and filter change for the 7 speed transmission.

Next day, there were engine issues (misfiring) and the spark plugs were changed (probably faulty batch) and this rectified that issue. Since the service, other than this obvious cured misfiring fault, I've noticed:

  • at ignition the mpg indicator falls from 27.0 mpg straight down to between 19 or 20 mpg, where it used to fall to halfway between that. I do alot of motorway driving and where my consumption was normally 29 or 30 mpg at the end of a trip it now peaks at 26 or 27 mpg at best. I've not done the maths accurately but I'm doing about 40 miles less on a full tank and spending about £10-15 per week extra in fuel.
  • at gear changes at higher speeds (60mph and above) the revs are higher and do not drop and I've had to take my foot off the gas sometimes for the gear to change and revs to drop for the cruising speed

Everything we can think of has been checked, e.g. plugs again, transmission oil level, the diagnosis assistance reading shows all control units correct etc, etc.

Has anyone got an ideas to explain these issues? Two things concern me - anything going on that is putting stress on the car/engine and may lead to a more major failure or costly repairs, and also bringing the fuel consumption back to normal and obviously reducing the fuel cost by doing this.

Thanks in advance....
 

television

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If only the sparking plugs were changed, were they the correct type, thats all I can think of
 

ianstaley

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Don't forget the transmission that has been cleaned and new filter, just a quick question at 70 mph are you doing the same revs as before or is that an unknown. If the ignition is fine and 100% then the only other thing is the box. A few years ago I had a problem with the AFM of a C180 estate automatic. I couldn't get kick down, other than that I just couldn't figure it, the AFM was at fault in sofar as it put the car in limp mode, everything worked okay bit sluggish but okay. So it went to the MB dealer fitted new AFM given the car back everything okay now, until I went out the door, they had cleared the engine fault but had forgotten to clear the associated gearbox ECU fault. Once the Gearbox ECU was reset and cleared no problem everything a ok. Maybe should look there,
 

Pathfinder

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Maybe the guy who serviced it has disturbed something by accident. Just a suggestion.
 
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Thanks for the responses all. The guy who does the service is an independent with a good business and who used to be a service tech and dept manager for MB. He uses only MB approved parts (plugs, oil etc). He's looked over the car several times to see if he nudged something but can't find what's changed.

@ianstaley - at 70mph the revs seem to be slightly higher with the main difference being the gear change holds a bit longer and it feels like I have to come off the gas for the gear change to take place.

Does anyone think there is mechanical damage taking place here? Although I don't want to, I guess I can live with the extra fuel consumption expense, but I don't want to be trashing anything else.
 

Oarsman

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Just a thought, you're not running it in S mode are you.
This would account for the gears being held longer during acceleration and cause higher fuel consumption.
 
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Hi Oarsman, I always run in S mode. Funnily enough when I first got the car, years ago, and I compared fuel consumption in S or C mode, there wasn't a difference! So thats not the issue - it was in S mode before the service.
 
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