I took my 09 MCS in for the yearly service and the dealer did all that was recommended. When i got the vehicle back i noticed that the drivers side window didnt dip and the passenger side would do strange things. So i took it back in and they reflashed the car. Problem fixed.... BUT, now i get 18-19 mpg's around the city and 25-27 on the highway driving conservatively. I do mostly highway driving so my average is around 24-25 MPG's. I noticed the onboard comp was really low for the first full fill up (i reset my MPG counter everytime i fill up). So, one the next fill up i checked the tire pressure since it is getting colder and zero'd out my Tripometer so i can get the actual milage myself. Still horrible MPG'S. Any thoughts?????

 

While that seems pretty extreme, I've also noticed my MPG dipping down after my most recent service visit/reflash. Mine has only gone down about 1-2 MPG though...

 

Thats horrid gas mileage. On my '09 MCS, I don't believe I ever got less than 27mpg when doing mostly city driving or 30mpg on the highway. I'd definitely take it back to the dealership as there is something thats not right.

 

On another note, mileage will usually be worse in the winter. The engine spends more time warming up with the mixture enriched, so it takes a while before the car runs at peak efficiency. Also, cold tires are stiffer, cold air is denser, etc.

That said, I don't know if these factors could cause the major difference you've seen. Just a theory!

Cheers,

Spridget

 

Yeah, I did check the air pressure in the tires. Had to add a little. Didn't see no where near that kinda change last winter though so i doubt the colder air has to do with that much of a milage decrease. Im gonna take her in after the holidays to get her checked out. I also probably could check the air filter while im at it....

 

18 MPG isn't right. I don't care if the tires are flat. I'd get on it with the dealer, because you know darn well you were getting way better than that before they messed with it. I get about 28-29 city in my MCS.