I recently purchased a used 08 Charger R/T with 12,000 miles on it. The check engine light was on when purchased. After two weeks the dealer i purchased the vehicle from brought it to a certified doge dealer. The replaced two sensors on the vehicle. I was told they were o2 sensors. Since getting the car back i've had a series of problems in three weeks. Poor gas mileage (averaging 13-14 city, nothing more). I baby the vehicle and have not got on it at all;long country roads. I can feel the car switch from the 8 cylinder down to 4 cylinders. When switching to the 4 cylinder theres a noticeable feel like the car downshifts very hard. Associated with this there is a vibration and a noticeable hum inside the vehicle. I now hear a very light knock throughout the engine being on and a smell of burning oil at an idle. Not sure whats going on , but none of this was occurring prior to going to the dealership. It goes back in in 1 week. If anyone could help it would be appreciated, it is a beautiful car!

 

In the stop and go of city driving you will be in 8 cylinder mode for the majority of the time. In my 06 I average around 15 MPG so you are not that far off on your city MPGs. However the burning oil concerns me. When they replaced the O2 sensors did they change the oil at all? At idle with the car in "park" get out of the vehicle and walk around to the back, is there the tell tale sign of the blue tinted smoke coming out of the tail pipe? Have someone rev the motor up a little, is it there now? I bought mine with the service engine light on and it was just a dummy light trip code for a vacuum leak from the fuel system...I replaced the gas cap and reset the light and it hasn't come back on since. It is very uncommon to have to replace the O2 sensors at 12k miles...Did you see the originals? Did your service writer mention anything about abnormal wear on them? What was the specific trip code that triggered the service engine light? Your discription of the problem is extremely vague other than the switching of the MDS system and even it may be related to the original problem that triggered the service engine light. The MDS system relies on a ton of information sent to it from the TPS, the O2 sensors in the exhaust, among various other sensors throughout the car. A little more information can help us out with this and maybe help you with diagnosing whats wrong with your car.

 

Welcome to the forum Little fish.

Sorry to hear of the issues. Your PCM may have been rest while at the dealership so give it a bit longer to learn your driving habits and settle into decent MPGs.

The burning oil smell would be a concern. Pop the hood and look for any pooling oil around the intake manifold. If you get a chance to see it on the lift, check around the valve covers to insure there isn't any oil seeping from the lower part of the valve cover gasket.

I can tell when MDS kicks in but only due to a tone change in the exhaust, the car sholdn't feel like it's downshifting.

It sounds like the place your purchased the car from is working with you on those issues. If you can't get those two items fixed in a reasonable amount of time I suggest you ask them to take the car back and look for something else to replace it.