M266 bad idle and stuttering at low speeds
Posted: Tue Jul 07, 2026
No one,
I am in dire need of assistance in determining the cause of an issue with my 2009 Mercedes A150, which is powered by an M266 petrol engine and has recently reached 90k miles. I can't figure out why it's happening, and the fact that there are no error codes is driving me crazy. Additionally, it varies from day to day, with some being better than others. There were only two issues with the charge air cooler sensor that I discovered during my initial scan of the vehicle when I purchased it. They have never come back since I cleared them. I had a misfire on cylinder 3 and the engine began running poorly about a week after I got the car (it was raining at the time, in case that matters). I ran the vehicle through a battery of high-quality fuel and fuel system cleaners because it had been sitting at the dealer for a while, and the misfire never occurred again. Poor low rpm operation, or the inability to maintain, say, 2,000 rpm without fluctuation, is the main problem. There is some hesitancy and jerkiness while driving, but these issues typically go away once the vehicle is warmed up and you get to higher RPMs. The vehicle has been performing considerably better ever since it was serviced three weeks ago; during that time, I also cleaned the air temperature and map sensors. I've attached a photo of the extracted plugs for your reference, just to demonstrate how terrible they were.
This hesitation and poor throttle response at low Rpms, along with a somewhat rough idle and the occasional little shudder, is still an issue, though it isn't as severe. There seems to have been no loss of power, as fuel economy is good and acceleration and top speed are in line with the stated figures. From where I'm sitting in the driver's seat, I can make out a single sound—a tapping or knocking—that seems to be emanating from the vehicle's left side (the RHD side), although I can't localise it under the bonnet; instead, it seems to be more in the middle. It appears and disappears, and I can't tell if it has anything to do with my issues.
Never letting fuel trim figures go into double digits, whether it's short or long term. In typical driving, the short term is almost always negative and only slightly positive when the throttle is fully extended. I even tried using my vape as a smoke machine, but I didn't see the smoke going anywhere, and I sprayed carb cleaner in an unsuccessful attempt to locate a hoover leak. One thing that Torque Pro constant scanning does wrong is fail at the time of switching from rich to lean sensors.
There are a few possible causes of my problems: a vacuum leak, a failed injector, a coil pack, or the leads. Because of the challenges in accessing the engine, I want to be sure of what could be wrong before choosing a course of action; however, if this were any other vehicle, I could simply test all of these things. I would be eternally grateful if someone could kindly inform me if they have any idea how to solve a problem like this using their expertise in engines.
I am in dire need of assistance in determining the cause of an issue with my 2009 Mercedes A150, which is powered by an M266 petrol engine and has recently reached 90k miles. I can't figure out why it's happening, and the fact that there are no error codes is driving me crazy. Additionally, it varies from day to day, with some being better than others. There were only two issues with the charge air cooler sensor that I discovered during my initial scan of the vehicle when I purchased it. They have never come back since I cleared them. I had a misfire on cylinder 3 and the engine began running poorly about a week after I got the car (it was raining at the time, in case that matters). I ran the vehicle through a battery of high-quality fuel and fuel system cleaners because it had been sitting at the dealer for a while, and the misfire never occurred again. Poor low rpm operation, or the inability to maintain, say, 2,000 rpm without fluctuation, is the main problem. There is some hesitancy and jerkiness while driving, but these issues typically go away once the vehicle is warmed up and you get to higher RPMs. The vehicle has been performing considerably better ever since it was serviced three weeks ago; during that time, I also cleaned the air temperature and map sensors. I've attached a photo of the extracted plugs for your reference, just to demonstrate how terrible they were.
This hesitation and poor throttle response at low Rpms, along with a somewhat rough idle and the occasional little shudder, is still an issue, though it isn't as severe. There seems to have been no loss of power, as fuel economy is good and acceleration and top speed are in line with the stated figures. From where I'm sitting in the driver's seat, I can make out a single sound—a tapping or knocking—that seems to be emanating from the vehicle's left side (the RHD side), although I can't localise it under the bonnet; instead, it seems to be more in the middle. It appears and disappears, and I can't tell if it has anything to do with my issues.
Never letting fuel trim figures go into double digits, whether it's short or long term. In typical driving, the short term is almost always negative and only slightly positive when the throttle is fully extended. I even tried using my vape as a smoke machine, but I didn't see the smoke going anywhere, and I sprayed carb cleaner in an unsuccessful attempt to locate a hoover leak. One thing that Torque Pro constant scanning does wrong is fail at the time of switching from rich to lean sensors.
There are a few possible causes of my problems: a vacuum leak, a failed injector, a coil pack, or the leads. Because of the challenges in accessing the engine, I want to be sure of what could be wrong before choosing a course of action; however, if this were any other vehicle, I could simply test all of these things. I would be eternally grateful if someone could kindly inform me if they have any idea how to solve a problem like this using their expertise in engines.