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So I own a 2015 gla250 4matic. I just had my rear differential replaced about 8 months ago which was over 5k. I haven’t had any issues with that but my car is acting weird and by that I mean some days I have to add more pressure to the gas pedal other days I can just barley tap it and it goes. I’ve reset the transmission, I’ve made sure my car is always full of gas I just don’t know what it can be. But like the days I have to add more pressure to the gas pedal the car takes longer to get up to speed and feels like it’s worker harder. But then again the next day I can barley tap the gas and it just takes off full power. What can cause such a dramatic difference for the car to be driving like this. It’s always in eco mode but the days it seems to be having a hard time I sometimes put it in sport mode to just keep up with other cars on the road. I seem to go through my gas a lot more quicker on those days as well. Any info anyone can give me that will be great! Thank you
 

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You should of course get a service scan but I would consider the turbo and/or related systemic components as it may now be having an intermittent issue. There is always a bit of turbo lag but those days that you feel underpowered even underway it may be struggling to provide normal boost. Do you notice more of a whining noise these days?

Especially in Eco mode where performance is already throttled in favor of greater fuel efficiency this may be especially noticeable.

However it could be water in the fuel. It's good to keep the tank full as it helps stave off ambient moisture condensation in the fuel, especially in humid weather. What type and octane level do you use? A Top Tier such as Shell or Mobil is generally better formulated and their stations typically more caring about water in their tanks. Also the ECU will retard timing, and thereby performance, if less than optimal octane is used (should be 91 but these high fuel cost days there are some opting to go with less expensive 87 or 89). This altering of timing is to self-protect the engine from pre-detonation (aka dieseling, knock, ping).

You may want to reset the transmission control unit (TCU) just to have a known diagnosis starting point.
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Possibly related: Strange whining noise when accelerating
 
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