I have no idea why they would think it is the air filter or the fuel filter if it ran fine when you brought it in. First suspicion is that they overfilled the AC and there is too much pressure so the AC pulley is dragging the engine down. That's if you have the air on. Does it idle different with the air on than with the air off?

Second shot at this is if you know how many miles it had on it when you brought it in and how many miles when you left with it in case they ran the Texas mile event--see how fast you can go? Did they test drive it before you picked it up and not notice?

What do you call a "full service"?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas