All fluids and filters need to be changed.
The valves need to be adjusted.

Its a 240D right? So no turbo boost to check.

Check the timing chain for stretch and I'd time the IP as well.
If the mileage still isn't that great pull the injectors and have them gone through.

If you think you have a fuel leak track it down and fix it. Fuel leaks will kill your mileage.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200