13l/100km=18 mpg (US). That's worse than my 300D. Something is wrong, but I'm not sure what. I'm having trouble thinking of a powertrain problem that would create those symptoms without throwing a code. Have you checked for dragging brakes?

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.