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156 2.0 poor fuel consumption

909 7
I've not driven the alfa much as I've been restoring her. Now noticed that I'm getting under 20mpg (when the fuel dial works - lol). Could this be because I have fitted a CF3 engine - still using the original CF2 ECU and CF2 exhaust? (Fitted all the CF2 bits to it) Could it need tuning? Could it be caused by a blowing exhaust manifold (hairline crack)? Where do I start?
99 156 2.0TS