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I had a bad fuel damper. Started with bad fuel mileage. Progressed to rough idle and then smoking( running rich). car relies on vacuum at lower rpm to pull fuel. Vacuum is highest at idle and lower at higher rpm. Could explain the mileage difference when car is operating at lower rpm(higher vacuum)during city driving. If you have a small tear in the diaphragm in the fuel damper, it will pull unmetered fuel into the intake manifold . Just a guess on your issue. maybe test the fuel damper?
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