I have a 1999 Ford Fiesta when driving on motorway theres a strong smell of petrol! I have just had a new back box fitted thinking that would sort problem and mechanic said everything great but problem still persists, any ideas??
There has got to be a fuel leak somewhere....by the 'back box' I presume the rear silencer? When the fitter was under there he should have had a good look round for any corrosion around the fuel tank / fuel filter and fuel pipe areas, but I think you need to get your car up on a ramp so that all of the fuel pipes can be traced, from back to front - then have a good look round in the engine bay for traces of leakage.
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