Clio 1.2 poor mpg/air leak/ exhaust pop

[usman01]

hi
i have a clio 1.2 16v which is really bad on fuel. it does around 27-28mpg around town but on the motorway it does around 42mpg
i have changed the lambda sensor after cat and also the coolant temp sensor aswell also used ngk sensor and a good quality coolant sensor.
If i rev the car the exhaust starts popping and also i can hear a hissing sort of sound every time i rev the car. i have cleaned the throttle body but did not change the gaskets on the manifold so could it be this?

need some ideas what to do next
thanks

 

[Alan]

Have you changed the spark plugs recently, the plugs maybe coked up, does the car sound like its running in 3 cylinders ?

 

[usman01]

yes iv changed plugs, oil and all filters around 4 months ago. when i start the car up from cold it feels as if its misfiring for about 10secounds then goes fine after that but have noticed the air filter is really dirty and covered in oil