my car petrol consumption is very high (around 5km/liter) and this is unusual.

Look at http://www.fueleconomy.gov/feg/findacar.htm to see what the gas mileage is expected to be. So many things could cause bad gas mileage, restricted air filter, low tire pressures, even the make of tire can affect gas mileage. A bad thermostat, temperature switch can also affect gas mileage. I would like to look at live sensor data on a professional grade scan tool to see what the fuel trim values are, and O2 sensors are reading and air flow meter readings are.