Can anybody please help me? My 1998 C180 is using a full tank of fuel to cover an average of185-210 miles! The car is used as a taxi so all the miles are urban, however a mate uses a bmw 520 and gets 350 per tank. I have recently changed the air flow meter and this problem started (I used to get 300ish) Is this likley to be the Lamba??? Or even worse a valve!!! Sometimes when idle the car will rev between 5-10Rpm up and down without me doing anythng.