Failure in replacing a dirty air filter on time can result in your engine running too rich. This is because a dirty air filter prevents (or even blocks) oxygen from entering the cylinders for the combustion process. As a result, a lesser than ideal amount of air mixes with fuel for compression, hence causing the engine to burn unnecessary fuel. All of this has an impact on your engine performance (in terms of acceleration) and fuel consumption. You immediately start feeling a lack of acceleration and a declining fuel average.