1. Strange engine noises
When your car is idling or stationary, you should feel and hear the smooth vibrations of an efficient engine. If you notice unusual noises, in particular a coughing, popping or spitting noise, it suggests that the engine isn’t getting enough airflow, which means your air filter needs to be replaced.
What has actually happened in your engine is the air filter has become dirty or clogged. This reduces the airflow, changing the air-fuel mixture. The rich fuel mixture creates a black soot residue which covers the spark plugs. The noise comes from the spark plugs not firing properly due to this residue. Dirty spark plugs can also cause problems with starting your car and misfiring.
2. Decreased performance
Does your car respond normally when you press the accelerator? Or is it slow and sluggish? If it’s the second, there’s a good chance a dirty air filter is preventing your engine from receiving the clean air it needs to perform optimally. Simply replacing your air filter can eliminate this issue.
3. Decreased fuel economy
Decreasing fuel economy is a clear sign of a bad or dirty air filter. A bad or dirty air filter restricts air flow, lowering the oxygen in the mixture. Your engine compensates for this by consuming more fuel to produce enough power to move the same distance or speed as you could with a clean filter.
4. Black smoke or flames in the exhaust
An insufficient air supply means your engine will be running on a fuel-rich mixture, which won’t burn completely before it enters the exhaust to leave the car as a black soot-like residue. This residue can be seen as black smoke. Alternatively, the heat in the exhaust might ignite the unburnt fuel, causing flames at the end of the exhaust and a popping sound.
5. Smell of petrol in the exhaust
If you smell petrol when starting the car, it’s because insufficient air is entering the fuel injection system and the excess unburnt fuel exits the car through the exhaust pipe (hence the smell). When you replace the air filter, the smell should go.
6. Air filter looks dirty
A brand new air filter is a white / off-white colour, which will slowly darken as it accumulates dust and dirt over time. A visual inspection of your air filter under bright light will show a lot of dirt, but not all the tiny particles can be easily seen.
7. Check engine light comes on
An inadequate supply of air can result in carbon deposits accumulating in the engine, which will eventually trigger your check engine light. If the light comes on, check your air filter to see if it needs replacing before you run other diagnostics. For this reason, most car manufacturers recommend changing your air filter every 12,000 miles (approximately 19,000 km) or every 12 months, whichever comes first, regardless of how dirty your air filter appears to be.
