What are common sources of air filter contamination and how does it affect engine performance?

• A. Dust, moisture, oil blow-by; reduces air flow, lowers power, increases wear, and can cause misfires.
• B. Fuel varnish and varnish buildup; improves air flow and increases power.
• C. Bright metal particles from bearings; improves engine cleanliness.
• D. Coolant leaks and rust; increases efficiency and power.

Answer: (A)

Air filter contamination mainly comes from dust and dirt in the intake air, moisture from humidity or condensation, and oil that enters the filter from blow-by or the PCV system. Each of these contaminants plugs or coats the filter media, which raises intake restriction. When the engine can’t draw enough air, it can’t burn fuel efficiently, so power and throttle response drop and fuel economy suffers.

Dust and dirt on the filter gradually clog the pores, making it harder for air to flow. Moisture can cause the media to swell or clump, further blocking air passages and, if water makes it into the intake, can cause rough running or misfires. Oil that coats the filter dramatically reduces its permeability and can also be drawn into the combustion chamber, fouling spark plugs and leaving deposits that worsen combustion and wear over time. The combined effect is a less-than-ideal air-fuel mix, which often shows up as reduced power, rough idle, misfires, and greater engine wear.

Other proposed contaminants don’t fit as common causes of air-filter contamination. Varnish buildup isn’t a normal result of air-filter contamination and wouldn’t typically improve performance. Particles from bearings or coolant leaks introduce different failure modes and don’t reflect the usual ways air filters get contaminated.