Technician A says the best practice is to replace air filters on turbocharged engines when the restriction gauge reads 20-25 inches of water. Technician B says the filter should be replaced around 10-15 inches of water. Who is correct?

• A. Technician A
• B. Technician B
• C. Both Technicians
• D. Neither Technician

Answer: (B)

The key idea is how the air filter restriction gauge relates to engine airflow. The gauge measures the pressure drop across the filter—the dirtier the filter, the higher the restriction. For turbocharged engines, maintaining adequate unrestricted air flow is crucial because the turbo relies on a steady, clean air supply to create boost. If you wait until the restriction climbs to very high levels, the turbo can’t pull in as much air, reducing boost, performance, and efficiency.

Replacing the filter when the gauge reads about 10–15 inches of water keeps airflow in the optimal range and prevents the bigger performance losses that occur if you let it rise to 20–25 inches. So the guidance to replace around 10–15 inches of water is the best practice for turbocharged engines.