What factor primarily determines when to change the air filter in a diesel engine?

The primary factor determining when to change the air filter in a diesel engine is the restriction gauge reading. This measurement provides a direct assessment of the air filter's condition by indicating how much airflow restriction is present. As the air filter traps contaminants, it becomes clogged, leading to increased pressure drop and reduced airflow to the engine. Monitoring the restriction gauge enables technicians to identify when the filter is compromised and needs replacement to maintain optimal engine performance and efficiency.

While time since last replacement, visual inspection, and manufacturer's guidelines can provide useful insights or general recommendations regarding filter changes, they do not offer an accurate, real-time indication of the filter's actual condition. The restriction gauge reading is the most practical and effective way to assess when a diesel engine air filter needs to be changed. This approach helps ensure proper air intake, which is crucial for effective combustion and engine operation.