Which of the following should be performed first when checking an engine that produces black smoke under load?

When dealing with an engine that produces black smoke under load, the first step often involves checking the air filter restriction. This is because black smoke typically indicates that the engine is burning an excessive amount of fuel, which can be a result of not receiving enough air for proper combustion. Inspecting the air filter helps determine if it is clogged or restricted. A restricted air filter can hinder airflow, leading to an imbalanced air-fuel mixture, which then causes incomplete combustion and results in black smoke.

By conducting an air filter restriction test first, you can quickly identify whether a simple issue—like a dirty or blocked air filter—is responsible for the symptoms. Addressing this potential problem early on can often lead to a straightforward solution without needing to delve into more complex areas such as injection timing or fuel quality. If the air filter is found to be clear, then further diagnostics can be pursued systematically, such as checking the injection timing, examining the DPF for blockages, or performing a chemical analysis of fuel quality.