Red Seal Truck and Transport Mechanic Practice Exam

What is a likely outcome of an engine running too lean?

1. Enhanced engine performance

2. Possible engine overheating

3. Increased fuel efficiency

4. Improved torque output

The correct answer is: Possible engine overheating

When an engine runs too lean, it means that there is too much air and not enough fuel in the combustion mixture. This condition can lead to several issues, with overheating being one of the most significant outcomes. When the mixture is excessively lean, it causes higher combustion temperatures, as the excess air can lead to more complete combustion of the fuel but generates additional heat. Continuous operation under these conditions can damage engine components, including the pistons and valves, due to the increased thermal stress and potential pre-ignition or detonation. Although some might think that a lean mixture could enhance performance, in reality, once the mixture becomes too lean, engine performance typically suffers due to improper combustion. Also, while it might seem that increased fuel efficiency would be an advantage, the risks associated with running lean often compromise engine longevity and reliability. Similarly, torque output will not necessarily improve; rather, it can actually decrease when running in a lean state due to inadequate fuel reaching the combustion chamber. Understanding the implications of running an engine too lean helps technicians recognize symptoms and take corrective actions before serious engine damage occurs.