What is a consequence of a malfunctioning fuel-water separator?

A malfunctioning fuel-water separator allows water to mix with the fuel supply. This component is crucial for separating water from diesel fuel, as the presence of water can lead to severe issues in diesel engines. Water contamination can cause various problems, including corrosion of engine parts, diminished fuel quality, and potential damage to the fuel system, such as injectors and the fuel pump. Additionally, water in the fuel can lead to incomplete combustion, resulting in a loss of power and efficiency, as well as increased emissions.

In contrast, the other options are generally outcomes associated with proper functioning or benefits of preventive maintenance. Improved fuel economy, increased engine horsepower, and better combustion efficiency would not result from a malfunctioning separator; rather, they are positive aspects of a well-maintained fuel system. Thus, the consequence of water contamination in the fuel supply due to a malfunctioning fuel-water separator is the correct focus here.