40. If the operating speed of a diesel engine increases without an apparent change in the engine control settings, you may suspect a __________.

An unexplained increase in diesel engine speed indicates a malfunctioning governor, as the governor regulates fuel to maintain the set speed. A leaking air starting valve, control air leak, or clogged intake air intercooler would not directly cause this symptom; instead, they typically result in power loss or erratic response. The governor's failure to limit fuel delivery is the most probable cause of the observed speed increase.