What is the probable cause of a racing fuel oil service pump?

The probable cause of a racing fuel oil service pump is water in the fuel. When water is present in the fuel system, it can disrupt the normal operation of the pump. Water may not ignite and can cause incorrect fuel delivery, leading to an erratic or increased operating speed of the pump—referred to as "racing."

Water can also cause corrosion and surface damage within the system components, further exacerbating the problem and potentially leading to pump failure. Proper fuel maintenance and testing are vital to prevent water contamination, as maintaining the integrity of the fuel system is essential for performance and reliability.

Other issues such as air in the fuel can also cause operational problems, but it typically leads to issues such as cavitation or loss of suction rather than an increased speed in operation. Low temperature generally affects the viscosity of fuel, and high pressure may indicate a need for relief but does not typically correlate with the racing condition unless combined with other factors. Understanding these distinctions helps in diagnosing and troubleshooting fuel-related issues effectively.