Fuel System Diagnosis and Repair Flashcards

A technician finds water in the clear bowl of the fuel/water separator on a common rail diesel. If the water is not removed and reaches the high-pressure components, which problem is the most likely result?

Answer: Corrosion and scoring of the pump and injector parts, because water provides poor lubrication

Water does not lubricate, and it corrodes the precision-fit parts of a high-pressure pump and injectors. The damage is costly and can happen quickly.

During a hard-start complaint on a common rail diesel, a back-leakage (return flow) test shows one injector returning far more fuel than the others while cranking. What does this indicate?

Answer: That injector has internal leakage, such as a worn control valve, which can keep rail pressure from building

Excess return flow from one injector means the control valve or its seat is leaking internally. That bleeds off rail pressure and causes hard starting.

A diesel stalls or loses power after running for a few minutes, and a hissing sound is heard when the fuel cap is loosened. Pump supply pressure returns to normal after the cap is opened. Which is the most likely cause?

Answer: A plugged fuel tank vent that creates a vacuum in the tank

A plugged tank vent lets a vacuum build as fuel is drawn out. The supply pump then cannot draw fuel, and loosening the cap relieves the vacuum.

Before loosening a high-pressure fuel line on a common rail system, what is the correct practice?

Answer: Shut the engine off and wait the time given in the service information so rail pressure can drop

Common rail systems can hold very high pressure after shutdown. Wait the specified time and follow the service procedure before opening any high-pressure fitting.

A common rail diesel cranks but starts hard. A scan tool shows actual rail pressure far below desired pressure while cranking, and supply (low-pressure) pressure and volume are normal. Which of the following would most likely explain this?

Answer: A rail pressure control valve stuck open

If the rail pressure control valve is stuck open, high-pressure fuel leaks back to the return, so pressure cannot build. Low-pressure supply is normal, so a supply restriction is not the cause.

At what point does diesel fuel begin to form wax crystals that can plug fuel filters in cold weather?

Answer: The cloud point

The cloud point is the temperature at which wax crystals first appear in diesel fuel. These crystals can clog the fuel filter.

Why should a high-pressure fuel line normally be replaced instead of reused or reshaped when it has been removed from a common rail diesel?

Answer: The sealing end can deform and the line can be damaged, which may lead to leaks or fatigue cracks at high pressure

Sealing surfaces deform when tightened and lines can be bent or stressed, so reuse risks leaks or cracks at very high pressure. Many manufacturers specify replacement.

A technician connects a vacuum gauge to the suction side of the fuel transfer pump and sees a much higher vacuum than specified while the engine runs. Which of the following is the most likely cause?

Answer: A restriction between the tank and the pump, such as a plugged pickup or filter

High vacuum on the suction side means the pump is working against a restriction upstream, such as a clogged pickup, a collapsed line or a restricted filter.

A diesel's fuel supply contains a significant amount of gasoline from a mis-fueling event. What is the main danger to the fuel system?

Answer: Reduced fuel lubricity and cetane, which can damage the high-pressure pump and injectors

Gasoline lowers the fuel's lubricity and cetane number. The pump and injectors depend on the fuel for lubrication, so they can be damaged quickly.

On a common rail diesel, the ECM reduces power when fuel temperature returned to the tank becomes very high. Which condition is most likely to be involved in this complaint?

Answer: Excess heat from high-pressure pump and injector return flow, with an inadequate fuel cooler or low tank level

Fuel that circulates through the pump and injectors and returns to the tank picks up heat. If the cooling is poor or the tank is nearly empty, fuel temperature rises and the ECM may derate the engine.