ASE A9 Fuel System Diagnosis and Repair — Questions and Answers
Question 1: 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?
- Increased fuel cetane number and smoother combustion
- Higher fuel pressure at the rail caused by thicker fuel
- Corrosion and scoring of the pump and injector parts, because water provides poor lubrication (Correct answer)
- Reduced fuel return flow caused by a plugged tank vent
Correct 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.
Diesel fuel lubricates the high-pressure pump and injectors, and common rail parts have extremely tight clearances and very high pressures. Water in the fuel displaces that lubricating film and causes rust and scoring. It can also flash to steam in the injector tip and cause erosion. Water separators and water-in-fuel sensors exist to catch it early, so drain the separator regularly and find out how the water got in.
Question 2: 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?
- That injector has internal leakage, such as a worn control valve, which can keep rail pressure from building (Correct answer)
- The fuel filter is restricted
- The lift pump is delivering too much volume
- The rail pressure sensor is out of calibration
Correct 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.
Common rail injectors normally pass only a small, controlled amount of fuel back to the return line. A worn control valve, seat or leaking internal seal lets high-pressure fuel escape to the return. Rail pressure then builds slowly or not at all during cranking. The test compares each injector's return volume over a set time and cranking period against the others and the specification. Replace any injector that is out of specification.
Question 3: 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?
- A leaking injector return line
- A failed rail pressure sensor
- A stuck-open pressure control valve
- A plugged fuel tank vent that creates a vacuum in the tank (Correct answer)
Correct 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.
As fuel is drawn from a sealed tank, air must enter to replace it. If the vent or vent valve is blocked, tank vacuum rises and starves the supply pump. Symptoms are stalling, power loss under load, and a rush of air at the cap. The fix is to clear or replace the vent, not to replace pumps or injectors. Always confirm with a vacuum or pressure check before replacing components.
Question 4: Before loosening a high-pressure fuel line on a common rail system, what is the correct practice?
- Loosen the line at the rail while the engine idles to bleed air out
- Shut the engine off and wait the time given in the service information so rail pressure can drop (Correct answer)
- Use a rag to feel for leaks while the engine is running
- Pinch the return line to hold pressure in the rail
Correct 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.
Rail pressure can be well over 20,000 psi and may remain after the engine stops. A high-pressure fuel jet can penetrate skin and cause severe injury. Technicians should follow service information, shut the engine off, wait for the specified time, and use a scan tool where the procedure calls for it. Never check for leaks with your hands, and never loosen lines while the engine is cranking or running.
Question 5: 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?
- A restricted fuel filter
- A plugged fuel tank vent
- A rail pressure control valve stuck open (Correct answer)
- Water in the fuel/water separator
Correct 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.
The pressure control valve, or a pump metering valve in some designs, regulates rail pressure. A valve stuck open or leaking dumps fuel to the return circuit, so the pump cannot raise rail pressure during cranking. Because the filter, tank vent and supply side checked normal, the problem is on the high-pressure side. Confirm with a scan tool actuator test and a return flow check.
Question 6: At what point does diesel fuel begin to form wax crystals that can plug fuel filters in cold weather?
- The cloud point (Correct answer)
- The flash point
- The cetane point
- The boiling point
Correct 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.
Diesel contains paraffin wax that comes out of solution as the fuel cools. The temperature at which it first appears is the cloud point, and the fuel looks hazy. If the fuel keeps cooling, it eventually reaches the pour point and stops flowing. Filters usually plug at or below the cloud point. Winter-blend fuel, additives and filter or fuel heaters help prevent the problem.
Question 7: 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?
- The line's internal diameter increases each time it is removed
- The line has a built-in one-time pressure sensor
- Used lines are always magnetic and attract debris
- The sealing end can deform and the line can be damaged, which may lead to leaks or fatigue cracks at high pressure (Correct answer)
Correct 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.
High-pressure lines are made to precise length, bend and end shape. Tightening deforms the sealing surface, and bending or reshaping a line adds stress that can lead to fatigue cracking. At extreme pressures, even a small leak is dangerous and can cause hard starting or a fire hazard. Always follow the manufacturer's replacement and torque specifications, and keep the line clamps in place so the line does not vibrate.
Question 8: 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?
- A weak injector return spring
- A restriction between the tank and the pump, such as a plugged pickup or filter (Correct answer)
- Excess fuel pressure in the rail
- A leaking fuel pressure regulator
Correct 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.
The transfer pump draws fuel through the tank pickup, lines and often the primary filter. If anything there is restricted, vacuum rises. High vacuum can cause aeration and low supply pressure, which leads to power loss and hard starting. Check or replace the filter first, then inspect the lines and the pickup. Normal vacuum readings are small and are given in the service information.
Question 9: 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?
- Reduced fuel lubricity and cetane, which can damage the high-pressure pump and injectors (Correct answer)
- Increased fuel lubricity that causes the injectors to stick open
- Higher cetane that causes the filter to swell
- Excess sulfur that clogs the tank vent
Correct 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.
Gasoline is a thin solvent with very little lubricity and a very low cetane number. Mixed into diesel, it thins the lubricating film in the high-pressure pump and injectors and causes premature wear. It also causes hard starting, rough running and knock. The tank should be drained, the fuel filters replaced, and the system flushed, and the engine should not be run any more than necessary.
Question 10: 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?
- Too much fuel flowing through the filter
- A high-resistance rail pressure sensor circuit that raises fuel density
- Excess heat from high-pressure pump and injector return flow, with an inadequate fuel cooler or low tank level (Correct answer)
- Cold fuel thickening in the tank
Correct 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.
Return flow from the pump and injectors carries heat back to the tank. Hot fuel is thinner, so it has less lubricity and the pump delivers less mass of fuel. To protect the pump and injectors, many ECMs reduce power when fuel temperature rises too high. Check the fuel cooler, fan or airflow, tank level and fuel temperature sensor when this happens.
ASE A9 Fuel System Diagnosis and Repair
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?
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