Heavy Duty Equipment Technician Red Seal Exam Prep · Question
After performing routine maintenance, a technician experiences difficulty starting a modern diesel engine equipped with a High-Pressure Fuel Pump (HPFP) and common rail system. The diagnostic scanner indicates insufficient rail pressure during cranking. What is the most likely cause?
Air in the fuel system is a very common issue after maintenance that involves opening fuel lines or changing filters. Air is compressible, preventing the High-P
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Question: After performing routine maintenance, a technician experiences difficulty starting a modern diesel engine equipped with a High-Pressure Fuel Pump (HPFP) and common rail system. The diagnostic scanner indicates insufficient rail pressure during cranking. What is the most likely cause?
Answer options: ✅ Air introduced into the fuel system, preventing the HPFP from building adequate pressure.
- A faulty crank position sensor, preventing the ECM from commanding injection.
- A seized high-pressure fuel pump (HPFP) relay, providing continuous power.
- Low battery voltage, inhibiting the operation of the engine's starter motor.
Correct answer: Air introduced into the fuel system, preventing the HPFP from building adequate pressure.
Explanation: Air in the fuel system is a very common issue after maintenance that involves opening fuel lines or changing filters. Air is compressible, preventing the High-Pressure Fuel Pump (HPFP) from effectively building the necessary high fuel pressure in the common rail for injection, leading to a no-start condition and a low rail pressure fault code. ECM faults or low battery would typically show different symptoms or codes related to those systems.
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