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Practice Test (A9: Light Vehicle Diesel Engines) 1 Flashcards

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  1. A technician is diagnosing a light-duty diesel engine that cranks but will not start in cold weather. The glow plug indicator light does not illuminate. What is the MOST likely cause?

    Answer: Failed glow plug control module

    The glow plug control module energizes the glow plugs and signals the dashboard indicator. If the module fails, neither the glow plugs nor the lamp will activate, making cold starts difficult or impossible.

  2. A common-rail diesel fuel system is operating at lower than specified rail pressure at idle. Which component should the technician inspect FIRST?

    Answer: Fuel pressure regulator/inlet metering valve

    The inlet metering valve (or fuel pressure regulator) on the high-pressure pump controls the amount of fuel entering the pump, directly determining rail pressure. A sticking or failed regulator is the most common cause of low rail pressure at idle.

  3. White smoke from a diesel engine that persists after the engine reaches normal operating temperature is MOST likely caused by:

    Answer: Incomplete combustion due to coolant entering the combustion chamber

    Persistent white smoke after warm-up typically indicates coolant (water vapor) entering the combustion chamber through a blown head gasket or cracked cylinder head. Cold-start white smoke clears once the engine warms, but coolant intrusion continues indefinitely.

  4. A diesel technician performs a cylinder contribution (cut-out) test and finds that dropping cylinder #3 produces no change in engine roughness or RPM. What does this indicate?

    Answer: Cylinder #3 injector is not contributing power to the engine

    During a cylinder contribution test, a normally firing cylinder will cause a noticeable RPM drop when cut out. If cutting out a cylinder causes no change, that cylinder is not producing power, indicating a failed injector, no compression, or no fuel delivery to that cylinder.

  5. After replacing a diesel high-pressure fuel injector, the technician must perform which of the following procedures using a scan tool before returning the vehicle to service?

    Answer: Injector quantity/trim code programming

    Modern common-rail injectors have unique flow-rate calibration codes (IMA, C2I, or QR codes) that must be programmed into the ECM so the engine management system can compensate for individual injector flow characteristics and maintain correct fueling balance.

  6. A diesel vehicle equipped with a selective catalytic reduction (SCR) system has a MIL on and a DTC for low NOx conversion efficiency. Which component is MOST likely at fault?

    Answer: Diesel exhaust fluid (DEF) dosing injector — clogged or failed

    The SCR system reduces NOx by injecting diesel exhaust fluid (DEF/urea) into the exhaust stream ahead of the SCR catalyst. A clogged or failed DEF dosing injector prevents urea from reaching the catalyst, drastically reducing NOx conversion efficiency.