ASE Practice Test (A9: Light Vehicle Diesel Engines) 2

Question 1

A light-duty diesel vehicle has a DTC for excessive diesel particulate filter (DPF) backpressure. What is the MOST likely cause?

Accepted Answer

Clogged DPF requiring regeneration or replacement

Answer

The DPF traps soot from diesel exhaust. Over time it becomes clogged and requires regeneration (burning off the soot at high temperatures). A DTC for excessive backpressure indicates the DPF is overloaded with particulate matter. If passive and active regenerations fail to clear it, the DPF may need forced regeneration or replacement.

Question 2

What is the function of the exhaust gas recirculation (EGR) system in a diesel engine?

Accepted Answer

Reduce NOx emissions by lowering combustion temperature

Answer

EGR reduces nitrogen oxides (NOx) emissions by recirculating a portion of exhaust gas back into the intake manifold. The inert exhaust gas dilutes the air-fuel mixture and absorbs heat, lowering peak combustion temperatures. Lower temperatures produce less NOx, which forms when nitrogen and oxygen combine at very high heat.

Question 3

During a diesel fuel system inspection, a technician finds air bubbles in the fuel return line. What should the technician check FIRST?

Accepted Answer

Fuel filter, supply lines, and lift pump for air leaks

Answer

Air in diesel fuel is a critical problem that causes hard starting, rough running, and power loss. Air enters through the low-pressure (supply) side of the fuel system. The technician should start at the fuel tank and work toward the injection pump, checking the fuel filter housing, supply lines, lift pump, and fittings for leaks that allow air ingestion.

Question 4

A turbocharged diesel engine produces black smoke under acceleration. Which of the following is the MOST likely cause?

Accepted Answer

Restricted air intake or over-fueling condition

Answer

Black smoke in a diesel engine indicates incomplete combustion caused by an excessively rich mixture — too much fuel or too little air. A restricted air filter, collapsed intake duct, or a turbocharger not producing sufficient boost will limit air supply. A faulty injection pump or stuck injectors can also cause over-fueling. The air/fuel ratio must be balanced for complete combustion.

Question 5

What does the term "pilot injection" refer to in a common rail diesel fuel system?

Accepted Answer

The first injection event per cycle, a small quantity before the main injection to reduce noise and combustion harshness

Answer

Common rail systems can perform multiple injection events per cycle. Pilot injection delivers a very small amount of fuel just before the main injection. This small pilot charge begins combustion and raises cylinder temperature and pressure slightly, so the main injection ignites more smoothly and gradually. This reduces the characteristic diesel knock and combustion noise.