310T Engine and Fuel Systems Questions and Answers — Questions and Answers
Question 1: A truck equipped with a Selective Catalytic Reduction (SCR) system is experiencing increased Diesel Exhaust Fluid (DEF) consumption and has an active P20EE fault code. What is the MOST likely cause of this issue?
- A clogged Diesel Particulate Filter (DPF)
- A malfunctioning upstream NOx sensor (Correct answer)
- Low-quality or contaminated DEF
- A failing turbocharger actuator
Correct answer: A malfunctioning upstream NOx sensor
The P20EE code indicates that the SCR catalyst efficiency is below the required threshold. The aftertreatment control module monitors NOx levels using both upstream (engine-out) and downstream (tailpipe-out) NOx sensors. If the upstream NOx sensor provides inaccurately high readings, the system will inject an excessive amount of DEF to compensate, leading to increased consumption and eventually triggering an efficiency code when the downstream sensor does not report the expected reduction in NOx.
Question 2: During a forced regeneration of a Diesel Particulate Filter (DPF), what is the primary method used by the engine control module (ECM) to increase the exhaust gas temperature to the required level for soot oxidation?
- Activating the engine's compression brake
- Closing the vanes on the variable geometry turbocharger (VGT)
- Injecting a small amount of fuel into the exhaust stream during the exhaust stroke (Correct answer)
- Increasing the engine idle speed to maximum RPM
Correct answer: Injecting a small amount of fuel into the exhaust stream during the exhaust stroke
Active and forced DPF regeneration require exhaust temperatures of around 600°C (1112°F) to burn off trapped soot. The most common method to achieve this is by post-combustion injection, where a small amount of diesel fuel is injected late in the exhaust stroke. This unburnt fuel travels to the Diesel Oxidation Catalyst (DOC) upstream of the DPF, where it oxidizes and generates significant heat, raising the exhaust temperature for DPF regeneration.
Question 3: A technician is diagnosing a low-power complaint on a diesel engine with a high-pressure common rail (HPCR) fuel system. A fuel rail pressure test reveals that the actual pressure is significantly lower than the desired pressure while under load. Which of the following is the LEAST likely cause of this condition?
- A leaking high-pressure fuel injector
- A restricted fuel return line (Correct answer)
- A worn high-pressure fuel pump
- A clogged fuel filter
Correct answer: A restricted fuel return line
Restricting the fuel return line on an HPCR system would cause housing pressure within the high-pressure pump to build excessively, potentially blowing out seals. It would not cause low rail pressure. Leaking injectors, a worn pump, or a clogged fuel filter are all common causes of low fuel rail pressure because they either prevent the system from building adequate pressure or introduce excessive leakage.
Question 4: What is the primary function of a Variable Geometry Turbocharger (VGT) at low engine speeds?
- To open its vanes to maximize exhaust flow and prevent over-speeding
- To act as an exhaust brake by creating high backpressure
- To close its vanes to increase exhaust gas velocity, improve turbine response, and build boost quickly (Correct answer)
- To bypass exhaust gas around the turbine to limit maximum boost pressure
Correct answer: To close its vanes to increase exhaust gas velocity, improve turbine response, and build boost quickly
At low engine speeds, the vanes inside a VGT move to a more closed position, narrowing the passage for exhaust gases. This restriction increases the velocity of the exhaust gas hitting the turbine wheel, causing it to spool up faster and generate boost more quickly, which reduces turbo lag and improves low-end torque.
Question 5: A coach arrives at the shop with the malfunction indicator lamp (MIL) on. The technician finds fault codes related to SCR catalyst contamination. Which of the following would be a direct cause of this type of failure?
- Using a non-OEM air filter
- Extended periods of engine idling
- Adding diesel fuel to the DEF tank (Correct answer)
- A faulty EGR valve
Correct answer: Adding diesel fuel to the DEF tank
The SCR catalyst is extremely sensitive to contamination. Adding any substance other than DEF, such as diesel fuel, coolant, or oil, into the DEF tank will cause irreversible damage to the catalyst substrate, leading to costly repairs and system failure.
Question 6: Which of the following symptoms is most commonly associated with a clogged fuel filter in a heavy-duty truck?
- Excessive black smoke under load
- Engine overheating
- Loss of power, especially during acceleration or when climbing grades (Correct answer)
- A loud knocking sound from the engine
Correct answer: Loss of power, especially during acceleration or when climbing grades
A clogged fuel filter restricts the flow of fuel to the engine. This fuel starvation becomes most apparent when the engine's demand for fuel is highest, such as during acceleration or when under heavy load (e.g., climbing a hill), resulting in a noticeable loss of power.
310T Engine and Fuel Systems Questions and Answers
A truck equipped with a Selective Catalytic Reduction (SCR) system is experiencing increased Diesel Exhaust Fluid (DEF) consumption and has an active P20EE fault code.
What is the MOST likely cause of this issue?