Engine and Fuel Systems Flashcards

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?

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.

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?

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.

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?

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.

What is the primary function of a Variable Geometry Turbocharger (VGT) at low engine speeds?

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.

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?

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.

Which of the following symptoms is most commonly associated with a clogged fuel filter in a heavy-duty truck?

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.