A9: Light Vehicle Diesel Engines (Sample Test)

The top of the cylinder bore is the most common location for wear. This is due to the high temperatures generated during combustion, which cause repeated expansion and contraction of metal surfaces. Combined with the force of combustion and friction from the piston rings, this leads to increased wear at the upper portion of the cylinder.

Cylinder taper wear refers to the difference in diameter between the top and bottom of the cylinder bore. It occurs over time as friction from the piston rings and exposure to combustion gases cause more material to wear away at the top than at the bottom. Excessive taper wear can lead to piston ring blow-by, oil consumption, engine misfires, and reduced fuel efficiency.

To measure taper wear, a dial bore gauge is used. Measurements are taken at the top, middle, and bottom of the cylinder bore. The difference between the top and bottom readings indicates the amount of taper.

The acceptable limit for taper wear varies by engine design, so it is essential to consult the manufacturer’s specifications to determine if wear is within acceptable limits.

Precombustion chambers in a cylinder head do not always require replacement if minor cracks are present. The need for replacement depends on the severity and location of the damage, as well as the specific engine design and manufacturer guidelines.

In many cases, minor surface cracks—especially those that do not affect structural integrity or sealing surfaces—may be considered acceptable or even repairable using approved methods. However, if the cracks are deep, growing, or located in critical areas, replacement may be necessary to ensure proper combustion, engine efficiency, and reliability.

When addressing a driveability concern, connecting a scan tool to the Diagnostic Link Connector (DLC) and checking the calibration number of the Powertrain Control Module (PCM) or Engine Control Module (ECM) is a critical diagnostic step.

The calibration number indicates the software version currently programmed into the control module. Manufacturers periodically release updated calibrations to correct known issues, enhance performance, improve fuel economy, or address emissions-related concerns.

If the calibration number on the vehicle does not match the latest version, reprogramming the PCM/ECM with the most recent calibration may resolve the issue without the need for physical repairs or part replacements.

Technician A is correct.
Using a weak solution of antifreeze can lead to corrosion build-up in the cooling system. Antifreeze contains corrosion inhibitors that protect metal components—such as the radiator, water pump, and engine block—from rust and deterioration. If the antifreeze concentration is too low, there may not be sufficient inhibitors to prevent corrosion, increasing the risk of internal damage over time.

Technician B is incorrect.
Most OEMs (Original Equipment Manufacturers) recommend changing engine coolant at specified intervals. This is because the corrosion inhibitors in coolant degrade over time, even if the coolant level appears adequate. Once these additives are depleted, the cooling system becomes more susceptible to rust, scale, and electrochemical damage. Failing to replace coolant as recommended can lead to overheating, component failure, and costly repairs.

Both Technician A and Technician B are correct.

• Technician A is right in stating that oil cooling nozzles should be removed and inspected during diesel engine disassembly. This is standard procedure during engine rebuilding to ensure the nozzles are clean, free of debris or blockages, and functioning properly. Any damaged or clogged nozzles should be cleaned, repaired, or replaced before reassembly.

• Technician B is also correct. Proper alignment of the nozzles during installation is critical. These nozzles are designed to spray oil onto the underside of the pistons for cooling. If misaligned, the oil spray may miss the intended target, resulting in insufficient piston cooling, which can lead to overheating and engine damage.

If a replacement fuel line is shorter or longer than the original, it can affect the timing and delivery of fuel—particularly to individual cylinders like cylinder #3. Variations in fuel line length or routing can influence fuel pressure and delivery timing, which may lead to uneven idling, misfires, or poor combustion performance.

A rough idle or misfire in a hot engine is not typically caused by a faulty glow plug. Glow plugs are primarily used to assist with cold starting by preheating the combustion chamber. Once the engine is running and warmed up, glow plugs play a minimal role in maintaining smooth operation.

Regarding fuel quality:

• High Cetane fuel improves ignition quality by reducing the ignition delay period. This leads to smoother combustion, better performance, and fewer misfires.

• Low Cetane fuel has poor ignition quality, resulting in a longer delay before combustion begins. This can cause hard starts, white smoke, and misfiring, especially in cold conditions or under light loads.

Overheating of brake fluid is one of the key factors that can contribute to brake fade—a condition where the braking system’s effectiveness is significantly reduced due to excessive heat.

When brake fluid becomes too hot, it can begin to boil, forming vapor pockets in the hydraulic system. Since vapor is compressible (unlike fluid), this leads to a loss of hydraulic pressure, resulting in a soft or spongy brake pedal and reduced braking performance.

In addition to overheated brake fluid, other common causes of brake fade include:

• Excessive friction and heat buildup from aggressive or prolonged braking (e.g., downhill driving)

• Worn brake pads or discs, which reduce heat dissipation and braking efficiency

• Inadequate cooling of braking components, especially in high-performance or heavy-duty applications

The brake rotor plays a critical role in the braking system by converting kinetic energy into heat to slow down the vehicle. When the brake pads clamp against the rotating rotor, friction is generated. This friction transforms the vehicle’s kinetic energy (motion) into thermal energy (heat), allowing the vehicle to decelerate and stop safely. Efficient heat dissipation by the rotor is essential to maintain consistent braking performance and prevent brake fade.

The hand brake, also known as the parking brake or emergency brake, is primarily used for parking or in emergency situations. It operates independently from the main hydraulic braking system and is typically manually engaged via a lever, pedal, or electronic switch.

The hand brake usually activates the rear brakes—often through a cable mechanism—to hold the vehicle stationary when parked. In the event of hydraulic brake failure, it can also serve as a backup system to assist in slowing or stopping the vehicle in an emergency.

In a high-pressure pump system, the intake pipe of the pump experiences the lowest pressure, as it draws fluid into the system. In contrast, positive pressure is found at the pump’s outlet, where the fluid is discharged under high force.

Similarly, positive pressure is present at the inlet of the secondary air filter and at the inlet side of the primary filter, as these points are upstream in systems designed to maintain airflow or fluid flow under pressure. This pressure differential is essential for proper system operation, ensuring that air or fluid flows efficiently through the filters and into the pump or engine.