A8: Engine Performance

A malfunctioning viscous fan clutch can indeed cause engine overheating at idle, as the fan may not engage properly to draw sufficient air through the radiator.

• Technician A is correct. Viscous fan clutches rely on a silicone-based fluid to engage the fan. If this fluid begins to leak or seep from the clutch assembly, the clutch may not function effectively, resulting in poor cooling performance, especially at low engine speeds or idle.

• Technician B is incorrect. Most viscous fan clutches are not controlled by a relay. Instead, they operate using a bi-metallic spring that responds to temperature changes. As the engine heats up, the spring expands, allowing the fluid to transfer torque to the fan. Therefore, a stuck relay is not typically a factor in viscous fan clutch operation.

An engine stumble or stall is not typically caused by a stuck closed EGR valve. When the EGR (Exhaust Gas Recirculation) valve is stuck closed, it prevents exhaust gases from entering the intake manifold. This can lead to increased combustion temperatures, potentially causing knocking or pinging, but it is more commonly associated with higher emissions, rough idle, or reduced efficiency, rather than immediate stalling.

Technician A is incorrect in associating a stuck closed EGR valve with engine stalling.

Similarly, Technician B is also incorrect. A stuck open EGR valve allows too much exhaust gas into the combustion chamber, which can displace oxygen, leading to a rich or unstable air-fuel mixture and resulting in engine stumbling, rough idle, or stalling—especially at low RPMs. However, it typically does not cause detonation. In fact, EGR systems are designed to reduce combustion temperatures, which helps prevent detonation, not cause it.

If a spark plug is disconnected, it can result in a misfire that is detected by the OBD II system. In such cases, the Malfunction Indicator Lamp (MIL) may be illuminated on the first trip, depending on the severity and frequency of the misfire.

The Engine Control Module (ECM) or Powertrain Control Module (PCM) monitors misfire activity within specific timeframes or driving cycles. If a significant number of misfires are detected during the first trip, the system may determine the issue is serious enough—such as one that could damage the catalytic converter—and turn on the MIL immediately.

However, the exact thresholds and criteria for MIL activation can vary between manufacturers. Some vehicles may require multiple trips with consistent misfire detection before illuminating the MIL, while others may activate it on the first trip if the fault is deemed critical.

Both Technician A and Technician B are correct.

• Technician A is correct in stating that a digital multimeter (DMM) can be used to check voltage values at the Powertrain Control Module (PCM) and other components within the fuel injection and ignition systems. Measuring these voltages helps diagnose issues related to power supply, sensor signals, or ground integrity.

• Technician B is also correct. Using a spark tester or similar diagnostic tool to check for spark at the spark plugs is a reliable method for identifying problems within the ignition system, such as faulty ignition coils, ignition modules, or control signals from the PCM.

Technician B is correct. Today, virtually all OEM service information is accessed online. Paper service manuals have become increasingly rare, with many manufacturers having discontinued printed manuals entirely.

Technician A is incorrect in believing that paper service manuals are still commonly used. While some technicians may still prefer physical manuals, the vast majority now rely on digital platforms for service and repair information.

There are several key reasons why online service information has become the industry standard:

• Real-time updates – Online resources are continuously updated by manufacturers to reflect the latest service procedures, technical bulletins, and vehicle changes.

• Comprehensive content – Online systems typically include detailed repair procedures, diagnostic flowcharts, torque specifications, wiring diagrams, and component locations—all in one place.

• Ease of access – Online manuals can be accessed from any device with internet connectivity, improving efficiency in the shop and supporting mobile diagnostics.

When searching for specific service information, it is essential to know the vehicle’s make (brand), model, and model year. This information is critical for accessing accurate and vehicle-specific resources such as service manuals, technical service bulletins (TSBs), wiring diagrams, diagnostic procedures, and repair instructions.

Because different makes and models often have significant differences in design, components, and service procedures—even within the same brand—having the exact make, model, and year ensures that the information retrieved is relevant and applicable to the vehicle being serviced.

This level of detail helps technicians perform accurate diagnostics, follow correct procedures, and use the appropriate tools and specifications, ultimately improving repair quality and efficiency.

Both Technician A and Technician B are correct.

• Technician A is correct in stating that a malfunctioning ignition coil can cause the engine to misfire under load. The ignition coil is responsible for generating the high voltage needed to ignite the air-fuel mixture. If the coil is weak or faulty, it may not deliver a strong or consistent spark—especially under load conditions—resulting in engine misfires and reduced performance.

• Technician B is also correct. The pickup coil (often part of the ignition system or distributor) provides crucial signals regarding crankshaft position and speed to the ECM or ignition control module. Applying dielectric grease under the pickup coil helps protect the electrical connections from moisture, corrosion, and contaminants, promoting better signal integrity and long-term reliability.

Both a faulty Engine Coolant Temperature (ECT) sensor and a faulty thermostat can cause the engine to run too rich, resulting in a high fuel-to-air ratio.

• The ECT sensor sends temperature data to the Engine Control Module (ECM), which uses that information to adjust the air-fuel mixture. If the ECT sensor provides incorrect readings—such as falsely indicating that the engine is cold—the ECM may enrich the fuel mixture unnecessarily, leading to a rich-running condition.

• The thermostat controls engine temperature by regulating coolant flow. If it’s stuck open, the engine may never reach proper operating temperature. As a result, the ECM may continue to operate in open-loop mode, enriching the fuel mixture to compensate for what it perceives as a cold engine.

Both Technician A and Technician B are correct.

• Technician A is correct in stating that the turbocharger boost pressure sensor measures the intake manifold’s boost pressure and sends this data to the Engine Control Module (ECM). The ECM uses this information to precisely adjust fuel injection and ignition timing, optimizing engine performance, power output, and efficiency under varying load and boost conditions.

• Technician B is also correct. A faulty boost pressure sensor can send inaccurate readings to the ECM, leading to incorrect fuel and timing adjustments. This can result in reduced engine power, poor acceleration, and lower fuel efficiency. In some cases, the ECM may even enter a limp mode to protect the engine, further limiting performance.

If an emissions test shows elevated levels of nitrogen oxides (NOx), the most likely cause is a stuck closed EGR (Exhaust Gas Recirculation) valve.

The EGR system is designed to reduce NOx emissions by recirculating a portion of exhaust gases back into the intake manifold. This process lowers combustion temperatures, which in turn reduces NOx formation, as NOx is primarily produced under high-temperature conditions. If the EGR valve is stuck closed, exhaust gases are prevented from re-entering the intake, resulting in higher combustion temperatures and increased NOx output.

Other issues—such as a bad ignition coil, a leaking fuel injector, or random misfires—can affect overall engine performance and emissions, but they are less likely to significantly impact NOx levels.