A8: Engine Performance

Both Technician A and Technician B are correct.

Technician A is right in stating that a digital multimeter (DMM) can be used to measure voltage at the Powertrain Control Module (PCM) and other electrical components involved in the fuel injection and ignition systems. These voltage checks are essential for diagnosing issues related to power supply and circuit integrity.

Technician B is also correct that using a spark tester or similar device to check for spark at the spark plugs is an effective way to determine if the ignition system is functioning properly. This test can reveal issues such as a failed ignition coil or faulty wiring.

Together, both techniques provide a comprehensive approach to diagnosing electrical and ignition system problems.

Technician B is correct. Most OEM service information is now accessed online, as paper service manuals have become increasingly rare. Many manufacturers have phased out printed manuals entirely.

Technician A is incorrect in believing that paper service manuals are still commonly used. Although some technicians may still prefer them, the majority now rely on digital resources for service information.

Online service information has become the standard for several reasons. It is more current, with regular updates reflecting the latest vehicle changes. It is also more comprehensive, often including step-by-step repair procedures, diagrams, and wiring schematics. Additionally, it is more accessible, allowing technicians to retrieve information from any location with internet access.

When looking for specific service information, it’s essential to know the vehicle’s make, model, and model year. This information is critical for finding accurate service manuals, technical bulletins, wiring diagrams, diagnostic procedures, and other vehicle-specific resources.

Since different makes and models can vary significantly in components, systems, and repair procedures, having the correct details ensures the information retrieved is relevant and accurate. It helps technicians access the most current and precise data needed for effective diagnostics, repairs, and maintenance.

Both Technician A and Technician B provide valid insights regarding ignition system maintenance and diagnostics.

Technician A is correct in stating that a faulty ignition coil can lead to engine misfires under load. The ignition coil generates the high voltage necessary to ignite the air-fuel mixture in the engine’s cylinders. If the coil fails to deliver a strong or consistent spark, misfires can occur—especially when the engine demands more power.

Technician B is also correct in recommending the use of dielectric grease under the pickup coil. The pickup coil—also known as the crankshaft position sensor or distributor pickup—relays crankshaft position and speed information to the ECM or ignition control module. Applying dielectric grease helps protect the electrical connections from moisture, corrosion, and contaminants, thereby improving reliability and performance.

Together, their recommendations support proper ignition system function and effective misfire diagnosis.

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

The ECT sensor sends data to the Engine Control Module (ECM) about the coolant temperature, which the ECM uses to adjust the fuel mixture. If the sensor provides inaccurate readings—such as indicating the engine is colder than it actually is—the ECM may deliver more fuel than needed, creating a rich mixture.

Similarly, the thermostat helps regulate the engine’s operating temperature by controlling coolant flow. If it’s stuck open, the engine may stay cooler than intended, prompting the ECM to enrich the fuel mixture unnecessarily.

In both cases, the result is an overly rich air-fuel ratio due to incorrect temperature data or improper thermal regulation.

Turbocharged engines use a turbocharger to compress and force more air into the engine, increasing power output. The turbocharger boost pressure sensor monitors the pressure of this incoming air and sends the data to the Engine Control Module (ECM).

Technician A is correct in stating that the boost pressure sensor provides the ECM with intake manifold boost pressure. The ECM uses this data to adjust fuel injection and ignition timing, helping the engine operate efficiently and perform optimally.

Technician B is also correct in noting that a faulty boost pressure sensor can negatively affect vehicle acceleration. If the sensor sends inaccurate data, the ECM may miscalculate fuel delivery and timing, leading to reduced power, poor acceleration, and lower fuel efficiency.

Both technicians accurately describe the critical role of the boost pressure sensor in maintaining turbocharged engine 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 helps reduce NOx emissions by redirecting a portion of exhaust gases back into the intake manifold. This process lowers peak combustion temperatures and reduces the amount of oxygen available for combustion, both of which help limit NOx formation. When the EGR valve is stuck closed, exhaust gases cannot be recirculated, resulting in higher combustion temperatures and increased NOx emissions.

While issues like a faulty ignition coil, a leaking fuel injector, or random misfires can affect engine performance, they typically do not lead to a significant rise in NOx emissions.

Technician A is correct in stating that an ignition misfire can lead to elevated carbon monoxide (CO) and hydrocarbon (HC) emissions and will typically trigger a Diagnostic Trouble Code (DTC). A misfire results in incomplete combustion, allowing unburned fuel to enter the exhaust system, which raises CO and HC levels. Modern vehicles are equipped with systems that monitor engine performance and detect misfires, prompting the ECM or PCM to set a corresponding DTC.

Technician B is incorrect in claiming that a misfire caused by a lean condition will increase HC and CO emissions without setting a DTC. While a lean misfire can raise these emissions, the vehicle’s onboard diagnostics system can still detect the abnormal combustion and will typically set a misfire-related DTC.

Therefore, Technician A’s explanation is accurate, while Technician B’s statement does not align with how modern engine diagnostics function.

Spark knock is a common symptom of an EGR (Exhaust Gas Recirculation) valve that is stuck closed. The other options are incorrect, as they are typically associated with an EGR valve that is stuck open.

Spark knock occurs when the air-fuel mixture in the cylinder ignites prematurely—before the spark plug fires. This premature combustion can be caused by excessive heat or a lean air-fuel mixture. A closed EGR valve contributes to this condition by preventing exhaust gases from recirculating, which leads to higher combustion temperatures and a leaner mixture, increasing the likelihood of spark knock.

A four-wire oxygen sensor signal that cycles between 0.150 volts and 0.850 volts indicates the sensor is working correctly and responding to changes in the exhaust’s oxygen content.

Therefore, any claim that the sensor has failed or is biased/stuck rich is incorrect. A sensor stuck rich would produce a consistently high voltage signal and would not cycle as expected, indicating it is not accurately measuring the exhaust gases.

However, the statement that this sensor includes a heater circuit is likely true. Most four-wire oxygen sensors feature an internal heating element to help the sensor reach operating temperature faster and maintain consistent performance for accurate readings.

The PCV (Positive Crankcase Ventilation) valve is designed to regulate the flow of gases between the crankcase and the intake manifold. It contains a spring-loaded internal valve that opens and closes as needed to ensure proper crankcase ventilation.

The rattling sound heard when shaking the PCV valve is typically caused by the movement of this internal valve and spring, indicating that the valve is not seized and is likely operating as intended.

If the PCV valve becomes clogged or stuck open, it cannot regulate airflow properly, which may lead to problems such as increased oil consumption, oil leaks, or rough idling. A broken spring inside the valve would also impair its function, causing similar issues.

Therefore, hearing a rattling sound when shaking the PCV valve generally suggests that it is functioning properly.

Technician B is correct in stating that allowing the engine to reach normal operating temperature before checking for diagnostic trouble codes (DTCs) is an important step in effectively using the vehicle’s onboard diagnostic system.

When the engine is cold, it operates in what is known as “open loop” mode. During this phase, the engine control module (ECM) uses preset parameters—rather than real-time sensor data—to manage the air-fuel ratio. The fuel mixture is richer, and idle speed is higher to aid cold starts. As a result, certain emissions-related tests are not active, and some issues may go undetected.

Once the engine reaches its normal operating temperature, the ECM switches to “closed loop” mode. In this mode, it relies on input from sensors such as the oxygen sensor, coolant temperature sensor, and mass airflow sensor to fine-tune engine performance. This is also when the ECM performs a full range of self-tests on components like the catalytic converter, oxygen sensors, and evaporative emissions system. These tests help detect faults that might not appear in open loop operation.

Testing for DTCs after the engine has warmed up ensures that the ECM has completed its self-diagnostics and can accurately detect potential issues.

As for the other options, it is incorrect to say the onboard diagnostic system only activates after five minutes of continuous driving. The OBD-II system begins monitoring as soon as the ignition is turned on, although certain tests only run under specific conditions—such as during closed loop operation. The scan tool shown in the picture is a standard OBD-II scanner capable of retrieving DTCs at any time, regardless of engine temperature or drive time.

Bank 1 Sensor 2 monitors the efficiency of the catalytic converter in reducing exhaust pollutants. Its signal typically shows a much smaller amplitude compared to Bank 1 Sensor 1. This is because the catalytic converter breaks down harmful emissions using oxygen from the exhaust gases and secondary air injection.

The Throttle Position Sensor (TPS) monitors the position of the throttle plate and sends this data to the engine control module (ECM), which uses it to adjust the air-fuel mixture and ignition timing for optimal performance.
If the TPS is faulty, it can send inaccurate signals to the ECM, leading to an incorrect air-fuel ratio. This often results in momentary hesitation during acceleration.
While issues with components like the EGR valve, oxygen sensor, or PCV valve can affect engine performance, they are less likely to cause hesitation during acceleration. A faulty EGR valve may lead to rough idling or stalling, a bad oxygen sensor can reduce fuel economy or increase emissions, and a faulty PCV valve may cause oil-related problems.
Therefore, a faulty TPS is the most likely cause of brief hesitation when accelerating.

Excessive blowby is not a direct cause of an inoperative PCV system. The PCV (Positive Crankcase Ventilation) system is designed to manage blowby gases from the crankcase. When the system fails, it can lead to symptoms such as oil leaks, oil in the air cleaner housing, and increased emissions.
In contrast, a plugged PCV valve, restricted or kinked hoses, and oil in the air cleaner housing are valid causes or indicators of a malfunctioning PCV system.

This diagram illustrates a waste spark ignition system, commonly used in older vehicles. In this system, each ignition coil serves two cylinders, firing both spark plugs at the same time—one during the compression stroke and the other during the exhaust stroke. Since the spark on the exhaust stroke isn’t needed for combustion, it’s considered a “wasted” spark, giving the system its name.

Technician A is correct—a crushed Positive Crankcase Ventilation (PCV) hose can affect engine idle and may lead to oil leaks. The PCV system regulates pressure within the engine crankcase, and a damaged or restricted hose can disrupt this balance. As a result, improper ventilation can cause increased pressure or vacuum, leading to rough idling, reduced fuel efficiency, and oil leakage.

Technician B is incorrect. While measuring crankcase pressure at 2500 RPM is a standard method for assessing PCV function, slight crankcase pressure at idle is not a reliable indicator of proper system operation. Crankcase pressure can vary due to factors such as engine design, component wear, and environmental conditions. Therefore, idle pressure alone is not a dependable diagnostic tool for PCV issues.

A faulty EGR solenoid on a four-cylinder EFI engine can lead to insufficient EGR flow, resulting in increased nitrogen oxide (NOx) emissions.
The EGR (Exhaust Gas Recirculation) system helps reduce NOx by directing a portion of exhaust gases back into the intake manifold. This lowers oxygen levels in the combustion chamber and reduces peak combustion temperatures. When the EGR solenoid fails, the system can’t function properly, allowing combustion temperatures to rise and NOx emissions to increase.

In the P0301 diagnostic trouble code, the “P” stands for Powertrain, indicating the issue is related to the engine, transmission, or related components. The “0” signifies that the code is a generic (SAE) code, not manufacturer-specific, and applies to all vehicle makes and models. The “3” designates a misfire-related problem, while “01” pinpoints the misfire to cylinder 1.

The PCV (Positive Crankcase Ventilation) valve allows crankcase gases to be routed back into the intake manifold, helping reduce emissions and maintain engine cleanliness.
While it doesn’t directly control the air/fuel mixture, it can influence it indirectly. By limiting oil vapors entering the intake, the PCV valve prevents dilution of the mixture, which could otherwise cause the engine to run rich.
Since the PCV valve is a mechanical—not electronic—component, it has no self-learned value. Therefore, there’s no need to reset or clear any learned data after replacing or repairing it.

Technician B is correct—a voltage drop test is useful for identifying excessive resistance in a circuit.
Technician A is incorrect in stating that the meter should be set to ohms. For a voltage drop test, the meter must be set to measure voltage, not resistance.
This test involves measuring the voltage difference across a component, such as a ground wire or switch. A significant voltage drop indicates excessive resistance, which may be caused by issues like loose connections, corrosion, or damaged wiring.

Once the technician has diagnosed and repaired the issue, the final step is to verify the repair. This involves retesting the vehicle to ensure the problem is resolved and all systems operate correctly.
Prior to this, the technician should have completed key steps such as verifying the customer concern, performing service checks, running diagnostic tests, inspecting components, and reviewing vehicle history.
Therefore, “Verify repair” is the correct answer, as it represents the final step in the diagnostic process after all necessary repairs have been made.

The correct answer is (a) Discussing the concern with the vehicle owner.
When diagnosing an engine performance issue, the first step should be gathering information directly from the owner. They can provide valuable details about the symptoms, when they occur, and any recent changes or events that may be related. This helps narrow down potential causes early in the process.
Although retrieving diagnostic trouble codes (b), consulting the service manual (c), and road testing the vehicle (d) are all important steps, they typically follow the initial conversation. Starting with the owner’s input can streamline the diagnostic process and focus attention on the most relevant areas.

Noncontinuous monitors are used to test the Heated Oxygen Sensor (HO2S).
The HO2S, also known as the front oxygen sensor, plays a key role in the emissions control system. It measures oxygen levels in the exhaust and sends this data to the engine control module (ECM), which adjusts the air-fuel mixture for optimal performance and efficiency.

Unlike continuous monitors, noncontinuous monitors operate only under specific driving conditions—such as steady cruising or deceleration. One of these monitors checks the performance of the HO2S by evaluating its response to changes in the air-fuel mixture.

If the sensor reacts too slowly or fails to respond correctly, the monitor will trigger a diagnostic trouble code (DTC) and activate the malfunction indicator lamp (MIL) on the dashboard.

Technician B is correct. A wet compression test involves adding a small amount of oil to each cylinder to temporarily seal gaps between the piston rings and cylinder walls. If compression pressure increases but still falls below specifications, it suggests the piston rings are worn and not sealing properly.

Worn rings allow combustion gases to escape, reducing compression. The added oil helps seal the gaps momentarily, causing a pressure increase. However, if the pressure still remains low, it indicates the rings are likely the issue and may need replacement.

Technician A is incorrect. While incorrect valve timing can affect compression and engine performance, it does not typically cause a consistent rise in cylinder pressure during a wet compression test.

Negative fuel trim values (–5% at idle and –25% while cruising) indicate that the engine control module (ECM) is reducing fuel delivery to compensate for a rich condition detected by the oxygen sensor.

However, when a vacuum hose is disconnected—a condition that should make the mixture lean—the oxygen sensor voltage does not change, suggesting it’s not responding to variations in the air-fuel ratio. This points to a possible fault in the sensor or its wiring.

Therefore, the most likely cause is a faulty oxygen sensor or an issue with its wiring. Further diagnostic testing is recommended to confirm the exact source of the problem.

The Malfunction Indicator Lamp (MIL), or check engine light, signals a problem with the vehicle’s emissions system. Manually cycling the MIL on and off does not help diagnose the issue and may actually erase Diagnostic Trouble Codes (DTCs), making diagnosis more difficult.

In contrast, the following are approved methods for retrieving DTCs:

• Using jumper wires at the data link connector (DLC)—commonly used on older vehicles.

• Cycling the ignition switch three times within five seconds—works on some older models.

• Using a scan tool—the most accurate and widely used method for reading DTCs on modern vehicles.

A loose connection at the crankshaft position sensor can cause the engine to stall, particularly during stops or turns when the engine is under load. This sensor provides critical data to the engine control module (ECM) about the crankshaft’s position, which is used to time spark and fuel injection. If the connection is faulty and the sensor sends inaccurate or intermittent signals, the ECM cannot manage engine operation properly, leading to stalling.

The engine starting process is electronically disabled by the vehicle’s security system. This system typically includes an ignition switch with an antenna ring and a unique transponder or chip key. If the signal from the key does not match the programmed data in the control module, the engine will not start.

On vehicles equipped with OBD II, catalytic converter efficiency is monitored using two oxygen sensors. The front (upstream) oxygen sensor is positioned before the catalytic converter, while the rear (downstream) sensor is located after it.
During closed-loop operation, the engine control module (ECM) compares the signals from both sensors to evaluate the converter’s performance.
The rear oxygen sensor is used solely for diagnostic purposes and typically does not affect engine performance or drivability.

In the United States, OBD-II (On-Board Diagnostics II) systems became mandatory in all new vehicles sold after January 1, 1996, as required by the Environmental Protection Agency (EPA) to reduce emissions and improve air quality.
OBD-II systems monitor key vehicle components—such as the engine, transmission, and emissions controls—using sensors and diagnostic trouble codes (DTCs). This enables technicians to quickly diagnose and repair issues, helping vehicles remain compliant with emissions standards throughout their lifespan.

Blue exhaust smoke is usually a sign of burning oil, and while worn valve seals can contribute, they are generally a less significant source of oil consumption compared to other issues. Valve seals prevent oil from leaking into the combustion chamber, but blue smoke is more often caused by worn piston rings, damaged cylinder walls, or worn valve guides, which allow oil to enter the combustion process and produce visible blue emissions.

If there is no power at the COP ignition coil’s Bat+ terminal, it likely indicates an open circuit between the coil and its 12-volt power source. This could be due to a blown fuse, faulty relay, broken wire, or poor wiring connection.

While resistance in the primary or secondary circuits can lead to ignition problems like weak spark or misfires, it typically does not result in a total loss of power at the Bat+ terminal. Similarly, a leak in the secondary insulation may cause high-voltage breakdowns and misfires, but it wouldn’t cut off power to the coil’s Bat+ terminal.

The knock sensor detects engine detonation, or “knocking,” which occurs when the air-fuel mixture ignites prematurely in the cylinder. To prevent engine damage, the sensor generates an AC voltage signal when detonation is detected and sends it to the engine control module (ECM). The ECM then responds by retarding ignition timing to stop further knocking.

In contrast, other sensors serve different functions:

• The MAP sensor measures intake manifold pressure and helps the ECM calculate the correct air-fuel ratio.

• The oxygen sensor monitors oxygen levels in the exhaust to fine-tune the air-fuel mixture.

• The throttle position sensor tracks throttle movement and helps the ECM control engine speed and power output.

On-board self-diagnostic test procedures may require different engine conditions depending on the vehicle. Some tests need a cold engine, while others require a warm engine or operation within a specific temperature range.

Technician B is correct—many diagnostic trouble codes (DTCs) only run when the engine is in closed loop operation. In closed loop, the engine control module actively adjusts the air-fuel mixture and other parameters using sensor feedback to meet emissions and performance standards. As a result, certain tests and DTCs are only triggered under these conditions.

In older engines, carbon buildup commonly occurs inside the distributor cap. This buildup can interfere with proper contact between the rotor and the terminals, leading to engine misfires and, in some cases, preventing the engine from starting.

The purge solenoid is part of the evaporative emission control system and regulates the flow of fuel vapors from the charcoal canister to the intake manifold. If the purge solenoid is faulty, it can disrupt the air-fuel mixture, leading to rough idling and hard starting. It may also trigger a fault code related to the EVAP system.

While issues like a clogged charcoal canister or an open vent valve can affect the EVAP system, they are less likely to cause these specific symptoms. Similarly, a faulty fuel level sensor is unlikely to be the source of the problem in this case.

Technician B is correct. A rattling sound from the PCV valve indicates it isn’t stuck and can move freely. However, this doesn’t guarantee the valve is functioning properly. Over time, PCV valves can become dirty or worn, reducing their effectiveness—even if they still rattle.

The PCV valve regulates gas flow between the crankcase and intake manifold. If it’s clogged or degraded, it may fail to control crankcase pressure or allow proper ventilation.

So, while rattling is a good sign, it’s not definitive. If symptoms like poor engine performance or increased oil consumption are present, the PCV valve may still need replacement.

A stuck-open EGR valve can cause a rough idle by allowing too much exhaust gas to enter the intake manifold. The EGR (Exhaust Gas Recirculation) valve is designed to recirculate a small amount of exhaust gas to lower combustion temperatures and reduce NOx emissions.

When stuck open, it recirculates excessive exhaust, reducing the oxygen available for combustion. This can lead to a lean air/fuel mixture, resulting in rough idling, misfires, and poor engine performance.

Low emissions are the least likely symptom of an evaporative emission system failure.
The evaporative emission system is designed to trap and store fuel vapors from the fuel tank, preventing them from escaping into the atmosphere. When the system fails, common symptoms include fuel odor, rough idle, and illumination of the Malfunction Indicator Lamp (MIL).

Rather than lowering emissions, a failure in this system typically results in increased emissions, as uncontained fuel vapors are released into the air.

Technician B is correct. A Long Term Fuel Trim (LTFT) of -19% at idle indicates that the Engine Control Module (ECM) is reducing the amount of fuel being delivered, not increasing it. This typically occurs when the ECM detects a rich condition—too much fuel and not enough air in the mixture.

In this scenario, a clogged catalytic converter is restricting exhaust flow, which disrupts engine breathing and can cause the air-fuel mixture to appear rich. As a result, the ECM compensates by reducing fuel delivery, reflected in the negative fuel trim value.

A severely clogged converter can also cause overheating, as trapped exhaust gases raise temperatures. While the catalytic converter’s role is to reduce emissions by converting harmful gases, excessive fuel in the exhaust can increase its operating temperature, potentially leading to damage.

Technician A is incorrect because an LTFT of –19% shows the ECM is pulling back fuel—not adding more.

A fluttering test light during cranking indicates that the ignition coil is being triggered properly. However, if the spark tester does not fire, it suggests the coil isn’t generating enough voltage to jump the spark plug gap—most likely due to a faulty ignition coil.

While the pickup coil and ignition module control the triggering of the ignition coil, they do not affect the coil’s output voltage. The spark plug is also unlikely to be the issue, as it only serves to ignite the air-fuel mixture, not generate the spark itself.

A minor evaporative emissions leak code can be triggered by something as simple as a small crack or improper seal in the gas cap. The gas cap is essential for sealing the fuel system and preventing fuel vapors from escaping. If it’s damaged or not fully tightened, a small leak can occur, setting a fault code.

Additionally, many modern EVAP systems include a low-pressure sensor that monitors fuel tank pressure. If the sensor detects pressure lower than expected, it may indicate a leak somewhere in the EVAP system.

A voltmeter reading of 0.00 volts at each (B+) power supply terminal of the ignition coil indicates that no power is reaching the coil. This suggests that the ECM is not providing the necessary signal to energize the coil and fire the spark plugs.

The absence of voltage at the coil’s power supply is likely the root cause of the no-start condition.

Excessive back pressure in the exhaust system can damage the EGR valve and its sensors. The EGR valve regulates the flow of exhaust gases into the intake manifold to reduce emissions. When back pressure is too high, it can cause the valve to malfunction, leading to poor engine performance and increased emissions.

In severe cases, excessive heat from back pressure can overheat or melt EGR sensors, especially those made of plastic, resulting in complete valve failure.

To prevent recurring issues, it’s important to inspect the exhaust system for restrictions or blockages whenever the EGR valve or its sensors fail. Identifying and addressing the root cause can help avoid further damage to the EGR system.

A faulty crankcase ventilation valve is not likely to cause the catalyst material in the catalytic converter to fail or break apart.

In contrast, issues like coolant or oil leaking into the combustion chamber or excessive fuel consumption can contaminate the catalyst, leading to overheating and potential damage or breakdown of the converter’s internal substrate.

While a faulty crankcase ventilation valve typically doesn’t directly affect the catalytic converter, it can still lead to increased oil consumption, reduced engine performance, and higher emissions, and should be repaired when necessary.

The most likely reason for poor vehicle performance after the battery has been disconnected or discharged is the loss of learned values stored in the RAM of the engine control module (ECM) or electronic control unit (ECU).

Modern vehicles rely on the ECM/ECU to manage engine functions like fuel mixture, ignition timing, and idle speed. These systems continuously adapt to sensor inputs and driving conditions, storing optimized values in RAM.

When the battery is disconnected, power to the ECM/ECU is lost, erasing these learned settings. As a result, the engine may run poorly—exhibiting rough idle, hesitation, or reduced power—until the system relearns and readjusts the parameters.

A drop in fuel economy may be linked to a problem with the variable valve timing (VVT) system, which plays a key role in optimizing engine efficiency. If the VVT solenoid is activated but there’s no noticeable change in engine performance, it could point to a faulty oil control valve. This valve regulates oil flow to the VVT system, and if it malfunctions, the system may fail to adjust valve timing correctly.

Additionally, contaminated engine oil can reduce fuel efficiency by affecting overall engine performance. Dirty oil can increase friction and make the engine work harder, leading to increased fuel consumption.

The wastegate is a valve that regulates how much exhaust gas flows through the turbocharger. When it opens, it diverts some exhaust around the turbocharger, reducing boost pressure.

If the wastegate diaphragm is leaking, the valve may remain open when it shouldn’t, allowing excess exhaust to bypass the turbocharger. This can actually lead to reduced boost pressure, not increased.

On the other hand, a stuck-closed wastegate can cause an overboost condition, which may damage the turbocharger and engine. Overboost can also lead to rough running and a loss of power.

The illustration shows the activity of “Checking for spark with an ST125.”

The ST125 is a spark tester used to visually confirm whether the ignition system is producing a spark. It connects to the end of the spark plug wire and lights up if a spark is present.

This test is commonly used when diagnosing a no-start condition or poor engine performance. If no spark is detected, possible causes include a faulty ignition coil, spark plug wire, distributor cap, rotor, or other ignition components. The ST125 helps technicians quickly identify whether the ignition system is functioning and pinpoint problem areas for further inspection.

The primary ignition system consists of components that generate and control the flow of current needed to create high voltage for the ignition process. These include the battery, ignition switch, ignition coil, and distributor.

The secondary ignition system delivers the high voltage from the ignition coil to the spark plugs, which ignite the air-fuel mixture in the combustion chamber. Key components include the spark plugs, spark plug wires, and distributor cap.

If the engine is backfiring and lacking power after replacing a leaking fuel injector, and DTC P0172 indicates a rich air-fuel mixture, it’s possible that the excess fuel has damaged the catalytic converter. A clogged converter can restrict exhaust flow, leading to reduced engine performance.

Using a vacuum gauge is a common method for diagnosing exhaust restrictions. Low or dropping vacuum readings can indicate a blockage, such as a clogged catalytic converter, which may be causing the backfiring and loss of power.

Technician A is correct in suggesting a check of the variable valve timing (VVT) solenoid and its circuit, as a malfunction in either can lead to rough idle and poor engine performance at low speeds. The VVT system relies on oil pressure to adjust valve timing, so issues with the solenoid or its circuit can disrupt timing control.

Technician B is also correct that contaminated or old engine oil could be the cause. The VVT system depends on clean oil to function properly, and dirty oil can affect the solenoid, actuator, or other components, leading to performance problems like rough idle.

Both suggestions are valid and should be considered during diagnosis.

Technician A is correct—when a cylinder is deactivated in a four-cylinder engine, it can cause the engine to stumble or run roughly. This happens because the engine loses one-quarter of its power strokes, affecting balance and smoothness.

Technician B is also correct—a noticeable drop in RPM is expected when a cylinder is turned off. Since each cylinder contributes to overall power output, deactivating one reduces engine power, resulting in a measurable decrease in RPM.

A cylinder power balance test helps identify which cylinder(s) are not contributing fully to the engine’s power output.

During the test, each cylinder is individually deactivated while the engine is running, and the change in engine speed is observed. If deactivating a cylinder causes little or no change in RPM, that cylinder is likely underperforming.

The test helps pinpoint which cylinder is affected but does not identify the exact cause. Further diagnosis is needed to determine the issue, which could stem from faulty spark plugs, clogged fuel injectors, vacuum leaks, or low compression due to worn piston rings or other internal problems.

In summary, a power balance test is a useful first step, but additional testing is required to determine the specific component that needs repair or replacement.

A kink in the fuel return line is not a likely cause of low fuel pressure in this scenario. The return line carries excess fuel back to the tank, and a restriction—such as a kink—would typically increase fuel pressure, not lower it. Therefore, in a sequential fuel injection (SFI) system, low fuel pressure is unlikely to result from a kinked return line.

A kink in the fuel return line is not a likely cause of low fuel pressure in this scenario. The return line carries excess fuel back to the tank, and a restriction—such as a kink—would typically increase fuel pressure, not lower it. Therefore, in a sequential fuel injection (SFI) system, low fuel pressure is unlikely to result from a kinked return line.

This illustration shows the connector for the MAP sensor’s signal circuit. The MAP (Manifold Absolute Pressure) sensor plays a vital role in engine operation by measuring intake manifold pressure and sending that data to the engine control module (ECM). The ECM uses this information to adjust the air/fuel mixture, ignition timing, and other parameters to optimize performance.

The connector, marked by the red-circled symbol, links the MAP sensor signal wire to the wiring harness and eventually to the ECM. It’s important to ensure this connection is clean and secure, as corrosion or looseness can lead to intermittent or inaccurate readings.

The solid black dot in the diagram represents a splice—a junction where two wires are joined. This splice may be located anywhere along the circuit, including inside a connector or within the wiring harness.

The “5V ref” label indicates the reference voltage circuit, where the ECM supplies a constant 5-volt signal to the MAP sensor. The sensor uses this voltage to generate an output that reflects the pressure in the intake manifold.

When performing a fuel injection balance test using a scan tool, a properly functioning injector will cause the engine RPM to drop when it is turned off. This test helps determine whether each injector is delivering the correct amount of fuel.

During the test, the scan tool sequentially disables each injector while monitoring engine RPM. If RPM decreases noticeably, the injector is working correctly. However, if there’s little or no change in RPM, it suggests that the injector is not delivering fuel properly and requires further diagnosis to identify the underlying issue.

When performing a fuel injection balance test using a scan tool, a properly functioning injector will cause the engine RPM to drop when it is turned off. This test helps determine whether each injector is delivering the correct amount of fuel.

During the test, the scan tool sequentially disables each injector while monitoring engine RPM. If RPM decreases noticeably, the injector is working correctly. However, if there’s little or no change in RPM, it suggests that the injector is not delivering fuel properly and requires further diagnosis to identify the underlying issue.