Engine Performance ASE Style Quiz

Technician A says the ignition system must change spark timing based on engine RPM. Technician B says maximum spark advance is needed under heavy engine loads. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

Spark timing needs to be adjusted at higher RPMs because as the engine speed increases, the time available for the air-fuel mixture to fully combust decreases, so the spark needs to be initiated earlier in the compression stroke to ensure the combustion process is completed before the piston reaches top dead center (TDC), maximizing power and efficiency at higher engine speeds. 

Spark timing needs to be changed at higher engine loads because as the load increases, the combustion process takes longer, so the spark needs to be initiated slightly earlier to ensure the combustion wave reaches peak pressure at the optimal point in the piston stroke, maximizing power and efficiency.

All of the following EFI systems are used on modern engines except:

• GDI

• PFI

• TBI

• CPI

Answer explanation

Cars no longer use TBI (Throttle Body Injection) systems because they are considered outdated technology compared to more advanced multi-port fuel injection systems, which offer better fuel efficiency, power delivery, and emissions control by precisely delivering fuel directly to each cylinder's intake port, unlike TBI which injects fuel into a single point near the throttle body, leading to less precise distribution. 

Which of the following is the ideal air-fuel ratio?

• 12:1

• 14.7:1

• 13.5:1

• 15.8:1

Answer explanation

A 14.7 to 1 ( 14.7 parts of air to 1 part of fuel ) air-to-fuel ratio is considered the "perfect" fuel mixture for gasoline engines because it represents the stoichiometric ratio, meaning it provides the ideal balance of air and fuel for complete combustion, maximizing fuel efficiency while minimizing harmful emissions. 

All of the following statements about OBD II are correct except:

• OBD II equipped vehicles have a generic DLC

• The main purpose of OBD II is emissions monitoring

• The powertrain control module is reprogrammable

• The MIL illuminates if emissions exceed 125% of the Federal standard

Answer explanation

A check engine light will typically illuminate when a vehicle's emissions exceed the federal standard by 1.5 times. This means the onboard diagnostics (OBD) system detects conditions likely to result in emissions significantly higher than the allowed limit, triggering the warning light on the dashboard. 

Technician A says open loop operation means the PCM is monitoring and correcting fuel control based on the O2 sensor. Technician B says some systems go into open loop during wide-open throttle. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

In fuel injection, "open loop" means that the engine control unit (ECU) determines the amount of fuel to inject based solely on pre-programmed fuel maps, without receiving feedback from sensors like the oxygen sensor to adjust the mixture in real-time.

A vehicle goes into open loop at wide open throttle (WOT) because under such extreme conditions, the engine's air intake is significantly increased, making it difficult for the oxygen sensors to provide accurate feedback, so the car's computer relies on its pre-programmed fuel map to ensure proper fuel delivery and engine operation, rather than constantly adjusting based on real-time sensor data; essentially prioritizing power and safety over precise fuel economy at high throttle positions. 

The MIL on an OBD II equipped vehicle is flashing at idle. Technician A says this indicates a Class A misfire is occurring. Technician B says this indicates that catalyst damage may occur. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

A flashing check engine light generally indicates a "Class A" misfire, meaning a severe engine misfire that requires immediate attention as it can potentially cause damage to the catalytic converter and other engine components if left unaddressed; it's considered a serious issue and should prompt you to pull over and get your car checked by a mechanic as soon as possible. 

Technician A says NTC engine coolant sensors have high resistance at high temperatures. Technician B says a NTC sensor has high resistance at low temperatures. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

An NTC ( Negative Temperature Coefficient ) coolant temperature sensor works by using a thermistor, which is a type of resistor that changes its resistance based on temperature; as the coolant temperature increases, the resistance of the NTC thermistor decreases, allowing the engine control unit (ECU) to calculate the coolant temperature based on the change in electrical resistance, thereby adjusting engine parameters like fuel injection timing to maintain optimal operating conditions. 

Technician A says a broken MAP sensor vacuum hose will cause a rich condition. Technician B says a broken MAP sensor vacuum hose will cause a lean condition. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

Under normal conditions, during acceleration manifold vacuum drops as the throttle opens allowing more air to flow into the engine. The map sensor reports this drop in vacuum to the PCM, the PCM uses this information to add fuel for the increased airflow into the engine. A broken map sensor vacuum hose would cause the map sensor to falsely report a low vacuum reading to the PCM, causing the PCM to add fuel to the engine even when there is no added airflow, leading to a rich condition.

Technician A says a contaminated MAF sensor will cause a lean condition. Technician B says a contaminated MAF sensor will cause a rich condition. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

A contaminated MAF (Mass Airflow Sensor) can cause a lean condition because when the sensor is dirty, it can't accurately measure the amount of air entering the engine, leading the car's computer to believe there is less air coming into the engine causing it to inject too little fuel, resulting in a lean air-fuel mixture; essentially, the engine is getting more air than the computer thinks it is, causing it to send less fuel than needed. 

All of the following statements about O2 sensors are correct except: a. O2 sensors generate a small voltage in response to exhaust oxygen content b. O2 sensors compare exhaust gas oxygen content to the outside air c. O2 sensors can operate with very lean air-fuel mixtures d. O2 output voltage should switch rapidly from low to high and back again

• O2 sensors generate a small voltage in response to exhaust oxygen content

• O2 sensors compare exhaust gas oxygen content to the outside air

• O2 sensors can operate with very lean air-fuel mixtures

• O2 output voltage should switch rapidly from low to high and back again

Answer explanation

Standard oxygen sensors have a range between 200 and 800 millivolts. Very lean conditions require a sensor that can operate well below 200 millivolts, these sensors are known as wide band oxygen sensors, aka air fuel ratio sensors, or lean air fuel sensors.