Engine Performance ASE Style Quiz 2

A magnetic-pulse type CKP sensor shows infinite resistance when tested with a DMM. Which of the following statements are correct?

• The sensor winding is shorted

• The sensor resistance is correct

• The sensor winding is open

• This sensor cannot be checked for resistance

Answer explanation

Infinite resistance indicates current cannot flow through the circuit due to an open in the circuit.

All of the following ways to depressurize the fuel system are correct except:

• Using a fuel pressure gauge attached to the fuel test port

• Removing the fuel pump fuse and running the engine

• Loosening the fuel filter connection

• Removing the fuel pump relay and running the engine

Answer explanation

Loosening the fuel filter connection will cause fuel to spray out which can harm you.

Technician A says the gas cap is part of the emission control system. Technician B says the gas cap is part of the fuel supply system. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Technician A says most late model vehicles mount the fuel pump in the fuel tank. Technician B says some vehicles use a frame mounted fuel pump. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

The PCM may turn the fuel pump circuit off for all of the following reasons except:

• Excessive vehicle speed

• SRS deployment

• Excessive fuel pressure

• High speed deceleration

Answer explanation

Pcm's are not programmed to turn the fuel pump circuit off for excessive fuel pressure, however they are programmed to turn it off under the three other conditions for the following reasons: SRS deployment and excessive vehicle speed for safety, and high speed deceleration to improve fuel efficiency by preventing unnecessary fuel delivery when the engine is not actively needing it.

Which of the following is not a cause of low fuel pressure?

• Restricted fuel filter

• Obstructed return line

• c. Stuck fuel pump check valve

  
  

• d. Low pump RPM

  
  

Answer explanation

An obstructed fuel return line raises the fuel pump pressure because it prevents excess fuel from returning to the fuel tank, causing the fuel pressure in the system to build up as the fuel pump continues to deliver fuel to the injectors, essentially creating a "backpressure" effect. 

Technician A says a speed density system uses RPM and a mass air flow sensor to calculate fuel delivery. Technician B says a mass air flow sensor is used to measure pressure in the intake manifold. Who is correct?

• Technician A

• Technician B

• Both A and B

• Neither A or B

Answer explanation

Speed Density is a tuning method where the ECU uses information based on manifold pressure and temperature using the MAP sensor and an added IAT sensor to calculate how much air is coming into the engine.

A Mass Air Flow (MAF) sensor measures the amount of air entering an engine, providing critical information to the engine control unit (ECU) which then calculates the precise amount of fuel needed to maintain an optimal air-fuel ratio for efficient combustion, ensuring smooth engine operation and optimal gas mileage; essentially, it tells the engine how much air is coming in so the correct amount of fuel can be injected to create the ideal mixture. 

All of these conditions require increased injector pulse width except:

• Cold engine start

• Increased electrical system load

• Engine deceleration

• Open loop conditions

Answer explanation

Injector pulse width" refers to the length of time a fuel injector actually sprays fuel, when decelerating, the injector pulse width decreases because the engine needs less fuel as it is slowing down.

Which of the following statements about fuel trim are correct?

• Positive fuel trim numbers indicate a rich exhaust condition

• Negative fuel trim numbers indicate a rich exhaust condition

• Negative fuel trim numbers are a command for increased fuel delivery

• Short term fuel trim responds to long term fuel trim trends

Answer explanation

A negative fuel trim indicates a rich exhaust because it means the engine control module (ECM) is actively trying to reduce the amount of fuel being injected into the engine.

Which of the following EFI systems is the most accurate?

• MPI

• SFI

• TBI

• CMFI

Answer explanation

MPI - Multi Port Injection

SFI - Sequential Fuel Injection

TBI - Throttle Body Injection

CMFI - Central Multi-Port Fuel Injection

Sequential fuel injection is considered the most accurate fuel injection system because it precisely times the fuel injection for each cylinder individually, just before the intake valve opens, allowing for better fuel atomization and a more controlled air-fuel mixture, resulting in optimal combustion and efficiency compared to systems that inject fuel for multiple cylinders simultaneously.