ASE4 Fuel, Air Induction, and Exhaust Systems 3

Question 1

A fuel trim diagnostic shows STFT (short-term fuel trim) swinging widely between +25% and -25%. This most likely indicates:

Accepted Answer

A slow-responding oxygen sensor

Answer

Wide STFT swings indicate the ECM is making large corrections due to a slow or lazy oxygen sensor not providing timely feedback.

Question 2

A scan tool shows both bank LTFT values at +18%. The MOST likely cause is:

Accepted Answer

A large vacuum leak affecting the entire engine

Answer

Equal positive fuel trims on both banks indicate a global lean condition, which points to a vacuum leak or low fuel pressure affecting the whole engine.

Question 3

A returnless fuel system maintains fuel pressure without a return line to the tank. The pressure regulator in this system is located:

Accepted Answer

Inside the fuel tank on the pump module

Answer

In a returnless fuel system, the pressure regulator is integrated into the in-tank fuel pump module to regulate pressure at the source.

Question 4

A catalytic converter efficiency code (P0420) is set. Before replacing the converter, the technician should first:

Accepted Answer

Check for misfires, oil burning, or coolant contamination that could have damaged the converter

Answer

A converter failing efficiency is often caused by an underlying engine problem; repairing those first prevents the new converter from failing again.

Question 5

A cold engine starts and idles but stalls when the throttle is opened. Fuel pressure is normal. The MOST likely cause is:

Accepted Answer

A dirty or failed mass airflow sensor

Answer

A dirty or failed MAF sensor can cause correct idle (using MAP backup) but stumble on throttle opening when accurate airflow data is needed.

ASE4 - ASE Automobile Engine Performance Practice Test

ASE4 - ASE Automobile Engine Performance Practice Test

ASE4 Fuel, Air Induction, and Exhaust Systems 3