A technician is testing a fuel injection system with a scan tool. While the engine is running in closed loop, he loosens a vacuum hose to create a small vacuum leak. Initially the STFT rises and the LTFT remains unchanged. After a few minutes the STFT returns to near zero and the LTFT rises. This test proves that ______. A)the system is running rich B)the system is running lean C)the O₂S sensor is faulty D)the system is working normally

Question

Quizplus · Accepted Answer

The behavior described in the scenario is normal for a fuel injection system. When a vacuum leak is introduced, the Short Term Fuel Trim (STFT) increases to compensate for the additional air entering the engine, indicating the system is running lean temporarily. Over time, the system adjusts, and the Long Term Fuel Trim (LTFT) increases to maintain the correct air-fuel ratio, while the STFT returns to near zero, showing that the system has adapted to the change. This is a normal response of a functioning fuel injection system to a vacuum leak.

A Technician Is Testing a Fuel Injection System with a Scan