The vacuum leak occurs mainly between the engine and its air flow sensor. In order to detect that leak, the popularly used smoke test vacuum leak inspection is applied. The airflow sensor is installed within an air filter box. Within the engine’s fuel injection system, the air flow sensor will be measuring the amount of air flowing into the engine. And an engine computer can be used to calculate how much fuel needs to be injected.
This calculation will be based on the mass air flow sensor movements. Should no vacuum leak be detected between the engine and the air flow sensor, unmetered air may have entered the system. Actual air flow will be higher than what the air flow sensor initially measures. A miscalculation of injected fuel could have occurred. The engine will then be running lean. This basically means too little fuel in the system.
And too much air. The detection of a vacuum leak is easier when the engine is idle and air flow is lower. There are, however, other noticeable signs that a vacuum leak may have occurred. All the driver needs to do is check his engine light on the dashboard. If not that, the car’s engine could be stalling. A hissing sound from the engine bay could be heard. An engine can stall when it stops. A scan tool can also be used to detect a vacuum leak.
Utilizing this tool, engineers or mechanics may refer to the detection of a vacuum leak as a long term fuel trim or LTFT. In spite of the smoke test vacuum leak inspection being a popular testing mechanism, it does need to be handled with caution. Unless the driver is a skilled motor mechanic, this test is best handled at a repair workshop.