Pressurizing a vehicle's fuel system can cause hidden leaks to be revealed. This is essentially what a leak detection pump does, though it is also capable of checking for blockage problems as well. There is also other valuable information to be had regarding this device.
The job of exposing leaks in an evaporative control system (EVAP) is an important one, as even a small leak would release vapors that are harmful to the environment. A leak detection pump's role is so vital that the U.S. government makes it mandatory that vehicles have their EVAP systems checked for leaks.
A leak detection pump (LDP) utilizes engine vacuum to help detect leaks. The LDP has a diaphragm that changes in shape as pressure in the vacuum rises. When the right level is reached, the LPD shuts off which also prevents any more buildup in the vacuum. The Powertrain Control Module (PCM) times this whole process and uses the information to determine if there is a problem in the system.
An LPD has checks performed on it by the PCM such as for electric or mechanical issues. There is a reed switch installed in the LPD, which allows the PCM to monitor it and control its functionality.
