All vehicle manufacturers are required to build in test sequences to detect a possible leak of fuel vapors into the atmosphere. They can do this two ways: by adding pressure to the system or by pulling a vacuum on the system. Starting in 1996 Chrysler used a pressure technique using a leak detection pump (LDP) to check for system leaks.
Ironically, LDP is basically a vacuum/spring driven pump. It uses engine vacuum to pull the diaphragm up and suck air into a lower chamber. The PCM energizes a solenoid valve to allow engine vacuum to the upper chamber of the pump. That vacuum sucks the diaphragm up. The diaphragm pulls air into the lower chamber through a filter and a one-way check valve. When the PCM shuts off engine vacuum, a spring forces the diaphragm down, and this is what forms the pumping action. The pump solenoid is what controls the on/off of engine vacuum.
The spring exerts 7.5″ of (H2O) pressure, so when the pressure inside the tank reaches that level, the diaphragm no longer moves down after vacuum is released. How does the PCM know that this pressure has been achieved? There’s a set of electrical contacts inside the LDP that open/close each time the diaphragm moves. When it stops moving and the PCM knows that is has released vacuum to the upper chamber, it assumes that the correct pressure is present in the tank and connecting hoses. That’s when the PCM starts a timer and continues to watch the electrical contacts. The system must build and hold pressure in certain time frame “windows.” If it doesn’t, the PCM sets a code.
Starting in 2002, Chrysler developed a vacuum testing technique referred to as natural vacuum leak detection (NVLD).
The procedure is based on the physical law that as you decrease the temperature of a fixed volume of gas, the pressure inside the container will drop in direct proportion, unless there is a leak in the container.
As fuel is pumped from the tank to the fuel rail, it picks up heat—heat from the pumping action and heat from the engine compartment. The excess fuel that bleeds off the fuel pressure regulator returns to the fuel tank, where it eventually helps raise the temperature of all the gas in the tank.
So, when the vehicle is turned off and the fuel starts to cool, the pressure in the tank should drop. It ultimately drops to less than atmospheric pressure, or negative pressure (vacuum)/
A mechanical vacuum switch closes at 1” of vacuum. The PCM sees that switch close and knows that the tank and all the connecting hoses are in good condition, so it “passes” the leak test.
However, if there is a leak in any of the hoses, atmospheric pressure will leak into the tank and prevent the system from reaching a vacuum.
Chrysler also designed a pressure/vacuum relief valve to vent fuel pressure during gas station fills. They wanted to prevent a buildup of pressure inside the tank that would interfere with the leak detection test on engine shutdown. The relief valve also opens to allow air into the tank if the purge solenoid or vent solenoid fails. The valve opens at 3-6” of vacuum to protect the tank from collapsing.
I have another article on vent and purge solenoids and carbon canisters, but rather than force you to search for it, I’ll recap it here.
Remember, the whole point of an EVAP system is to prevent the release of raw gas vapors into the atmosphere. As you fill your tank with fuel, the gas vapors in the tank are forced into a canister of activated charcoal where they are absorbed. This obviously can’t go on forever—there’s a physical limit to how much gas vapor the canister can hold. So, when the PCM determines that the canister might be full, it commands a PURGE cycle.
The purge cycle starts when the computer sends power to the vent solenoid. That’s literally a valve that opens the system to the atmosphere. Once that’s open, the PCM sends power to the purge solenoid, another valve. The purge valve is connected by a vacuum line to the intake manifold. Once the valve is open, engine vacuum sucks the gas vapors out of the canister and fresh air “purges” the canister—the fresh air that came in through the vent solenoid. The gas vapors are burned by the engine and the PCM monitors engine performance during this time to make sure the engine doesn’t choke on all the extra fuel. At the end of the purge, the PCM closes the purge and vent solenoids. The next time you shut down the engine, the PCM conducts the leak test discussed above.
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© 2012 Rick MuscoplatPosted on by Rick Muscoplat