Mazda, Other Asian Manufacturers Changing Evaporative System TestingPosted 3/08/2005
By Guy Dibble
If you've been around Chrysler and European cars, pressure-based evaporative emissions system testing is nothing new. But in the "land of the rising sun," we are just starting to see these systems appear on the horizon. In the past, testing evaporative systems for leaks in the Asian world meant applying vacuum to the system via a purge solenoid to see if it would hold vacuum for a period of time. Now, Mazda and some other Asian manufacturers are doing just the opposite - they are testing their evaporative systems for leaks by applying pressure to the system through a component called a leak detection pump (LDP).
Mazda's first generation of evaporative emissions system testing used a purge solenoid, charcoal canister, canister drain cut valve, tank pressure control valve and a pressure sensor. The pressure sensor was the "eyes" of the power control module (PCM) - without it there was no way to tell if the evaporative system had a leak.
Today's pressure-based evaporative system has fewer parts. Two parts that remain unchanged on the new system are the purge solenoid and the charcoal canister. The other components have been replaced with one component: the LDP. If the pressure sensor was the eyes of the old system, the LDP is the "heart" of the new system. The LDP takes the place of the tank pressure control valve by controlling the vapors from the fuel tank and directing them into the charcoal canister. The LDP has a built-in changeover valve, which takes the place of the canister drain cut valve and seals the canister vent during testing. The pressure sensor has been eliminated, and the PCM monitors milliamps of current flow through the LDP to determine if there is a leak. The theory is that it takes more current flow to run an electric pump into a system that is sealed and building pressure than it does to run the pump into a system with a leak building no pressure.
Testing for leaks: On the early systems, the PCM ran a test during the drive because it relied on engine vacuum to run the test. One of the drawbacks of this early system is that conditions had to be just right before the PCM could test the system. With the new system, the PCM will run the test after you have driven the vehicle, shut it off, removed the key and walked away! I monitored the ground side of the LDP on a test vehicle after completing a drive cycle, and the PCM actually ran a test. (Note: The vehicle has to be started from cold for the PCM to consider it a drive cycle, and the fuel level still has to be between 1/4 and 3/4 of a tank for it to run a test.)
During a test, the PCM initially runs the leak detection pump for a few seconds prior to turning on the changeover valve. During that time, LDP pressure is pumped through a fixed orifice of 0.5mm that is built into the pump. The PCM establishes a base line of current flow that is needed to run the pump through that 0.5mm fixed orifice (on a test vehicle, this was about 25.5 milliamps). After the pump has run and base line current flow is established, the PCM energizes the changeover valve, which seals the canister vent and directs LDP pressure into the canister. To determine if there is a leak in the evaporative system, the PCM monitors the current flow of the LDP and the amount of time it takes for current flow to reach that of the established base line current flow. With no leaks, the current flow ramps up to the base line quickly as pressure builds in the system.
With a leak, the current flow stays pretty low as no pressure builds in the system. Running the test with the gas cap off, the current flow of the LDP stayed around 18 milliamps. These readings were gathered from a worldwide diagnostic system (WDS) scan tool that was running a programmed test. In the real world, a low amp current probe could be used to obtain these readings while manually energizing the LDP and changeover valve. If it is established that there is a leak, Mazda uses a special tool, a K-Line tester, which pumps pressure and smoke into the evaporative system to pinpoint the source of the leak. You should be able to rely on vacuum testing by energizing the purge solenoid and changeover valve with the engine idling, and then feeling and listening for a vacuum leak.
During a simulation test, I decided to play with the PCM, and I started unplugging and plugging the vacuum hose at the purge solenoid. Strangely enough, the PCM set a code P2407. This code is for the LDP heater circuit. I learned a heater circuit is built into the LDP to prevent freezing. The LDP heater gets power from the same source as the LDP. I assumed the code set because the PCM saw quick erratic changes in the current flow through the circuit, and flagged the heater circuit as the logical cause.
I also did some bench testing on the leak detection pump with a battery and some jumper wires. When I energized the pump circuit, the pump ran quietly. I detected an audible clicking sound when energizing the changeover valve. I hooked up my 20-year-old vacuum/pressure gauge to the 5/8-sized hoses on each side of the LDP and ran the pump with the changeover valve energized. On one side, it created about 1.75 psi; on the other side, it pulled about 4 inches of vacuum.
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