Basically, if volume is held constant and either of the other two changes, so will the other. NVLD takes advantage of Boyle's Gas Law, which describes the relationship between volume, temperature and pressure of a gas. The big advantage of NVLD is that it can detect small leaks with great accuracy because the test runs with the vehicle parked and the engine turned off. Like existing Ford and General Motors (GM) EVAP systems, it uses solenoid valves at each end of the system, so DCX will gradually phase out the old Leak Detection Pump (LDP). DCX and other manufacturers are gradually switching over to this type of EVAP test as new models are introduced. Called Natural Vacuum Leak Detection (NVLD), it was introduced by DaimlerChrysler (DCX) on the 2002 LH platform. However, there is one way to leak test an EVAP system using test conditions that are very easy to meet. If the monitor does run and detect a small leak, it must do so twice before it will set a hard fault code and turn on the malfunction indicator lamp (MIL). The time required to run the test to completion – attain the target vacuums – varies with fuel level, and test accuracy cannot be assured when the level is above or below certain limits.ĭepending on where and how the vehicle is typically used, it's common that the EVAP monitor will not run to completion for months at a time, and it's even possible that the test will never be completed. Finally, fuel evaporates more easily at high ambient temperature and at low atmospheric pressure found at higher altitudes. Vapor pressure also is increased by fuel sloshing around in the tank when the vehicle is driven in stop-and-go traffic. Fuel vapors are generated when warm fuel is returned to the tank after passing through the fuel rail of a warm engine. Engine at idle or steady speed/load for a specific period of time.īecause the EVAP system is tested with vacuum, the test must run under conditions least likely to generate fuel vapor in the tank, which would increase pressure in the fuel tank and cause vacuum decay that is unrelated to a possible leak.Altitude below 8,000 feet (barometer greater than 75 kpa).Coolant temperature close to ambient (cold start). Ambient temperature between 40☏ and 86☏.Fuel level between 15 percent and 85 percent.Test vacuum is typically about 7 inches of water (H 2O), and because the OBD system is looking for a leak equivalent to a hole only 20 thousandths of an inch, measuring vacuum decay requires very specific test conditions that are hard to meet in the real world. Test vacuum is easy to generate, using either manifold vacuum or a leak detection pump (LDP), but the test vacuum must be very small to avoid the possibility of creating a leak in an otherwise tight system. OBD II checks for leaks by putting the EVAP system under vacuum, closing all the valves to seal the system and then measuring the rate of vacuum decay. Powertrain engineers will tell you that's why testing the EVAP system is the most difficult part of OBD. Add in the on-board refueling vapor recovery system (OVR), and the on-board diagnostics (OBD) system that checks the whole EVAP system for leaks, and things can get complicated in a hurry.ĭesigning any system that can be tested while it's actually in use is a big challenge. It's also opened every time the tank is refilled, introducing the opportunity for malfunction caused by human error. That's because the EVAP system is opened at both ends at least once every drive cycle to purge the vapor storage canister. It seems simple enough, but as it turns out, engineering an effective and legal evaporative emissions control system (EVAP) is not as easy as designing a leak-free fuel system. When the engine is running, the vapors are drawn into the intake manifold and burned in the engine. Today, the pressure is vented through a tube that leads to a canister filled with activated charcoal that absorbs fuel vapors. Sealing the fuel tank to prevent evaporative emissions was not a solution because pressure builds up with heat, making the emissions problem potentially a lot worse. The newest EVAP leak detection system is designed to perform its tests right there on the driveway.īefore cars had emission controls, gasoline evaporating from the fuel system generated almost as much unburned hydrocarbon emissions as the engine did.
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