This article was originally published Feb. 21, 2012. Some of the information may no longer be relevant, so please use it at your discretion.
Chrysler's NVLD, or Natural Vacuum Leak Detection, system was first introduced in 2002 as an alternative to its Leak Detection Pump (LDP) method for the onboard testing of the evaporative emissions system. This method of leak detection is based on the "Ideal Gas" law that states, in part, that the pressure in a sealed vessel will change linearly as a function of the temperature of the gas in that vessel. Any loss of seal will allow the internal pressure to equalize with the atmospheric pressure outside the container.
In an automotive application, the fuel system is sealed when the vehicle is in a key off condition. The powertrain control module (PCM) monitors the state of a switch contained within the NVLD assembly. As the temperature in the fuel system drops, whether due to the cooling of the fuel or diurnal temperature change, pressure in the sealed system will drop. The NVLD switch, normally open, closes with as little as 1" /H20 pressure drop, which is seen by the PCM. (For comparison, 1"/H
If the PCM does not see this change of state within a calculated time limit, it registers a failure and then tests to determine the size of the leak. This method is much easier to run than an EVAP monitor that doesn't take hours of specific driving conditions to complete. It also allows the adoption of an in-bay verification of system integrity.
Chrysler products use two versions of natural vacuum leak detection. NVLD is the name of the first, and an updated version was added in 2007. This system is called ESIM, for Evaporative System Integrity Monitor, and there are some important differences between the two.
First, NVLD
The NVLD pump is really nothing of the kind. It is an assembly mounted either remotely or directly on the charcoal canister and is attached to the vent side. Contained in the NVLD assembly are a vacuum operated switch, a vent solenoid and a vent/pressure diaphragm valve.
The vent solenoid is powered by a high side driver in the PCM, and is energized only when the engine is running. In its normally off state, it is the vent/pressure valve that actually seals the system and protects it from over pressurization or excessive vacuum. The vacuum switch receives 12V when the key is on and 5V when the key is off. Both the solenoid and the switch are grounded to chassis ground.
When the vehicle is shut down, the PCM monitors the switch state. If a change of state occurs within the monitored time frame, the PCM knows that there are no system leaks and no further tests are needed. No change in state is recorded as a fault. Since the PCM is not taking any active role that would affect system pressures, this is referred to as a non-intrusive test.
If a fault is detected, the PCM will perform an intrusive test, taking an active role by operating the purge solenoid and powering the NVLD solenoid to seal the system, then pulling the system into a vacuum on its own. If there is still no change of state, the ECM will set a general system failure code. If the PCM is able to pull enough vacuum to get the switch to close, it will then close the purge solenoid and start an internal decay timer to determine the actual size of the leak.
Now, ESIM
ESIM functions much the same as the NVLD system. The main, noticeable difference is in the ESIM assembly itself. There are no solenoids in the ESIM, only the wiring for the vacuum switch. Also mounted on the vent side of the canister, it performs pressure/vacuum relief via a weighted valve contained in the unit. For this reason, it is imperative that the assembly be properly installed in the car or operation of the valve will be affected.
Unlike the NVLD, the switch reference voltage is relatively constant at 4.5V, key on or off. Resistance across the switch is lower as well, specified at less than 1 ohm, compared to the 130 ohms across the NVLD switch when closed.
Like its NVLD cousin, it also has an inline air filter that should be inspected for any restrictions that would interfere with normal venting. Contaminants passing by this filter can also interfere with the operation of the weighted valves. Both monitors can be suspended if the barometric pressure is below 22.2"/Hg, ambient temperature is below 19 degrees F, battery voltage is below 11V, or the fuel tank is under 12 percent or over 88 percent filled.
Testing tips
Sticking switches in the NVLD assembly are relatively common, and it most commonly results in a P0441 EVAP purge performance code. However, it can also be at the heart of a P0440 EVAP system performance code as well as a few others. The first step is to verify that there are no leaks present in the system using an EVAP system tester.
The best place to connect is directly at the filter side of the NVLD or ESIM assembly. The reason for connecting here is that most EVAP system testers use pressure to test a system that uses vacuum to self-test. The pressure generated by the machine exceeds the blow off levels of the internal relief valves, and testing at the supplied test port would require sealing that vent, either manually or by scan tool activation.
Look closely at the condition of all the rubber lines in the system. Often, decay of the rubber is enough to cause a small leak that may be hard to locate using another method.
Once all leaks have been located and repaired, you can use an appropriate scan tool to run the Forced Monitor test. This test, however, will not detect small leaks, only medium and large leak rates, but since the monitor test for a small leak occurs on engine shutdown and is non-intrusive, allowing the PCM to retest the system for you may be the best choice when verifying a small leak repair.
If you suspect a stuck switch, you can monitor the switch state on your scan tool or directly with a digital multimeter (DMM) or scope while vacuum is applied at the filler neck, but it only takes a very small amount to trip the switch.
