Diagnosing and repairing NVLD systems can present unique challenges - Checking this assembly often can diagnose problems faster than an EVAP monitor. - Motor Age - Automotive training, certification &
The NVLD module uses an electrical solenoid to seal the system only for testing. Otherwise, tank pressure control is handled
by the vacuum/pressure relief valve. (Illustration: Chrysler Corp)
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 inch/H20 pressure drop, which is seen by the PCM. (For comparison, 1 inch/H2O is the equivalent of 0.0735 inch/Hg — not very much).
The NVLD can be mounted directly to the charcoal canister, or remotely, like this one off of a Town and Country minivan. (Photos:
Pete Meier)
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 white module to the left of the photo is the ESIM assembly on this 2008 Dodge Caliber SXT.
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 12 volts when the key is on and 5 volts when the key is off. Both the solenoid and the
switch are grounded to chassis ground.
The ESIM serves the same purpose as the NVLD, without the need for the solenoid.
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.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
