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.
Now, ESIM
General EVAP Testing Tips
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 4.5 volts, 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.
Deterioration of the rubber lines can cause a "small" leak and be hard to find. Be sure to do a thorough visual inspection
of all the components.
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 inches/Hg, ambient temperature is below 19 degrees Fahrenheit,
battery voltage is below 11 volts or the fuel tank is under 12 percent or over 88 percent filled.
Testing Tips
You can see the weighted check valves inside the ESIM. They correspond to "A" and "B" in the diagram. The ESIM must be properly
oriented to work.
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.
Pete Meier is an ASE certified Master Technician and sponsoring member of iATN. He has over 35 years practical experience as a technician and educator, covering a wide variety of makes and models. Now, as technical editor for Motor Age, his primary goal is to bring working technicians the information they need to succeed.
Articles by Peter F. Meier
Got something to say about it? Post it here!
GRIZZ / OMAHA, NE, UNITED STATES
Posted 2011-03-17 17:40:49.0
I JUST BOUGHT AN '03 DODGE RAM 1500 5.7LT. THE CHECK ENGINE LIGHT WAS ON WHEN I BOUGHT IT. AFTER SCANNING THE TRUCK I RECEIVED CODE "P0499--NVLD CANISTER VENT VALVE SOLENOID CIRCUIT HIGH." AFTER REPLACING THE ENTIRE NVLD ASSEMBLY I SCANNED THE TRUCK AGAIN AND RECEIVED CODE "P0499--UNIDENTIFIED EMISSION CONTROL ERROR." I RESET THE CHECK ENGINE LIGHT TO SEE IF IT WOULD STAY OFF, BUT SURE ENOUGH AFTER ONLY ABOUT 60 SECS IT WAS BACK ON. DOES ANYONE HAVE AN IDEA AS TO WHAT MY NEXT MOVE SHOULD BE OR WHAT THE PROBLEM IS? THANK YOU IN ADVANCE.
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