tion pump as the method of evaporative system leak
detection. The current CARB requirement is to
detect a leak equivalent to a 0.020
9 (0.5 mm) hole.
This system has the capability to detect holes of this
size very dependably.
The basic leak detection theory employed with
NVLD is the
9Gas Law9. This is to say that the
pressure in a sealed vessel will change if the tem-
perature of the gas in the vessel changes. The vessel
will only see this effect if it is indeed sealed. Even
small leaks will allow the pressure in the vessel to
come to equilibrium with the ambient pressure.
In addition to the detection of very small leaks,
this system has the capability of detecting medium
as well as large evaporative system leaks.
THE NVLD UTILIZES THE GAS LAW
PRINCIPLES
A vent valve seals the canister vent during engine
off conditions. If the vapor system has a leak of less
than the failure threshold, the evaporative system
will be pulled into a vacuum, either due to the cool
down from operating temperature or diurnal ambi-
ent temperature cycling. The diurnal effect is con-
sidered one of the primary contributors to the leak
determination by this diagnostic. When the vacuum
in the system exceeds about 1
9 H2O (0.25 KPA), a
vacuum switch closes. The switch closure sends a
signal to the PCM. The PCM, via appropriate logic
strategies (described below), utilizes the switch
signal, or lack thereof, to make a determination of
whether a leak is present.
THE NVLD DEVICE AND HOW IT FUNCTIONS
The NVLD Assembly is designed with a normally
open vacuum switch, a normally closed solenoid,
and a seal, which is actuated by both the solenoid
and a diaphragm. The NVLD is located on the
atmospheric vent side of the canister.
The normally open vacuum switch will close with
about 1
9 H2O (0.25 KPA) vacuum in the evaporative
system. The diaphragm actuates the switch. This is
above the opening point of the fuel inlet check valve
in the fill tube so cap off leaks can be detected.
Submerged fill systems must have recirculation
lines that do not have the in-line normally closed
check valve that protects the system from failed
nozzle liquid ingestion, in order to detect cap off
conditions.
The normally closed valve in the NVLD is in-
tended to maintain the seal on the evaporative
system during the engine off condition. If vacuum in
the evaporative system exceeds 3
9 to 69 H2O (0.75 to
1.5 KPA), the valve will be pulled off the seat,
opening the seal. This will protect the system from
excessive vacuum as well as allowing sufficient
purge flow in the event that the solenoid was to
become inoperative. The solenoid actuates the valve
to unseal the canister vent while the engine is
running. It also will be used to close the vent during
the medium and large leak tests and during the
purge flow check. This solenoid requires initial 1.5
amps of current to pull the valve open but after 100
ms. will be duty cycled down to an average of about
150 mA for the remainder of the drive cycle.
Another feature in the NVLD Assembly is a
diaphragm that will open the seal with pressure in
the evaporative system. The seal will be opened at
about 0.5
9 H2O (0.12 KPA) pressure to permit the
venting of vapors during refueling. An added bene-
fit to this is that it will also allow the tank to
9breathe9 during increasing temperatures, thus lim-
iting the pressure in the tank to this low level. This
is beneficial because the induced vacuum during a
subsequent declining temperature will achieve the
switch closed (pass threshold) sooner than if the
tank had to decay from a built up pressure.
The NVLD Assembly itself has 3 wires: Switch
sense, solenoid driver and ground. It also includes a
resistor to protect the switch from a short to battery
or a short to ground. The PCM utilizes a high-side
driver to energize and duty-cycle the solenoid.
THE PCM’S ROLE IN NVLD DIAGNOSIS:
The integral part of the diagnostic system that
makes engine-off leak detection possible is a special
circuit in the PCM controller. After the vehicle is
turned off, a special part of the controller stays alive
and monitors for an NVLD switch closure. This
circuit within the PCM is very specific in its func-
tion and consumes very little power. If a switch
closure is detected, it will log the event and time
from key-off, and then power down. This informa-
tion will be processed at the next key cycle.
NVLD LEAK DETECTION
Small Leak Test (Passive)
If, after a specified delay after key off (perhaps 5
minutes), the switch closes or is closed, the test will
pass, indicating that there is no leak. The PCM
records the switch closure. The NVLD circuit in the
PCM will shut down for the remainder of that
particular engine off (soak) period. When the engine
is started, the switch closure is recorded as a
9Pass,9
and the timers that are recording accumulated time
are reset.
This diagnostic test can take at least a week to
mature a leak fault. A week has been chosen for this
because the vehicle will have been exposed to the
largest possible drive scenarios before a decision is
made (most vehicles should see both daily work and
weekend driving cycles). This also satisfies CARB’s
stated goal of getting 3 MIL illuminations within a
month for 0.020
9 (0.5 mm) leak detection diagnostic.
6
GENERAL INFORMATION