sion of fuel tank vapors into the atmosphere. When
fuel evaporates in the fuel tank, the vapors pass
through vent hoses or tubes to an activated carbon
filled evaporative canister. The canister temporarily
holds the vapors. The Powertrain Control Module
(PCM) allows intake manifold vacuum to draw
vapors into the combustion chambers during certain
operating conditions (Fig. 1).

All engines use a proportional purge solenoid sys-

tem. The PCM controls vapor flow by operating the
purge solenoid. Refer to Proportional Purge Solenoid
in this section.

NOTE: The evaporative system uses specially man-
ufactured hoses. If they need replacement, only use
fuel resistant hose. Also the hoses must be able to
pass an Ozone compliance test.

NOTE: For more information on Onboard Refueling
Vapor Recovery (ORVR), refer to the Fuel Delivery
section.

EVAPORATIVE EMISSIONS

25 - 7

Fig. 1 ORVR System Schematic (PZEV)

1 - FUEL TANK (PLASTIC)

11 - NATURAL VACUUM LEAD DETECTION (NVLD)

2 - FUEL FILLER TUBE

12 - LIQUID SEPARATOR (IF EQUIPPED)

3 - FUEL CAP (PRESSURE/RELIEF)

13 - ENGINE WIRING HARNESS TO NVLD

4 - FILL TUBE TO FUEL TANK CONNECTOR (ELASTOMERIC)

14 - VAPOR CANISTER

5 - TANK VENT/ROLLOVER VALVE(S)

15 - PURGE LINE

6 - VAPOR RECIRCULATION LINE

16 - PURGE DEVICE

7 - TANK VAPOR LINE

17 - WITHOUT NVLD

8 - VAPOR LINE TO CANISTER

18 - BREATHER ELEMENT

9 - CHECK VALVE (N/C)

19 - FLOW CONTROL ORIFICE

10 - CONTROL VALVE

20 - SERVICE PORT
21 - WITH NVLD

25 - 8

EVAPORATIVE EMISSIONS

EVAPORATIVE EMISSIONS (Continued)

EVAP/PURGE SOLENOID

REMOVAL

(1) Disconnect the negative battery cable.
(2) Unlock and disconnect the electrical connector

(Fig. 2).

(3) Release tab and pull solenoid from bracket

(Fig. 3).

(4) Remove the vacuum lines from solenoid.

INSTALLATION

(1) Connect the vacuum lines to solenoid (Fig. 4).
(2) Install solenoid to bracket.

(3) Connect the electrical connector to the solenoid

and lock (Fig. 5).

(4) Connect the negative battery cable.

Fig. 2 EVAP PURGE SOLENOID

Fig. 3 PURGE SOLENOID REMOVED

Fig. 4 PURGE SOLENOID REMOVED

Fig. 5 EVAP PURGE SOLENOID

EVAPORATIVE EMISSIONS

25 - 9

FUEL FILLER CAP

DESCRIPTION

The plastic fuel fill cap is threaded/quarter turn

onto the end of the fuel filler tube. It’s purpose is to
retain vapors and fuel in the fuel tank.

OPERATION

The fuel filler cap incorporates a two-way relief

valve that is closed to atmosphere during normal
operating conditions. The relief valve is calibrated to
open when a pressure of 17 kPa (2.5 psi) or vacuum
of 2 kPa (0.6 in. Hg) occurs in the fuel tank. When
the pressure or vacuum is relieved, the valve returns
to the normally closed position.

CAUTION: Remove the fuel filler cap to release fuel
tank pressure before disconnecting any fuel system
component.

NATURAL VAC LEAK
DETECTION ASSY

OPERATION

The Natural Vacuum Leak Detection (NVLD) sys-

tem is the next generation evaporative leak detection
system that will first be used on vehicles equipped
with the Next Generation Controller (NGC). This
new system replaces the leak detection pump as the
method of evaporative system leak detection. This 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 pres-

sure in a sealed vessel will change if the temperature
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 equi-
librium 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 evap-
orative system leaks.

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 ambient
temperature cycling. The diurnal effect is considered
one of the primary contributors to the leak determi-
nation 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 NGC. The NGC, 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 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 NVLD assembly may
be mounted on top of the canister outlet, or in-line
between the canister and atmospheric vent filter. The
normally open vacuum switch will close with about 1

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 pro-
tects the system from failed nozzle liquid ingestion,
in order to detect cap off conditions.

The normally closed valve in the NVLD is intended

to maintain the seal on the evaporative system dur-
ing the engine off condition. If vacuum in the evapo-
rative 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 can-

ister 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 sole-
noid 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 device is a diaphragm that

will open the seal in the NVLD with pressure in the
evaporative system. The device will

9blow off9 at

about 0.5

9 H2O (0.12 KPA) pressure to permit the

venting of vapors during refueling. An added benefit
to this is that it will also allow the tank to

9breathe9

during increasing temperatures, thus limiting the
pressure in the tank to this low level. This is benefi-
cial because the induced vacuum during a subse-
quent declining temperature will achieve the switch
closed (pass threshold) sooner than if the tank had to
decay from a built up pressure.

The device itself has 3 wires: Switch sense, sole-

noid driver and ground. The NGC utilizes a high-side
driver to energize and duty-cycle the solenoid.

25 - 10

EVAPORATIVE EMISSIONS

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