chapter 26 evaporative emission control systems

VAPOR CANISTER STORAGEFIGURE 26-3 The evaporative emission control system includes all of the lines, hoses, and valves, plus the charcoal canister... VAPOR PURGINGComputer-Controlled Pu

After studying Chapter 26, the reader will be able to:

1 Prepare for the ASE Engine Performance (A8)

certification test content area “D” (Emission

Control Systems).

2 Describe the purpose and function of the

evaporative emission control system.

3 Discuss how the evaporative emission control

system is tested under OBD-II regulations.

4 Explain methods for diagnosing and testing faults

in the evaporative emission control system.

NEED FOR EVAPORATIVE EMISSION CONTROL

• The purpose of the evaporative (EVAP) emission control

system is to trap and hold gasoline vapors

• The charcoal canister is part of an entire system of hoses

and valves

• Before the early 1970s, most gasoline fumes were simply

vented to the atmosphere.

• The purpose of the EVAP system is to trap gasoline vapors

—volatile organic compounds, or VOCs—that would

otherwise escape into the atmosphere

• These vapors are instead routed into a charcoal canister,

from where they go to the intake airflow so they are burned

in the engine.

NEED FOR EVAPORATIVE EMISSION CONTROL

Common Components

• Most EVAP fuel tank filler

caps have

pressure-vacuum relief built into

them

• When pressure or vacuum

exceeds a calibrated value,

the valve opens

• Once the pressure or

vacuum has been relieved,

the valve closes.

FIGURE 26-1 A typical bayonet-type gas cap.

VAPOR CANISTER STORAGE

• The canister is located

under the hood or

underneath the vehicle, and

is filled with activated

charcoal granules that can

hold up to one-third of their

own weight in fuel vapors

• A vent line connects the

canister to the fuel tank.

FIGURE 26-2 Charcoal canister as

mounted under the hood (Jeep) Not all charcoal canisters are this accessible; in fact, most are hidden under the hood or in other locations on the vehicle.

VAPOR CANISTER STORAGE

FIGURE 26-3 The evaporative emission control system includes all of the lines,

hoses, and valves, plus the charcoal canister.

VAPOR PURGING

• During engine operation, stored vapors are drawn from the canister into the engine through a hose

connected to the throttle body or the air cleaner

• This “purging” process mixes HC vapors from the

canister with the existing air-fuel charge.

VAPOR PURGING

Computer-Controlled Purge

• Canister purging on engines with electronic fuel

management systems is regulated by the

powertrain control module (PCM).

• Control of this function is particularly important

because the additional fuel vapors sent through the purge line can upset the air–fuel ratio provided by a fuel-injection system

• Since air–fuel ratio adjustments are made many

times per second, it is critical that vapor purging is controlled just as precisely.

VAPOR PURGING

Computer-Controlled Purge

FIGURE 26-4 A typical evaporative

emission control system Note that when the computer turns on the canister purge solenoid valve, manifold vacuum draws any stored vapors from the canister into the engine Manifold vacuum also is applied to the pressure control valve When this valve opens, fumes from the fuel tank are drawn into the charcoal canister and eventually into the engine When the solenoid valve is turned off (or the engine stops and there is

no manifold vacuum), the pressure control valve is spring-loaded shut to keep vapors inside the fuel tank from escaping to the atmosphere.

EVAPORATIVE SYSTEM PRESSURE

• Pressures can build inside the fuel system and are usually measured in units of inches of water,

abbreviated in H2O (28 inches of water equals one PSI)

• Pressure buildup is a function of:

– Fuel evaporation rates (volatility)

– Gas tank size (fuel surface area and volume)

– Fuel level (liquid versus vapor)

– Fuel slosh (driving conditions)

– Temperature (ambient, in-tank, close to the tank)

– Returned fuel from the rail

NONENHANCED EVAPORATIVE CONTROL

SYSTEMS

• Nonenhanced evaporative

systems use either a

canister purge solenoid or a

vapor management valve to

control purge vapor.

FIGURE 26-5 A typical OBD-II EVAP

system which uses fuel tank pressure

and purge flow sensors to detect leaks

and measure purge flow The purge

flow sensor is similar to a mass air flow

sensor and measures the amount of

flow into the engine.

ENHANCED EVAPORATIVE CONTROL SYSTEM

• All vehicles built after 1995 have enhanced

evaporative systems that have the ability to detect purge flow and system leakage

• If either of these two functions fails, the system is

required to set a diagnostic trouble code and turn

on the MIL light to warn the driver of the failure.

• Vent Valve

• Purge Valve

ENHANCED EVAPORATIVE CONTROL SYSTEM

FIGURE 26-6 The vapor

management valve (VMV) and the canister purge valve (CPV) are both PCM

controlled and check for proper flow through the evaporative control system

The fuel tank pressure (FTP) sensor monitors vapor pressure inside the fuel tank.

ONBOARD REFUELING VAPOR RECOVERY

• The primary feature of most ORVR systems is the restricted tank filler tube, which is about 1 inch (25 mm) in diameter.

• This reduced filler tube creates an aspiration effect, which tends to draw outside air into the filler tube

• During refueling, the fuel tank is vented to the

charcoal canister, which captures the gas fumes:

with air flowing into the filler tube, no vapors can

escape to the atmosphere.

STATE INSPECTION EVAP TESTS

• In some states, a periodic

inspection and test of the

fuel system are mandated

along with a dynamometer

test

• The emissions inspection

includes tests on the

vehicle before and during

the dynamometer test.

FIGURE 26-7 A gas cap being testing by

an inspector at an inspection station.

DIAGNOSING THE EVAP SYSTEM

• Before vehicle emissions testing began in many

parts of the country, little service work was done on the evaporative emission system

• Common engine-performance problems that can be caused by a fault in this system include:

– Poor fuel economy

– Poor performance

DIAGNOSING THE EVAP SYSTEM

FIGURE 26-8 A typical purge

flow tester connected in series between the intake manifold (or control solenoid) and the charcoal canister

Most working systems should

be capable of flowing at least

1 L/min Some vehicles must

be test driven because their computers only purge after a certain road speed has been achieved.

LOCATING LEAKS IN THE SYSTEM

• Leaks in the evaporative emission control system will cause the malfunction check gas cap indication light to come on in some vehicles.

FIGURE 26-9 Some vehicles will display a message if an evaporative control

system leak is detected that could be the result of a loose gas cap.

LOCATING LEAKS IN THE SYSTEM

• There are two methods that can be used to check for leaks in the evaporative system.

– Smoke machine testing

– Nitrogen gas pressurization.

LOCATING LEAKS IN THE SYSTEM

FIGURE 26-10 (a) A typical EVAP diagnostic tester (b) A smoke test shows a

leaking gas cap.

LOCATING LEAKS IN THE SYSTEM

FIGURE 26-11 An emission tester that uses nitrogen to pressurize the fuel system.

EVAPORATIVE SYSTEM MONITOR

• The EVAP system monitor

tests for purge volume and

leaks

• Most applications purge the

charcoal canister by venting

the vapors into the intake

manifold during cruise

• To do this, the PCM

typically opens a

solenoid-operated purge valve

installed in the purge line

leading to the intake

manifold FIGURE 26-12 Typical evaporative system

showing the valves and the normal position.

EVAPORATIVE SYSTEM MONITOR

Engine Off Natural Vacuum

• System integrity (leakage) can also be checked

after the engine is shut off

– The premise is that a warm evaporative system will cool down after the engine is shut off and the vehicle is stable.

• A slight vacuum will be created in the gas tank

during this cooldown period

– If a specific level of vacuum is reached and maintained, the system is said to have integrity (no leakage).

GENERAL MOTORS ENHANCED EVAP

• The PCM will run the EVAP monitor when the

following enable criteria are met

• Typical enable criteria include:

– Cold start

– BARO greater than 70 kPa (20.7 in Hg or 10.2 PSI)

– IAT between 39°F and 86°F at engine start-up

– ECT between 39°F and 86°F at engine start-up

– ECT and IAT within 39°F of each other at engine start-up – Fuel level within 15% to 85%

– TP sensor between 9% and 35%

GENERAL MOTORS ENHANCED EVAP

Running the EVAP Monitor

• There are four tests which are performed during a typical GM EVAP monitor

• A DTC is assigned to each test.

– Weak Vacuum Test (P0440—large leak)

– Small Leak Test (P0442—small leak)

– Excess Vacuum Test (P0446)

– Purge Solenoid Leak Test (P1442).

ALWAYS TIGHTEN “THREE CLICKS”

FIGURE 26-13 Some newer Fords use a fuel system that does not use a cap

Carefully check the opening for dirt or debris if an evaporative system DTC is set.

FORD ENHANCED EVAP

• The PCM will run the EVAP monitor when the

following enable criteria are met.

– Cold start

– Inlet air temperature between 40°F and 100°F

– 6- to 8-hour engine soak timer must expire

• A PCM reset (without turning the ignition off) will

bypass the 6- to 8-hour engine soak timer.

– Altitude less than 8,000 feet

– Fuel level within 15% to 85% (40% min for 0.020 in leak) – 40 to 65 mph steady cruise

– 30-minute time limit for the monitor to run

FORD ENHANCED EVAP

Typical Ford Strategy

• Phase 0—Initial Vacuum Pulldown.

• Phase 1—Vacuum Stabilization.

• Phase 2—Vacuum Hold and Decay.

• Phase 3—Vacuum Release.

• Phase 4—Vapor Generation.

KEEP THE FUEL TANK PROPERLY FILLED

FIGURE 26-14 The fuel level

must be above 15% and below 85% before the EVAP monitor will run on most vehicles.

LEAK DETECTION PUMP SYSTEM

• Many Chrysler and some

other brands of vehicles

use a leak detection pump

(LDP) as part of the

evaporative control system

diagnosis equipment.

FIGURE 26-15 A leak detection pump

(LDP) used on some Chrysler vehicles

to pressurize (slightly) the fuel system

to check for leaks.

LEAK DETECTION PUMP SYSTEM

• The shorter the pump period, the larger the leak

• The longer the pump period, the smaller the leak.

– EVAP large leak (>0.080): less than 0.9 seconds

– EVAP medium leak (0.040 to 0.080): 0.9 to 1.2 seconds – EVAP small leak (0.020 to 0.040): 1.2 to 6 seconds

EVAP SYSTEM-RELATED DIAGNOSTIC TROUBLE CODES (DTCs)

1 The purpose of the evaporative emission (EVAP)

control system is to reduce the release of volatile organic compounds (VOC) into the atmosphere.

2 A carbon (charcoal) canister is used to trap and

hold gasoline vapors until they can be purged and run into the engine to be burned.

3 Pressures inside the EVAP system are low and

are measured in inches of water (1 PSI = 28 in

4 A typical EVAP system uses a canister

purge valve, which is normally closed, and a canister vent valve, which is normally open.

5 OBD-II regulation requires that the

evaporative emission control system be

checked for leakage and proper purge flow rates.

6 External leaks can best be located by

pressurizing the fuel system with

low-pressure smoke.

REVIEW QUESTIONS

1 What components are used in a typical

evaporative emission control system?

2 How does the computer control the purging of the

vapor canister?

3 What is the difference between an enhanced and

nonenhanced evaporative control system?

4 How is a flow gauge used to diagnose the

evaporative emission control system?

5 What are the parameters (enable criteria) that

must be met for the evaporative system monitor to run?

CHAPTER QUIZ

1 What is the substance used in a vapor canister to

absorb volatile organic compounds?

a) Desiccant

b) Organic absorber

c) Pleated paper

d) Carbon

CHAPTER QUIZ

1 What is the substance used in a vapor canister to

absorb volatile organic compounds?

a) Desiccant

b) Organic absorber

c) Pleated paper

d) Carbon

CHAPTER QUIZ

2 Which valve(s) is (are) normally closed?

a) Canister purge valve

b) Canister vent valve

c) Both canister purge and canister vent valve

d) Neither canister purge nor canister vent valve

CHAPTER QUIZ

2 Which valve(s) is (are) normally closed?

a) Canister purge valve

b) Canister vent valve

c) Both canister purge and canister vent valve

d) Neither canister purge nor canister vent valve

CHAPTER QUIZ

3 All of the following can increase the pressure in the

evaporative emission control system except _

a) Fuel temperature

b) Returned fuel from the fuel-injection system

c) Inlet fuel to the fuel pump

d) RVP of the fuel

CHAPTER QUIZ

3 All of the following can increase the pressure in the

evaporative emission control system except _

a) Fuel temperature

b) Returned fuel from the fuel-injection system

c) Inlet fuel to the fuel pump

d) RVP of the fuel

CHAPTER QUIZ

4 Evaporative emission control systems operate on

low pressure measured in inches of water (in

CHAPTER QUIZ

4 Evaporative emission control systems operate on

low pressure measured in inches of water (in

CHAPTER QUIZ

6 Two technicians are discussing a P0442 DTC

Technician A says that it can be caused by a

leaking EVAP line Technician B says that a

leaking gas cap can cause a failure of the EVAP test Which technician is correct?

a) Technician A only

b) Technician B only

c) Both Technicians A and B

d) Neither Technician A nor B

CHAPTER QUIZ

6 Two technicians are discussing a P0442 DTC

Technician A says that it can be caused by a

leaking EVAP line Technician B says that a

leaking gas cap can cause a failure of the EVAP test Which technician is correct?

a) Technician A only

b) Technician B only

c) Both Technicians A and B

d) Neither Technician A nor B

CHAPTER QUIZ

7 A flow gauge is being used to check for proper

canister pump flow rate What is the specification used by most emission test sites?

a) 10 liters per minute

b) 5 liters per minute

c) 1 liter in 4 minutes

d) 0.5 liter per hour

CHAPTER QUIZ

7 A flow gauge is being used to check for proper

canister pump flow rate What is the specification used by most emission test sites?

a) 10 liters per minute

b) 5 liters per minute

c) 1 liter in 4 minutes

d) 0.5 liter per hour

CHAPTER QUIZ

8 Before an evaporative emission monitor will run,

the fuel level must be where?

CHAPTER QUIZ

8 Before an evaporative emission monitor will run,

the fuel level must be where?

CHAPTER QUIZ

9 Technician A says that low pressure smoke

installed in the fuel system can be used to check for leaks Technician B says that nitrogen under

low pressure can be installed in the fuel system to check for leaks Which technician is correct?

a) Technician A only

b) Technician B only

c) Both Technicians A and B

d) Neither Technician A nor B

CHAPTER QUIZ

9 Technician A says that low pressure smoke

installed in the fuel system can be used to check for leaks Technician B says that nitrogen under

low pressure can be installed in the fuel system to check for leaks Which technician is correct?

a) Technician A only

b) Technician B only

c) Both Technicians A and B

d) Neither Technician A nor B

CHAPTER QUIZ

10.A large leak is detected by the evaporative

emission control system monitor that could be

caused by a loose gas cap Which DTC will likely