Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems

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VEHICLE EMISSION STANDARDS AND TESTING 311

FIGURE 24–6 A partial stream sampling exhaust probe

being used to measure exhaust gases in parts per million

(ppm) or percent (%).

A popular method of engine analysis, as well as emission test-

ing, involves the use of five-gas exhaust analysis equipment.

 SEE FIGURE 24–6. The five gases analyzed and their signifi-

cance are discussed next.

HYDROCARBONS Hydrocarbons (HC) are unburned gas-

oline and are measured in parts per million (ppm). A correctly

operating engine should burn (oxidize) almost all the gasoline;

therefore, very little unburned gasoline should be present in

the exhaust. Acceptable levels of HC are 50 ppm or less. High

levels of HC could be due to excessive oil consumption caused

by weak piston rings or worn valve guides. The most common

cause of excessive HC emissions is a fault in the ignition sys-

tem. Items that should be checked include the following:



Spark plugs



Spark plug wires



Distributor cap and rotor (if the vehicle is so equipped)



Ignition timing (if possible)



Ignition coil

CARBON MONOXIDE Carbon monoxide (CO) is unsta-

ble and will easily combine with any oxygen to form stable

carbon dioxide (CO

). The fact that CO combines with oxy-

gen is the reason that CO is a poisonous gas (in the lungs, it

combines with oxygen to form CO

and deprives the brain of

oxygen). CO levels of a properly operating engine should be

less than 0.5%. High levels of CO can be caused by clogged

EXHAUST ANALYSIS AND

COMBUSTION EFFICIENCY

What Does NMHC Mean?

NMHC means nonmethane hydrocarbon, and it

is the standard by which exhaust emission testing

for hydrocarbons is evaluated. Methane is natural

gas and can come from animals, animal waste, and

other natural sources. By not measuring methane

gas, all background sources are eliminated, giving

better results as to the true amount of unburned

hydrocarbons that are present in the exhaust

stream.

FREQUENTLY ASKED QUESTION

or restricted crankcase ventilation devices such as the PCV

valve, hose(s), and tubes. Other items that might cause exces-

sive CO include the following:



Clogged air filter



Incorrect idle speed



Too-high fuel-pump pressure



Any other items that can cause a rich condition

CARBON DIOXIDE Carbon dioxide (CO

) is the result

of oxygen in the engine combining with the carbon of the

gasoline. An acceptable level of CO

is between 12% and

15%. A high reading indicates an efficiently operating engine.

If the CO

level is low, the mixture may be either too rich or

too lean.

OXYGEN The next gas is oxygen (O

). There is about 21%

oxygen in the atmosphere, and most of this oxygen should be

“used up” during the combustion process to oxidize all the

hydrogen and carbon (hydrocarbons) in the gasoline. Levels of

should be very low (about 0.5%). High levels of O

, especially

at idle, could be due to an exhaust system leak.

NOTE: Adding 10% alcohol to gasoline provides addi-

tional oxygen to the fuel and will result in lower levels of

CO and higher levels of O

in the exhaust.

OXIDES OF NITROGEN An oxide of nitrogen (NO) is

a colorless, tasteless, and odorless gas when it leaves the

engine, but as soon as it reaches the atmosphere and mixes

with more oxygen, nitrogen oxides (NO

) are formed. NO

reddish-brown and has an acid and pungent smell. NO and

are grouped together and referred to as NO

, where x

represents any number of oxygen atoms. NO

, the symbol used

to represent all oxides of nitrogen, is the fifth gas commonly

tested using a five-gas analyzer. The exhaust gas recircula-

tion (EGR) system is the major controlling device limiting the

formation ofNO

312 CHAPTER 24

13 16 1514 17

1,400

1,200

1,000

800

600

400

200

1,600

CO, CO

, & O

PERCENTAGE

PPM  100 NO

AIR/FUEL RATIO BY WEIGHT

ALL EXHAUST GASES BEFORE CONVERSION

14.7:1

RICHER

O2S

LEANER

DESIRED RANGE IN CLOSED LOOP

FIGURE 24–7 Exhaust emissions are very complex. When

the air-fuel mixture becomes richer, some exhaust emissions

are reduced, while others increase.

Acceptable exhaust emissions include the following:

Without Catalytic

Converter

With Catalytic

Converter

HC 300 ppm or less 30 to 50 ppm or less

CO 3% or less 0.3% to 0.5% or less

0% to 2% 0% to 2%

12% to 15% or higher 12% to 15% or higher

Less than 100 ppm at

idle and less than

1,000 ppm at WOT

Less than 100 ppm

at idle and less than

1,000ppm at WOT

 SEE FIGURE 24–7.

How Can My Worn-Out, Old, High-Mileage

Vehicle Pass an Exhaust Emission Test?

Age and mileage of a vehicle are generally not factors

when it comes to passing an exhaust emission test.

Regular maintenance is the most important factor for

passing an enhanced inspection and maintenance

(I/M) exhaust analysis test. Failure of the vehicle

owner to replace broken accessory drive belts, leak-

ing air pump tubes, defective spark plug wires, or a

cracked exhaust manifold can lead to failure of other

components such as the catalytic converter. Tests

have shown that if the vehicle is properly cared for,

even an engine that has 300,000 miles (483,000km)

can pass an exhaust emission test.

FREQUENTLY ASKED QUESTION

CO TOO HIGH

HC TOO HIGH

High hydrocarbon exhaust emissions are usually caused by an

engine misfire. What burns the fuel in an engine? The ignition

system ignites a spark at the spark plug to ignite the proper

mixture inside the combustion chamber. If a spark plug does

not ignite the mixture, the resulting unburned fuel is pushed out

of the cylinder on the exhaust stroke by the piston through the

exhaust valves and into the exhaust system. Therefore, if any of

the following ignition components or adjustments are not cor-

rect, excessive HC emission is likely:

1. Defective or worn spark plugs

2. Defective or loose spark plug wires

3. Defective distributor cap and/or rotor

4. Incorrect ignition timing (either too far advanced or too far

retarded)

5. A lean air-fuel mixture can also cause a misfire. This condi-

tion is referred to as a lean misfire. A lean air-fuel mixture

can be caused by low fuel-pump pressure, a clogged fuel

filter, or a restricted fuel injector.

NOTE: To make discussion easier in future reference to

these items, this list of ignition components and checks

can be referred to simply as “spark stuff.”

Excessive carbon monoxide is an indication of too rich an air-

fuel mixture. CO is the rich indicator. The higher the CO read-

ing, the richer the air-fuel mixture. High concentrations of CO

indicate that not enough oxygen was available for the amount of

fuel. Common causes of high CO include the following:



Too-high fuel-pump pressure



Defective fuel-pressure regulator



Clogged air filter or PCV valve

VEHICLE EMISSION STANDARDS AND TESTING 313

LOW-PRESSURE AREAS

BEHIND EXHAUST PULSES

“SLUGS” OF EXHAUST

OUTSIDE

AIR IS

DRAWN

INTO HOLE

EXHAUST

LEAKS OUT

HOLE

HOLE IN

EXHAUST

EXHAUST PIPE

FIGURE 24–8 A hole in the exhaust system can cause

outside air (containing oxygen) to be drawn into the exhaust

system. This extra oxygen can be confusing to a service tech-

nician because the extra O

in the exhaust stream could be

misinterpreted as a too-lean air-fuel mixture.

NOTE: One technician remembers “CO” as meaning

“clogged oxygen” and always looks for restricted

airflow into the engine whenever high CO levels are

detected.



Defective injectors

TECH TIP

CO Equals O

If the exhaust is rich, CO emissions will be higher

than normal. If the exhaust is lean, O

emissions will

be higher than normal. Therefore, if the CO reading

is the same as the O

reading, then the engine is op-

erating correctly. For example, if both CO and O

are

0.5% and the engine develops a vacuum leak, the

will rise. If a fuel-pressure regulator were to mal-

function, the resulting richer air-fuel mixture would

increase CO emissions. Therefore, if both the rich in-

dicator (CO) and the lean indicator (O

) are equal, the

engine is operating correctly.

TECH TIP

How to Find a Leak in the Exhaust System

A hole in the exhaust system can dilute the exhaust

gases with additional oxygen (O

).  SEE FIGURE 24–8.

This additional O

in the exhaust can lead the

service technician to believe that the air-fuel mixture

is too lean. To help identify an exhaust leak, perform

an exhaust analysis at idle and at 2,500 RPM (fast

idle) and compare with the following:

• If the O

is high at idle and at 2,500 RPM, the mix-

ture is lean at both idle and at 2,500 RPM.

• If the O

is low at idle and high at 2,500 RPM, this

usually means that the vehicle is equipped with a

working AIR pump.

• If the O

is high at idle, but okay at 2,500 RPM, a

hole in the exhaust or a small vacuum leak that is

“covered up” at higher speed is indicated.

Two gas exhaust analyzers (HC and CO) work well, but both

HC and CO are consumed (converted) inside the catalytic con-

verter. The amount of leftover oxygen coming out of the tailpipe

is an indication of leanness. The higher the O

level, the leaner

the exhaust. Oxygen therefore is the lean indicator . Acceptable

levels of O

are 0% to 2%.

NOTE: A hole in the exhaust system can draw outside

air (oxygen) into the exhaust system. Therefore, to be

assured of an accurate reading, carefully check the

exhaust system for leaks. Using a smoke machine is an

easy method to locate leaks in the exhaust system.

Carbon dioxide (CO

) is a measure of efficiency. The higher

the level of CO

in the exhaust stream, the more efficiently the

engine is operating. Levels of 12% to 15% are considered to

be acceptable. Because CO

levels peak at an air-fuel mixture

of 14.7:1, a lower level of CO

indicates either a too-rich or a

too-lean condition. The CO

measurement by itself does not

indicate which condition is present. An example follows:

 8% (This means that efficiency is low and that the

air-fuel mixture is not correct.)

Look at O

and CO levels.

A high O

indicates lean and a high CO indicates rich.

MEASURING OXYGEN

AND CARBON DIOXIDE

Oxides of nitrogen are formed by high temperature—over 2,500°F

(1,370°C)—and/or pressures inside the combustion chamber.

Oxides of nitrogen contribute to the formation of photochemical

smog when sunlight reacts chemically with NO

and unburned

hydrocarbons (HC). Smog is a term derived by combining the

words smoke and fog . Ground-level ozone is a constituent of

smog. Ozone is an enriched oxygen molecule with three atoms

of oxygen (O

) instead of the normal two atoms of oxygen (O

Ozone in the upper atmosphere is beneficial because it

blocks out harmful ultraviolet rays that contribute to skin can-

cer. However, at ground level, this ozone (smog) is an irritant to

the respiratory system.

PHOTOCHEMICAL

SMOG FORMATION

314 CHAPTER 24

TECH TIP

Your Nose Knows

Using the nose, a technician can often identify a ma-

jor problem without having to connect the vehicle to

an exhaust analyzer. An example follows:

• The strong smell of exhaust is due to excessive

unburned hydrocarbon (HC) emissions. Look for

an ignition system fault that could prevent the

proper burning of the fuel. A vacuum leak could

also cause a lean misfire and cause excessive HC

exhaust emissions.

• If your eyes start to burn or water, suspect exces-

sive oxides of nitrogen (NO

) emissions. The ox-

ides of nitrogen combine with the moisture in the

eyes to form a mild solution of nitric acid. The acid

formation causes the eyes to burn and water. Ex-

cessive NO

exhaust emissions can be caused by

the following:

•

A vacuum leak causing higher-than-normal

combustion chamber temperature

•

Overadvanced ignition timing causing higher-

than-normal combustion chamber temperature

•

Lack of proper amount of exhaust gas recircula-

tion (EGR) (This is usually noticed above idle on

most vehicles.)

• Dizzy feeling or headache. This is commonly

caused by excessive carbon monoxide (CO) ex-

haust emissions. Get into fresh air as soon as pos-

sible. A probable cause of high levels of CO is an

excessively rich air-fuel mixture.

TECH TIP

Check for Dog Food?

A commonly experienced problem in many parts of

the country involves squirrels or other animals plac-

ing dog food into the air intake ducts of vehicles.

Dog food is often found packed tight in the ducts

against the air filter. An air intake restriction reduces

engine power and vehicle performance.

Because the formation of NO

occurs mostly under load, the

most efficient method to test for NO

is to use a portable exhaust

analyzer that can be carried in the vehicle while the vehicle is

being driven under a variety of conditions.

SPECIFICATIONS FOR NO

From experience, a maxi-

mum reading of 1,000 parts per million (ppm) of NO

under

loaded driving conditions will generally mean that the vehicle

will pass an enhanced I/M roller test. A reading of over 100 ppm

at idle should be considered excessive.

TESTING FOR OXIDES

OF NITROGEN

The Case of the Retarded Exhaust Camshaft

A Toyota equipped with a double overhead camshaft

(DOHC) inline 6-cylinder engine failed the state-

mandated enhanced exhaust emission test for NO

The engine ran perfectly without spark knocking

(ping), which is usually a major reason for excessive

emissions. The technician checked the following:

•

The ignition timing, which was found to be set to

specifications (if too far advanced, can cause ex-

cessive NO

)

•

The cylinders, which were decarbonized using top

engine cleaner

•

The EGR valve, which was inspected and the EGR

passages cleaned

After all the items were completed, the vehicle

was returned to the inspection station, where the ve-

hicle again failed for excessive NO

emissions (better

but still over the maximum allowable limit).

After additional hours of troubleshooting, the

technician decided to go back to basics and start

over again. A check of the vehicle history with the

owner indicated that the only previous work per-

formed on the engine was a replacement timing belt

over a year before. The technician discovered that

the exhaust cam timing was retarded two teeth, re-

sulting in late closing of the exhaust valve. The proper

exhaust valve timing resulted in a slight amount of

exhaust being retained in the cylinder. This extra

exhaust was added to the amount supplied by the

EGR valve and helped reduce NO

emissions. After

repositioning the timing belt, the vehicle passed the

emissions test well within the limits.

REAL WORLD FIX

VEHICLE EMISSION STANDARDS AND TESTING 315

Exhaust Gas Summary Chart

Gas Cause and Correction

High HC

Engine misfire or incomplete burning

of fuel caused by the following:

1. Ignition system fault

2. Lean misfire

3. Too low an engine temperature

(thermostat)

High CO

Rich condition caused by the following:

1. Leaking fuel injectors or fuel-pressure

regulator

2. Clogged air filter or PCV system

3. Excessive fuel pressure

High HC

and CO

Excessively rich condition caused by the

following:

1. All items included under high CO

2. Fouled spark plugs causing a misfire

to occur

3. Possible nonoperating catalytic

converter

High NO

Excessive combustion chamber

temperature caused by the following:

1. Nonoperating EGR valve

2. Clogged EGR passages

3. Engine operating temperature too

high because of cooling system re-

striction, worn water pump impeller,

or other faults in the cooling system

4. Lean air-fuel mixture

5. High compression caused by exces-

sive carbon buildup in the cylinders

O2S Shows Rich, but Pulse Width Is Low

A service technician was attempting to solve a drive-

ability problem. The computer did not indicate any

diagnostic trouble codes (DTCs). A check of the oxy-

gen sensor voltage indicated a higher-than-normal

reading almost all the time. The pulse width to the

port injectors was lower than normal. The lower-than-

normal pulse width indicates that the computer is

attempting to reduce fuel flow into the engine by de-

creasing the amount of on-time for all the injectors.

What could cause a rich mixture if the injectors

were being commanded to deliver a lean mixture?

Finally, the technician shut off the engine and took

a careful look at the entire fuel-injection system.

Although the vacuum hose was removed from the

fuel-pressure regulator, fuel was found dripping

from the vacuum hose. The problem was a defective

fuel-pressure regulator that allowed an uncontrolled

amount of fuel to be drawn by the intake manifold

vacuum into the cylinders. While the computer tried

to reduce fuel by reducing the pulse width signal to

the injectors, the extra fuel being drawn directly from

the fuel rail caused the engine to operate with too

rich an air-fuel mixture.

REAL WORLD FIX

4. Carbon dioxide (CO

) levels indicate efficiency. The higher

the CO

, the more efficient the engine operation.

5. Oxygen (O

) indicates leanness. The higher the O

, the

leaner the air-fuel mixture.

6. A vehicle should be driven about 20 miles, especially

during cold weather, to allow the engine to be fully warm

before an enhanced emissions test.

1. Excessive hydrocarbon (HC) exhaust emissions are cre-

ated by a lack of proper combustion, such as a fault in the

ignition system, too lean an air-fuel mixture, or too-cold

engine operation.

2. Excessive carbon monoxide (CO) exhaust emissions are

usually created by a rich air-fuel mixture.

3. Excessive oxides of nitrogen (NO

) exhaust emissions are

usually created by excessive heat or pressure in the com-

bustion chamber or a lack of the proper amount of exhaust

gas recirculation (EGR).

SUMMARY

3. List two causes of a lean exhaust.

4. List those items that should be checked if a vehicle fails an

exhaust test for excessive NO

emissions.

1. List the five exhaust gases and their maximum allowable

readings for a fuel-injected vehicle equipped with a cata-

lytic converter.

2. List two causes of a rich exhaust.

REVIEW QUESTIONS

316 CHAPTER 24

6. All of the gases are measured in percentages except

______________ .

a. HC c. CO

b. CO d. O

7. After the following exhaust emissions were measured, how

was the engine operating?

HC  766 ppm CO

 8.2% CO  4.6% O

 0.1%

a. Too rich

b. Too lean

8. Technician A says that carbon inside the engine can cause

excessive NO

to form. Technician B says that excessive

could be caused by a cooling system fault causing the

engine to operate too hot. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

9. A clogged EGR passage could cause excessive

______________ exhaust emissions.

a. HC c. NO

b. CO d. CO

10. An ignition fault could cause excessive ______________

exhaust emissions.

a. HC c. NO

b. CO d. CO

1. Technician A says that high HC emission levels are often

caused by a fault in the ignition system. Technician B says

that high CO

emissions are usually caused by a richer-

than-normal air-fuel mixture. Which technician is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

2. HC and CO are high, and CO

and O

are low. This could

be caused by a ______________ .

a. Rich mixture

b. Lean mixture

c. Defective ignition component

d. Clogged EGR passage

3. Which gas is generally considered to be the rich indica-

tor? (The higher the level of this gas, the richer the air-fuel

mixture.)

a. HC c. CO

b. CO d. O

4. Which gas is generally considered to be the lean indica-

tor? (The higher the level of this gas, the leaner the air-fuel

mixture.)

a. HC c. CO

b. CO d. O

5. Which exhaust gas indicates efficiency? (The higher the

level of this gas, the more efficient the engine operates.)

a. HC c. CO

b. CO d. O

CHAPTER QUIZ

EVAPORATIVE EMISSION CONTROL SYSTEMS 317

chapter

EVAPORATIVE EMISSION

CONTROL SYSTEMS

OBJECTIVES: After studying Chapter 25 , the reader should be able to: • Prepare for the ASE Engine Performance (A8)

certification test content area “D” (Emission Control Systems). • Describe the purpose and function of the evaporative emission

control system. • Discuss how the evaporative emission control system is tested under OBD-II regulations. • Explain methods

for diagnosing and testing faults in the evaporative emission control system.

KEY TERMS: Adsorption 318 • Evaporative control (EVAP) system 317 • Fuel tank pressure (FTP) 323 • Leak detection

pump (LDP) 321 • Onboard refueling vapor recovery (ORVR) 321 • Purge valve 321 • Vent valve 321 • Volatile organic

compounds (VOC) 317

PURPOSE AND FUNCTION The purpose of the evapora-

tive emission control system is to trap and hold gasoline vapors,

also called volatile organic compounds (VOCs). The evapo-

rative control (EVAP) system includes the charcoal canister,

hoses, and valves. These vapors are routed into a charcoal

canister, then into the intake airflow, where they are burned in

the engine instead of being released into the atmosphere.

COMMON COMPONENTS The fuel tank filler caps used

on vehicles with modern EVAP systems are a special design.

Most EVAP fuel tank filler caps have a built-in pressure-vacuum

relief valve. When pressure or vacuum exceeds a calibrated

value, the valve opens. Once the pressure or vacuum has been

relieved, the valve closes. If a sealed cap is used on an EVAP

system that requires a pressure-vacuum relief design, a vacuum

may develop in the fuel system, or the fuel tank may be damaged

by fuel expansion or contraction.



SEE FIGURE 25–1 .

EVAP SYSTEM OPERATION The canister is located

under the hood or underneath the vehicle and is filled with acti-

vated charcoal granules that can hold up to one-third of their

own weight in fuel vapors.



SEE FIGURE 25–2 .

NOTE: Some vehicles with large or dual fuel tanks may

have dual canisters.

EVAPORATIVE EMISSION

CONTROL SYSTEM

When Filling My Fuel Tank, Why Should I Stop

When the Pump Clicks Off?

Every fuel tank has an upper volume chamber that

allows for expansion of the fuel when hot. The vol-

ume of the chamber is between 10% and 20% of

the volume of the tank. For example, if a fuel tank

had a capacity of 20 gallons, the expansion cham-

ber volume would be from 2 to 4 gallons. A hose is

attached at the top of the chamber and vented to

the charcoal canister. If extra fuel is forced into this

expansion volume, liquid gasoline can be drawn into

the charcoal canister. This liquid fuel can saturate

the canister and create an overly rich air-fuel mixture

when the canister purge valve is opened during nor-

mal vehicle operation. This extra-rich air-fuel mixture

can cause the vehicle to fail an exhaust emissions

test, reduce fuel economy, and possibly damage

the catalytic converter. To avoid problems, simply

add fuel to the next dime’s worth after the nozzle

clicks off. This will ensure that the tank is full yet not

overfilled.

FREQUENTLY ASKED QUESTION

Activated charcoal is an effective vapor trap because of

its great surface area. Each gram of activated charcoal has a

surface area of 1,100 square meters, or more than 1/4 acre.

Typical canisters hold either 300 or 625 grams of charcoal

with a surface area equivalent to 80 or 165 football fields. By

318 CHAPTER 25

electric vacuum solenoid and one or more purge valves.

Under normal conditions, most engine control systems

permit purging only during closed-loop operation at

cruising speeds. During other engine operation condi-

tions, such as open-loop mode, idle, deceleration, or

wide-open throttle, the PCM prevents canister purging.

EVAPORATIVE PRESSURES Pressures can build inside

the fuel system and are usually measured in units of inches of

water, abbreviated “in. H

O” (28 in. H

O equals 1 pound per

square inch, or 1 PSI). Pressure buildup can be caused by the

following:



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

Some scan tools display other units of measure for the

EVAP system that makes understanding the system diffi-

cult.



SEE CHART 25–1 for pressure conversions between

pounds per square inch (PSI), inches of mercury (in. Hg), and

inches of water (in. H

O).

a process called adsorption, the fuel vapor molecules adhere

to the carbon surface. This attaching force is not strong, so the

system purges the vapor molecules quite simply by sending a

fresh airflow through the charcoal:



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 unburned gasoline (hydrocar-

bons, abbreviated HCs) vapors from the canister with the

existing air-fuel charge.



SEE FIGURES 25–3 AND 25–4 .



Computer-controlled purge. The PCM controls when

the canister purges on most engines. This is done by an

CHARCOAL CANISTER

FIGURE 25–2 A charcoal canister can be located under the

hood or underneath the vehicle.

FUEL TANK

VAPOR

CANISTER

VAPOR

RESTRICTOR

PURGE

LINE

PURGE

VALVE

VENT

LINE

FIGURE 25–3 The EVAP system includes all of the lines,

hoses, and valves, plus the charcoal canister.

FIGURE 25–1 A capless system from a Ford Flex does not

use a replaceable cap; instead, it is spring-loaded closed.

Prior to 1996, evaporative systems were referred to as non-

enhanced evaporative control (EVAP) systems. This term

refers to evaporative systems that had limited diagnostic

capabilities. While they are often PCM controlled, their di-

agnostic capability is usually limited to their ability to detect

NONENHANCED

EVAPORATIVE

CONTROL SYSTEMS

EVAPORATIVE EMISSION CONTROL SYSTEMS 319

if purge has occurred. Many of these older systems have a

diagnostic switch that could sense if purge is occurring and

set a code if no purge is detected. This system does not

check for leaks. On some vehicles, the PCM also has the

capability of monitoring the integrity of the purge solenoid

and circuit. These systems’ limitations are their ability to

check the integrity of the evaporative system on the vehicle.

They could not detect leaks or missing or loose gas caps

that could lead to excessive evaporative emissions from

the vehicle. Nonenhanced evaporative systems use either

a canister purge solenoid or a vapor management valve to

control purge vapor.

VALVE OPEN

VALVE CLOSED

TO CANISTER

FROM TANK

RUBBER

DIAPHRAGM

SPRING

FUEL TANK

ROLLOVER

CHECK VALVE

CHARCOAL

CANISTER

PRESSURE CONTROL

VALVE

MANIFOLD

VACUUM

CANISTER PURGE

SOLENOID VALVE

THROTTLE

BODY

FIGURE 25–4 A typical EVAP 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.

ENHANCED EVAPORATIVE

CONTROL SYSTEM

BACKGROUND Beginning in 1996 with OBD-II vehicles,

manufacturers were required to install systems that are able to

detect both purge flow and evaporative system leakage:



The systems on models produced between 1996

and2000 must be able to detect a leak as small as

0.040inches in diameter.

320 CHAPTER 25

CHART 25–1

Pressure conversions.

NOTE: 1 PSI  28 in. H

0.25 PSI  7 in. H

PSI INCHES HG INCHES H

14.7 29.93 407.19

1.0 2.036 27.7

0.9 1.8 24.93

0.8 1.63 22.16

0.7 1.43 19.39

0.6 1.22 16.62

0.5 1.018 13.85

0.4 0.814 11.08

0.3 0.611 8.31

0.2 0.407 5.54

0.1 0.204 2.77

0.09 0.183 2.49

0.08 0.163 2.22

0.07 0.143 1.94

0.06 0.122 1.66

0.05 0.102 1.385

EVAPORATIVE

SYSTEM

CANISTER

SERVICE

PORT

PURGE SOLENOID

(NORMALLY CLOSED)

VENT SOLENOID

(NORMALLY OPEN)

FUEL TANK

PRESSURE

SENSOR

FUEL

CAP

THROTTLE

BODY

PCM

FIGURE 25–5 An enhanced EVAP system is able to perform system and leak detection diagnosis.



Beginning in the model year 2000, the enhanced systems

started a phase-in of 0.020-inch diameter leak detection.



All vehicles built after 1995 have enhanced evaporative

systems with 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 (DTC) and

turn on the malfunction indicator lamp (MIL) to warn the

driver of the failure.



SEE FIGURE 25–5 .

VENT VALVE The canister vent valve is a normally open

valve and is closed only when commanded by the PCM during

testing of the system. The vent valve is closed only during test-

ing by the PCM as part of the mandated OBD-II standards. The

vent solenoid is located under the vehicle in most cases and is

exposed to the environment, making this valve subject to rust

and corrosion.

PURGE VALVE The canister purge valve, also called

the canister purge (CANP) solenoid, is normally closed and is

pulsed open by the PCM during purging. The purge valve is

connected to intake manifold vacuum using a rubber hose to

draw gasoline vapors from the charcoal canister into the engine

when the purge valve is commanded open. Most purge valves

are pulsed on and off to better control the amount of fumes

being drawn into the intake manifold.