Evaporative emission control system.

Automotive Fuel and Emissions Control Systems, 2/eBy James D. Halderman and Jim Linder

© 2009 Pearson Higher Education, Inc.Pearson Prentice Hall - Upper Saddle River, NJ 07458





OBJECTIVESAfter studying Chapter 26, the reader will be able

to:

.1Prepare for the ASE Engine Performance (A8) certification test content area “D” (Emission

Control Systems).

.2Describe the purpose and function of the evaporative emission control system.

.3Discuss how the evaporative emission control system is tested under OBD-II regulations.

.4Explain 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 CONTROLCommon 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 STORAGEThe 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 PURGINGDuring 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 PURGINGComputer-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 PURGINGComputer-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 PRESSUREPressures 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 ValvePurge 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 TESTSIn 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 SYSTEMBefore 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 economyPoor 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 SYSTEMLeaks 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 SYSTEMThere 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 MONITORThe 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 MONITOREngine 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 startBARO greater than 70 kPa (20.7 in. Hg or 10.2 PSI)IAT between 39°F and 86°F at engine start-upECT between 39°F and 86°F at engine start-upECT and IAT within 39°F of each other at engine

start-upFuel level within 15% to 85%TP sensor between 9% and 35%



GENERAL MOTORS ENHANCED EVAPRunning 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 EVAPThe PCM will run the EVAP monitor when the

following enable criteria are met.Cold startInlet air temperature between 40°F and 100°F6 -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 feetFuel level within 15% to 85% (40% min for 0.020

in. leak)40 to 65 mph steady cruise30-minute time limit for the monitor to run



FORD ENHANCED EVAPTypical 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 SYSTEMMany 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 SYSTEMPump Period

The time between LDP solenoid off and LDP switch close is called the pump period .

This time period is inversely proportional to the size of the leak .

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 secondsEVAP medium leak (0.040 to 0.080): 0.9 to 1.2

secondsEVAP small leak (0.020 to 0.040): 1.2 to 6 seconds



EVAP SYSTEM-RELATED DIAGNOSTIC TROUBLE CODES (DTCs)



SUMMARY.1The purpose of the evaporative emission

(EVAP) control system is to reduce the release of volatile organic compounds (VOC)

into the atmosphere.

.2A carbon (charcoal) canister is used to trap and hold gasoline vapors until they can be

purged and run into the engine to be burned.

.3Pressures inside the EVAP system are low and are measured in inches of water (1 PSI

= 28 in. H2O).



SUMMARY.4A typical EVAP system uses a canister purge

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

.5OBD-II regulation requires that the evaporative emission control system be checked for leakage

and proper purge flow rates..6External leaks can best be located by

pressurizing the fuel system with low-pressure smoke.



REVIEW QUESTIONS.1What components are used in a typical

evaporative emission control system?.2How does the computer control the purging

of the vapor canister?.3What is the difference between an enhanced

and nonenhanced evaporative control system?

.4How is a flow gauge used to diagnose the evaporative emission control system?

.5What are the parameters (enable criteria) that must be met for the evaporative system

monitor to run?



CHAPTER QUIZ.1What is the substance used in a vapor

canister to absorb volatile organic compounds?

(aDesiccant(bOrganic absorber(cPleated paper(dCarbon



CHAPTER QUIZ2.Which valve(s) is (are) normally closed?

(aCanister purge valve(bCanister vent valve(cBoth canister purge and canister vent valve(dNeither canister purge nor canister vent valve



CHAPTER QUIZ3.All of the following can increase the pressure

in the evaporative emission control system except _____

(aFuel temperature(bReturned fuel from the fuel-injection system(cInlet fuel to the fuel pump(dRVP of the fuel



CHAPTER QUIZ4.Evaporative emission control systems operate

on low pressure measured in inches of water (in. H2O). One PSI is equal to how many

inches of water?(a1

(b10

(c18

(d28



CHAPTER QUIZ4.Evaporative emission control systems operate

on low pressure measured in inches of water (in. H2O). One PSI is equal to how many

inches of water?(a1

(b10

(c18

)d28



CHAPTER QUIZ5.Inadequate purge flow rate will trigger which

DTC?(aP0440(bP0441(cP0300(dP0301



CHAPTER QUIZ6.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?

(aTechnician A only(bTechnician B only(cBoth Technicians A and B(dNeither Technician A nor B



CHAPTER QUIZ7.A flow gauge is being used to check for proper

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

sites?(a10 liters per minute(b5 liters per minute(c1 liter in 4 minutes(d0.5 liter per hour



CHAPTER QUIZ7.A flow gauge is being used to check for proper

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

sites?(a10 liters per minute(b5 liters per minute)c1 liter in 4 minutes(d0.5 liter per hour



CHAPTER QUIZ8.Before an evaporative emission monitor will

run, the fuel level must be where?(aAt least 75% full(bOver 25%(cBetween 15% and 85%(dThe level of the fuel in the tank is not needed to

run the monitor test



CHAPTER QUIZ9.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?(aTechnician A only(bTechnician B only(cBoth Technicians A and B(dNeither Technician A nor B



CHAPTER QUIZ10.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 be set?(aP0440(bP0442(cP0446(dEither P0440 or P0442

Evaporative emission control system.

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