2. OBJECTIVES After studying Chapter 26, the reader will 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.

3. 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.

4. 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.

5. 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.

6. VAPOR CANISTER STORAGE
FIGURE 26-3 The evaporative emission control system includes all of the lines, hoses, and valves, plus the charcoal canister.

7. 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.

8. 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.

9. 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.

10. 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

11. 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.

12. 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

13. 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.

14. 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.

15. 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.

16. 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

17. 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.

18. 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.

19. 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.

20. LOCATING LEAKS IN THE SYSTEM
FIGURE (a) A typical EVAP diagnostic tester. (b) A smoke test shows a leaking gas cap.

21. LOCATING LEAKS IN THE SYSTEM
FIGURE An emission tester that uses nitrogen to pressurize the fuel system.

22. 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 Typical evaporative system showing the valves and the normal position.

23. 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).

24. 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%

25. 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).

26. ALWAYS TIGHTEN “THREE CLICKS”
FIGURE 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.

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

28. 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.

29. KEEP THE FUEL TANK PROPERLY FILLED
FIGURE The fuel level must be above 15% and below 85% before the EVAP monitor will run on most vehicles.

30. 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 A leak detection pump (LDP) used on some Chrysler vehicles to pressurize (slightly) the fuel system to check for leaks.

31. LEAK DETECTION PUMP SYSTEM Pump 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 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

32. EVAP SYSTEM-RELATED DIAGNOSTIC TROUBLE CODES (DTCs)

33. SUMMARY
The purpose of the evaporative emission (EVAP) control system is to reduce the release of volatile organic compounds (VOC) into the atmosphere.
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.
Pressures inside the EVAP system are low and are measured in inches of water (1 PSI = 28 in. H2O).

34. SUMMARY
A typical EVAP system uses a canister purge valve, which is normally closed, and a canister vent valve, which is normally open.
OBD-II regulation requires that the evaporative emission control system be checked for leakage and proper purge flow rates.
External leaks can best be located by pressurizing the fuel system with low-pressure smoke.

35. REVIEW QUESTIONS
What components are used in a typical evaporative emission control system?
How does the computer control the purging of the vapor canister?
What is the difference between an enhanced and nonenhanced evaporative control system?
How is a flow gauge used to diagnose the evaporative emission control system?
What are the parameters (enable criteria) that must be met for the evaporative system monitor to run?

36. CHAPTER QUIZ
What is the substance used in a vapor canister to absorb volatile organic compounds?
Desiccant
Organic absorber
Pleated paper
Carbon

37. CHAPTER QUIZ
What is the substance used in a vapor canister to absorb volatile organic compounds?
Desiccant
Organic absorber
Pleated paper
Carbon

38. CHAPTER QUIZ 2. Which valve(s) is (are) normally closed?
Canister purge valve
Canister vent valve
Both canister purge and canister vent valve
Neither canister purge nor canister vent valve

39. CHAPTER QUIZ 2. Which valve(s) is (are) normally closed?
Canister purge valve
Canister vent valve
Both canister purge and canister vent valve
Neither canister purge nor canister vent valve

40. CHAPTER QUIZ
3. All of the following can increase the pressure in the evaporative emission control system except _____
Fuel temperature
Returned fuel from the fuel-injection system
Inlet fuel to the fuel pump
RVP of the fuel

41. CHAPTER QUIZ
3. All of the following can increase the pressure in the evaporative emission control system except _____
Fuel temperature
Returned fuel from the fuel-injection system
Inlet fuel to the fuel pump
RVP of the fuel

42. CHAPTER QUIZ
4. 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? 1 10 18 28

43. CHAPTER QUIZ
4. 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? 1 10 18 28

44. CHAPTER QUIZ 5. Inadequate purge flow rate will trigger which DTC?

45. CHAPTER QUIZ 5. Inadequate purge flow rate will trigger which DTC?

46. 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?
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B

47. 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?
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B

48. 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?
10 liters per minute
5 liters per minute
1 liter in 4 minutes
0.5 liter per hour

49. 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?
10 liters per minute
5 liters per minute
1 liter in 4 minutes
0.5 liter per hour

50. CHAPTER QUIZ
8. Before an evaporative emission monitor will run, the fuel level must be where?
At least 75% full
Over 25%
Between 15% and 85%
The level of the fuel in the tank is not needed to run the monitor test

51. CHAPTER QUIZ
8. Before an evaporative emission monitor will run, the fuel level must be where?
At least 75% full
Over 25%
Between 15% and 85%
The level of the fuel in the tank is not needed to run the monitor test

52. 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?
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B

53. 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?
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B

54. 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 be set?
P0440
P0442
P0446
Either P0440 or P0442

55. 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 be set?
P0440
P0442
P0446
Either P0440 or P0442