What Is an Evaporative Emissions Control System?

20-01 Identify evaporative emissions (EVAP) system components and their uses.

The evaporative emissions (EVAP) control system is used to control the amount of gasoline and engine vapor dispersed into the surrounding environment. To complete this task, the system links the fuel tank to the engine so that those excess gasoline/ethanol fumes can be burned completely within the engine’s intake track. This system consists of a purge valve located near the intake manifold and a vent valve that allows the fuel tank to breathe, so it can be filled and emptied. The charcoal canister helps to clean the hydrocarbons (HCs) that are a part of the environment inside the system before that air/HC mixture is released into the atmosphere and the FTP sensor, which helps with monitoring the integrity of the EVAP system. All of these components work in concert with the other systems in the vehicle to create the best possible outcome for vehicle emissions.

Evaporative Emissions System Usage

The EVAP system has been a requirement ever since the implementation of On-Board Diagnostics second generation (OBD II) 1996. The recovery of fuel vapors is necessary to decrease the exposure of those toxic fumes to the environment and at the same time to recover some of the HCs that have been evaporated from the fuel in the tank. This fuel vapor, which consists of aldehydes, aromatics, olefins, and higher paraffins, is still highly combustible. These compounds react with sunlight to form smog. Smog is a type of air pollution that includes oxides of nitrogen (NOx), sulfur oxides, ozone, smoke, and dirt particles that create a fog over an area (FIGURE 20-1). Smog is toxic to human and animal life, so it must be limited as much as possible. When certain chemicals are exposed to sunlight, the conversion of those chemicals into harmful substances accelerates further, compounding the issue of smog. Limiting the emissions of automobiles can help with decreasing the effect and increasing the air quality anywhere they operate.

FIGURE 20-1 Smog is a toxic fog that is present in areas that create a lot of carbon monoxide (CO) or other toxic fumes from the combustion process. Combining these fumes with sunlight increases the toxicity of smog and creates a more toxic environment.

These HCs need to be captured and reintroduced to the combustion process so that they can be burned instead of released into the atmosphere. The lighter the HC is, the more likely that it will burn completely, so the fumes that are reused in this process will help to support combustion. By increasing the efficiency of the combustion process, the production of the harmful byproducts from the engine is decreased.

EVAP leaks are categorized according to their different standards. A 0.020" diameter leak is considered a small leak. Anything larger than a 0.040" diameter leak is considered a large leak. Describing which is which can allow the technician to determine their diagnostic procedure on the size of the leak. Of course, a larger leak should be easier to find because it should be obvious, but a smaller one may take the technician increased time to identify.

Evaporative Emissions Components

The EVAP components are vehicle-specific, which means that they will not fit other vehicles that are similar. The orifice size in the components is calibrated to the engine size, the emission system that is present on the vehicle, and the control features that operate the system. Mismatching components that came from the same manufacture will not work (FIGURE 20-2). With the development of California emission standards and US federal emission standards, vehicles that meet the US federal guidelines may not meet the California ones. This can be because of the EVAP components, fuel type, or engine system that controls these components meet only the federal standards, which are not as rigorous as the California ones. Understanding what standard a vehicle was built to meet will help the technician diagnose it correctly (FIGURE 20-3). Because of the different standards, components may be different based on the standard the vehicle was built to meet. These different components are developed to change the output of the engine to meet those standards. This could be different sizes of orifices in the system or different types of vent valves. Ensure to verify that the correct components have been acquired to fix the vehicle.

FIGURE 20-2 The same components that were designed to fit the vehicle must be used so that the technician can maintain the operational condition of the EVAP system.

FIGURE 20-3 Determining what type of EVAP system is present on the vehicle should be the first step in diagnosing the vehicle. Realizing what type of system is being diagnosed can help with determining what type of fix is needed.

Purge Valve

The purge valve is located on or near the intake manifold and is how the fuel vapors are introduced into the intake tract (FIGURE 20-4). Along with introducing the fuel vapors to the engine, it also allows vacuum to enter into the EVAP system so that it can check for any leaks within the system. The purge valve is controlled by the powertrain control module (PCM), which it uses to run the EVAP monitor to verify the integrity of the EVAP system (FIGURE 20-5). The purge valve is a simple solenoid that is very similar to the fuel injector (FIGURE 20-6). The size of the purge valve and the size of the orifice that the HCs pass through is metered to the engine so that operation will not cause the engine to stall. When the PCM operates this valve in the intake, it acts like a controlled vacuum leak so that the engine will suck in the fumes from the charcoal canister. The fuel tank is put into a negative pressure (vacuum) environment when the valve is opened. This negative pressure causes the fumes to be sucked into the intake tract. Once they are in the intake tract, they are then mixed with the air-fuel mixture in the cylinders so that they can be burned and expelled through the exhaust system. Not allowing these fumes into the environment in their natural form reduces the impact that HCs have on the environment.

FIGURE 20-4 A purge valve connects the EVAP system to the engine, which allows HC fumes to be introduced into the intake so that they can be used in the combustion process.

FIGURE 20-5 The PCM uses the purge valve as a source of vacuum from the engine so that it can put the EVAP system in a vacuum. The PCM then uses the FTP sensor to monitor decay rate to determine whether the system is operating correctly.

FIGURE 20-6 Like all the other electrical components on the engine, the purge solenoid is a simple coil that creates an electromagnetic to move a pintle to allow flow. Checking one of these is similar to checking other electrical components on the vehicle in that any one of the components can fail.

TECHNICIAN TIP

Vent valves are usually positioned underneath the rear of the vehicle, which is why they have a common failure tendency. All of the dirt, salt, road debris, and moisture linger within the valve, causing it to not seal or physically break because of the lack of movement. Examine the condition of the vent valve before conducting any diagnostics.

Evaporative Emissions Vent Valve

The vent valve is present on the EVAP system so that atmospheric pressure can be present within the fuel tank to help with filling and using fuel from the tank (FIGURE 20-7). To stop the fumes from coming through the EVAP vent valve, the valve is positioned after the charcoal canister in the EVAP system. Positioning of this valve will allow atmospheric pressure through, which acts as a cleaning agent so that the HCs are trapped in the system and allows for pressure to be vented to the atmosphere when the temperature changes pressure in the tank (FIGURE 20-8). The location of the vent valve is near the fuel tank, which means that it is exposed to everything that the vehicle travels over—such as snow, water, salt, dirt, and road debris—which is the main reason for vent valve failures. Visual inspections should be conducted when a failure code is stored in the PCM for the vent valve system. The vent valve defaults to open so that in the event of a vent valve failure, the vehicle operator can still fill the fuel tank since it allows for the pressure built up in the tank to be vented. When the PCM determines the conditions are correct for an EVAP monitor test, it closes the vent valve and then opens the purge valve to bring the EVAP system into a negative pressure environment. If the valve cannot seal the system or if it will not operate electronically, the PCM will set an emissions-related diagnostic trouble code (DTC). Once the technician has retrieved that DTC, they can conduct their diagnostic routine to find and fix the issue.

FIGURE 20-7 The vent valve is a typical electrical solenoid that is controlled by the PCM to close the system so that a monitor event can take place.

FIGURE 20-8 The location of the vent valve is such that the fumes from the fuel tank cannot escape from the system without going through the charcoal canister first, scrubbing them of their toxic nature.

Fuel Tank Pressure Sensor

The FTP sensor is a vacuum transducer that measures the level of negative pressure created inside the EVAP system when the PCM is ready to conduct a monitor test (FIGURE 20-9). This type of sensor is very simple. Like other sensors on the vehicle, the FTP sensor has three wires: one voltage reference (5 Vref), one signal ground (-), and one for the return signal to the PCM (FIGURE 20-10). The sensitivity of an FTP sensor must be maintained because it is used to measure the decay rate at which the EVAP system will lose its inches of mercury (inHg) or kilopascals (kPa) of vacuum. This feature is what the PCM uses to gauge the integrity of the EVAP system. The FTP sensor is usually located on the top of the fuel pump assembly so that it has a prime location for measuring the vacuum in the fuel tank (FIGURE 20-11). The FTP sensor has the same function as the manifold absolute pressure (MAP) sensor has on the engine’s intake system, measuring vacuum in the system. The PCM uses the information from the FTP sensor differently than the MAP sensor does.

FIGURE 20-9 The FTP sensor takes a pressure reading and turns it into an electrical signal that can be interpreted by the PCM.

FIGURE 20-10 Using a wiring schematic to diagnosis the failures of a FTP sensor will help the technician understand what it is not doing. If the power and ground are present but no signal is created, then it is most likely the FTP sensor. Understanding the system is key to diagnosing the system.

FIGURE 20-11 The FTP sensor is centrally located on the fuel pump assembly so that it has an unobstructed access to the fuel tank.

Charcoal Canister

The charcoal canister (vapor canister) is used on a vehicle to trap the harmful vapors before they are able to leave the vehicle’s fuel system. Just as the name implies, the charcoal canister has activated charcoal in it with an inlet and outlet position (FIGURE 20-12). As the vapors go into the charcoal canister, they attach to the charcoal, which traps the vapors until the purge valve reintroduces them to the intake system. As with any component that is near the fuel tank, underneath the vehicle, the elements that the charcoal canister is exposed to can cause the component to fail. In the winter time, snow, ice and salt can infiltrate the canister or cause it to freeze, thus not allowing the pressure to flow through it to the vent valve (FIGURE 20-13). This can cause a no-fill issue, with the driver not being able to fill up the fuel tank. In the other environmental situations, water, dirt, dust, and debris can damage the canister by cracking it, clogging it, or otherwise destroying the system. With the charcoal canister failing, the EVAP monitor will not successfully run, causing a DTC to be set. When the canister starts to fail, the charcoal could become detached from the component, allowing it to travel through the EVAP system. The most common place to see the detached charcoal is the vent valve, as it is very near to the canister and directly connected to it (FIGURE 20-14). Pieces of the canister can cause the valve to either become clogged, not allowing fuel to be put into the tank, or not seal, which will not allow the EVAP monitor to operate, causing a DTC.

FIGURE 20-12 The charcoal canister is a simple component that uses activated charcoal to trap fuel vapors so that they will not be released into the atmosphere.

FIGURE 20-13 The location of the charcoal canister can lead to its ultimate demise because of all the environmental conditions that it is exposed to underneath the vehicle.

FIGURE 20-14 The charcoal canister can cause the other components to fail when broken charcoal is sent through the system. Verifying why the component failed will help to prevent failures from repeating within the EVAP system.

Fuel Fill Neck and Fuel Cap

One of the most overlooked parts of the EVAP system is the fuel fill neck and the fuel cap (FIGURE 20-15). It is overlooked because it consists of a couple of metal tubes, hoses, and a fuel cap system. These components can cause a leak in the EVAP system, causing it to not be able to pass the EVAP monitor. They also can cause the driver to not be able to fill the fuel tank. Inspecting and diagnosing this area is also challenging because the components are usually tucked tightly into the body of the vehicle. The best way to diagnose a fuel fill neck leak is to close the vent valve and use a smoke machine to fill the system with smoke (FIGURE 20-16). If the fuel neck is leaking, smoke will come out of the body near the neck. Then disassembly can continue to help pin down the exact location of the smoke leak on the fuel neck.

FIGURE 20-15 The fuel fill neck is an integral component to the EVAP system. Over time, because of its location, corrosion and debris can deteriorate the neck, causing it to become porous, which will lead to vacuum leakage when the EVAP monitor runs.

FIGURE 20-16 Using a EVAP smoke machine, find a hole to help decrease the time necessary to diagnose the suspected area.

Fuel caps have been a large issue with EVAP system leaks because that component is constantly taken off and put back on when the tank is filled. Because of this frequent usage (or age), the rubber seal used to seal the cap to the fill neck starts to wear out (FIGURE 20-17). To minimize the failure of a fuel cap’s sealing ability, original equipment manufacturers (OEMs) have been moving toward a capless fuel fill neck (FIGURE 20-18). This type of system uses a spring-loaded valve that allows the valve to seal completely every time, to eliminate the possibility of a fuel cap not being installed or not being installed correctly. This minimizes the large leak DTCs because of the improper installation of the fuel cap; it also limits the amount of fuel vapors that escape into the atmosphere.

FIGURE 20-17 Simple failures of EVAP components can cause the system to not pass the monitor test and will trip a DTC, which will not allow the vehicle to pass an emission test before it is repaired.

FIGURE 20-18 The capless system allows the driver to not worry about whether they screwed on the fuel cap tight enough or whether they installed it at all.

Natural Vacuum Leak Detection

The natural vacuum leak detection (NVLD) pump system checks the system’s integrity when the engine is off. When the monitor is commanded to run, the PCM seals the vent valve (FIGURE 20-19). If there is a leak smaller than the failure counter, the EVAP system will be pulled into a vacuum because of the change of pressure in the fuel tank caused by the cooling fuel. As the fuel cools down, it causes a vacuum in the system—usually 1–3 inHg (10–34 kPa). When this happens a vacuum switch closes, which sends a signal to the PCM to determine whether a leak is present. In a perfectly sealed system, the switch will close, indicating that there isn’t a leak big enough to cause an issue. The vacuum switch will normally close with only about 1 inHg (3.4 kPa) vacuum. If the vacuum in the system exceeds 3–6 inHg (10–20 kPa), the valve will be pulled off of the seat, which also protects the system in the event of a solenoid failure. When the engine is running, the solenoid actuates the vent valve to open it so that the EVAP system will be venting and allowing fuel to flow and will remain open for the remainder of the drive cycle. The diaphragm within the NVLD will open with 0.5 inHg (1.7 kPa)of pressure to permit the refueling of the vehicle.

FIGURE 20-19 The NVLD pump is used to determine whether the EVAP system has any leaks in it when the vehicle is off. Using the changing pressures of the fuel in tank to generate a negative pressure in the EVAP system allows for the decay or leakage to be measured.

Leak Detection Pump

The leak detection pump (LDP) is very commonly used on Chrysler vehicles for their EVAP control system diagnostics. This pump is used to verify the EVAP system’s integrity, and it controls engine vacuum to the EVAP system (FIGURE 20-20). When the PCM initiates the LDP, the vent valve closes and the purge solenoid is energized (normal purge solenoids control the ground side of the solenoid, and in this situation, the PCM controls the power side). As the control valve moves in the LDP manifold, vacuum is allowed into the upper portion of the pump, which then moves the lower portion through a diaphragm spring to allow fresh air into the system. As that spring is pulled open, the contacts in a switch contact together, sending a signal to the PCM that states that the system is operating correctly. Once this happens, the PCM deenergizes the purge solenoid, which allows fresh air into the top of the LDP. This causes the valve to close and seals the system. After this event, the PCM closes the purge valve, then it cycles the LDP solenoid, watching the response of the sensor in the LDP to determine whether there is a leak. The time between when the pump deenergizes and when the switch is tripped can determine the size of the leak in the system.

FIGURE 20-20 The LDP is a common way for Chrysler vehicles to determine the integrity of their EVAP systems. When testing these components, using a scan tool should provide accurate diagnosis.

Evaporative System Integrity Monitor

The evaporative system integrity monitor (ESIM) operates like an NVLD but does not require a solenoid to operate it (FIGURE 20-21). Because the housing consists of a small and large weight that serve as check valves (which requires the assembly to be positioned vertically to get these to operate correctly), the ESIM eliminates the need for a solenoid. There is also a diaphragm and a switch located within the assembly. The larger weight seals for pressure, and the smaller one seals for vacuum. The system operates like an NVLD: When the system cools with the vehicle off, a vacuum becomes present in the system, which causes the diaphragm to move and connect the contacts. This indicates that the system is sealed and in operational condition. When the vacuum overcomes the small weight check valve, it opens and allows fresh air to enter the system and neutralizes the vacuum. During refueling as the fuel goes into the tank, the pressure increases and unseats the larger weight (check valve), which allows the fuel tank to be filled unhindered. If the key off test fails, this forces the PCM to run a more intrusive test, where engine vacuum is applied to the EVAP system by actuating the purge solenoid. Once the system is under a vacuum, the PCM closes the purge valve and then measures the rate of decay within the system to determine the size of the leak. As the size is determined, the PCM illuminates the check engine light (CEL) so that the driver is made aware that they will need to service their vehicle.

FIGURE 20-21 The ESIM valve must be in a vertical condition to operate properly. Also, this type of system has two different ways of determining whether a fault is in the system to quickly make the driver aware, so that they can get it repaired.

SKILL DRILL 20-1Performing a EVAP Service Bay Test

1. If the vehicle comes in with a check engine light on and an EVAP DTC, the technician should perform a service bay test to verify that the component is failing.

   

2. To put the vehicle in service bay mode, the fuel tank level should be between 15% and 85%, the electronic throttle control (ECT) temperature should be between 39°F and 86°F, the intake air temperature should be between 39°F and 86°F, and turn off all accessories (blower, wipers, radio, and the HVAC system).

   

3. Navigate through the scan tool to verify which code illuminated the CEL. Document the code and freeze-frame data, and then clear it. The code needs to be cleared because the test will not run if there are active DTCs in the PCM.

   

4. Once the codes are clear, the technician should navigate to the special functions menu to access the service bay test.

   

5. Working through the scan tool prompts, follow the instructions to force the vehicle to run the EVAP monitor. By running this monitor in the service bay, the technician can verify the code that was previously set.

   

6. Once the test is complete, if there was a failure of a component in the system, the corresponding code will have been set in the PCM. The technician can now diagnose the fault of this code because they know that an outside influence did not cause this code to set.

   

7. Now that the repair has been completed, the technician should repeat this process so that they can verify that they have correctly fixed this vehicle.