EVAP

Evaporative Emissions Control

When we think of controlling vehicle emissions, our first thoughts are usually about cleaning up tailpipe emissions. However, with today’s gasoline powered vehicles, if left unchecked, the gasoline fumes evaporating from the fuel tank and engine are also a major source of hydrocarbon pollution. That means if the fuel system is open to the atmosphere, it can pollute 24 hours a day even if the engine is not running. The Evaporative Emission Control System (EVAP) is used to prevent gasoline vapors from escaping into the atmosphere from the fuel tank and fuel system at all times.

EVAP systems have been required on gasoline powered vehicles since the 1970s. A typical system consists of the fuel tank, an EVAP vapor storage canister full of charcoal, valves, hoses and a sealed fuel tank gas cap. The EVAP system is designed to stop fuel system fumes from leaking directly into the atmosphere. Vent lines from the fuel tank pass vapors to the vapor canister, where they are trapped and stored until the engine is started. When the engine is warm and the vehicle is going down the road, the PCM then opens a purge valve allowing the vapors to be drawn from the storage canister into the intake manifold. The fuel vapors are then burned in the engine along with the air/fuel mixture.

EVAP systems are active systems that require very little maintenance. On pre-OBD II vehicles the PCM software had no method to verify that the EVAP system was working as designed. The only time this system was paid attention to was when there was a failure of a component that allowed for fuel from the tank to be drawn into the engine creating an extremely rich condition or when a component failure created a vacuum leak creating a lean mixture condition. Both of these issues were typically diagnosed as performance issues, not emissions. On OBD II equipped vehicles the PCM monitors the fuel system for fuel vapor leaks to make sure no hydrocarbons are escaping into the atmosphere. The EVAP monitor does two things: it verifies there is airflow from the EVAP canister to the engine, and that there are no leaks in the fuel tank, EVAP canister or fuel system vapor lines.

The PCM runs an EVAP diagnostic monitor on OBD II equipped vehicles under certain driving conditions to detect fuel vapor leaks, and if it finds any it will set a DTC fault code and illuminate the Check Engine light. However, the EVAP monitor only runs under very precise operating conditions including engine running characteristics, ambient temperature requirements and fuel tank volume requirements. The diagnostic routine run by the PCM is capable of detecting extremely small leaks which may be difficult to isolate and repair. Let’s take a deeper look at some of the evaporative emissions components and their functions.

Fuel Tank

The fuel tank stores fuel until it is needed by the injectors for combustion in the engine. The tank also is constructed to be air-tight including tank seals at access points for fuel delivery and EVAP components. The tank is also designed to store some volume of fuel vapor prior to its delivery to the EVAP storage canister.

Ruptures or failures to the lower portions of the fuel tank result in obvious fuel leaks. Failures located on top of the tank may not result in visible fuel leakage but will allow evaporative emissions (gasoline fumes) to escape into the atmosphere. On OBD II equipped vehicles these failures will be identified during EVAP monitoring.

Fuel Pump

Fuel injection systems operate at high fuel pressures, typically in the 40-60 psi range, or higher for direct injection. To achieve proper pressure and volume flow fuel pumps are generally electric motors located in the fuel tank, utilizing the fuel in the tank to cool the pump and to ensure a steady supply of fuel.

On OBD II equipped vehicles the PCM controls the power to the fuel pump. The PCM on most systems operates the pump through the fuel pump relay during normal engine operation and can shut the pump off if the vehicle is in a collision or if low oil pressure is indicated. With some OBD II vehicles the PCM controls fuel pressure through pulse-width modulation of the voltage to the pump; this allows a smaller and lighter electric motor to be used reducing electrical load.

In many fuel systems the pump is an integral part of the fuel tank sending unit assembly. The fuel sending unit assembly may be a combination of the electric fuel pump, the filter, the strainer, and electronic sensors used to measure the amount of fuel in the tank and tank pressure. Data from these sensors is used by the PCM and the dash fuel level gauge.

Low fuel system pressure due to the fuel pump can be caused by a defective pump or poor electrical connects to the pump or fuel pump relay. Partially clogged fuel filters or strainers or defective fuel pressure regulators can also cause low fuel pressure. High fuel pressure can be caused by restricted fuel return lines back to the tank or a defective fuel pressure regulator.

Fuel Tank Filler Neck

The fuel tank filler neck is usually a vented metal or rigid plastic pipe attached to the fuel tank through an air-tight flexible junction on one end; with the inlet end configured with fuel restriction hardware and access for re-fueling vapor venting. The top of the filler neck may be flanged and threaded/keyed to accept and seal a gas cap. Newer filler necks may be capless in design with a spring loaded self sealing flapper in place of a traditional gas cap.

Gas cap

Gas caps are among the most critical components of the fuel system and if not correctly designed, calibrated and installed can cause the OBD II system to display a warning message to the motorist. Gas caps can be vented or non-vented and need to be replaced with the appropriate style cap for the EVAP system to operate without causing performance or Check Engine light issues.

Most caps on OBD II equipped vehicles allow fresh air into the fuel tank equalizing internal and atmospheric pressure, compensating for the fuel volume lost during normal emptying of the tank through driving. The gas caps also allow virtually no gasoline vapors or liquid fuel to push back into the atmosphere from built up vapor pressure from gasoline evaporation in the fuel tank or during a vehicle rollover. This is accomplished through air-tight sealing of the cap to the filler neck and versatile internal sealing diaphragms and sensitive springs.

On OBD II vehicles, gas cap failures are pretty common. Gas cap failures are discovered when the PCM runs the EVAP monitor during normal engine operation. The EVAP monitor fails to run correctly and the PCM stores a DTC fault code and illuminates the Check Engine light. For failed or loose gas caps the PCM usually sets a P0440 DTC code indicating a large leak is present.

Fuel Tank Pressure Sensor

The fuel tank pressure sensor is part of the fuel pump sending unit assembly and is mounted on top of the fuel tank or inside the tank. The fuel tank pressure sensor measures the positive and negative pressure in the fuel tank. The sensor reads pressure in the fuel tank primarily during EVAP system monitoring. The PCM uses the pressure reading to detect evaporative leaks. When readings from the sensor indicate a leak, or if the sensor itself fails, the PCM sets a DTC code and illuminates the Check Engine light. Pinpoint testing or replacing a faulty sensor usually requires removing the fuel tank.

Fuel Lines

Fuel lines connect all components of the fuel system. The rigid lines are usually made of Steel tubing that has been zinc plated, on some systems rigid plastic tubing is being used. Fuel lines are secured to the frame and engine, minimizing vibration and keeping them away from exhaust manifolds, exhaust pipes and mufflers.

In attachment points where there is a lot of movement, for instance, between the firewall and engine short lengths of flexible fuel lines are used. These flexible lines are made from high pressure gasoline resistant rubber, braided steel or high pressure plastic fuel line. It is extremely important to replace fuel lines with suitable replacement components/materials and connection hardware. Leaking or damaged fuel lines can cause problems achieving proper fuel system pressure and safe system operation.

EVAP Canister

The EVAP canister is usually rectangular plastic container mounted somewhere in the vehicle. The canister is filled with activated charcoal, which absorbs and stores gasoline vapors. The vapors are stored in the canister until the engine is being driven. The PCM then opens the canister purge valve, which allows intake vacuum to draw the vapors into the engine. The EVAP canister is connected to the fuel tank through the tank vent line. EVAP canister problems include faulty purge control or vent solenoids or canister housing leaks.

Canister Purge valve / Solenoid

The canister purge valve or purge solenoid is an electrically operated valve that allows engine vacuum to draw gasoline vapors from the EVAP canister.

The PCM energizes the purge solenoid during normal driving conditions and also operates the valve during EVAP system monitoring. The PCM can detect any electrical issues with the solenoid whenever the engine is running through a process known as continuous component monitoring.

The PCM can assess the valve's ability to hold and release vacuum during EVAP system monitoring. In either case if the PCM detects an issue, the PCM sets a DTC code and illuminates the Check Engine light.

Canister purge valves sometimes become clogged or held partially open from debris or charcoal particles drawn through the EVAP canister.

Leak Detection Pump (LDP)

Some vehicle manufacturers use a leak detection pump as a pressure source for conducting positive pressure testing of the EVAP system during EVAP system monitoring. The LDP is a diaphragm pump with solenoids and check valves that pumps air into the fuel tank and charcoal canister. The PCM controls the operation of the LDP during EVAP system monitoring. Once the EVAP system is pressurized the PCM can measure the system's pressure decay.

The PCM can detect any electrical issues with the leak detection pump whenever the engine is running through a process known as continuous component monitoring. The PCM can assess the LDPs ability to create and hold pressure during EVAP system monitoring. In either case if the PCM detects an issue, the PCM sets a DTC code and illuminates the Check Engine light.

EVAP Vent Lines and Hoses

EVAP vent lines are similar to fuel lines only smaller in diameter and operate under very low pressure or vacuum situations. The EVAP lines connect all components of the EVAP system and the engine's intake manifold together. The rigid lines can be steel or rigid plastic tubing.

In attachment points where there is a lot of movement, for instance, between the firewall and engine, short lengths of flexible vent line are used. Leaking or damaged EVAP lines can cause problems during EVAP system monitoring. If the PCM detects a leak during testing, the PCM sets a DTC code and illuminates the Check Engine light. Unfortunately the PCM only indicates that there is a leak present, not where the leak actually is. Small EVAP leaks such as a porous or cracked vent line can be difficult to isolate and repair.

Powertrain Control Module

The PCM on OBD II vehicles is responsible for testing the integrity of the EVAP system and properly metering stored gas vapors back into the engine during certain vehicle operating conditions. The PCM accomplishes these tasks by monitoring engine operating conditions, vehicle operating conditions, fuel level, time between key cycles and ambient temperature.

To test for EVAP system integrity, the PCM runs an EVAP diagnostic monitor under certain driving conditions to detect fuel vapor leaks, and if it finds any it will set a DTC fault code and illuminate the Check Engine light. However, the EVAP monitor only runs under very precise operating conditions including engine running characteristics, ambient temperature requirements and fuel tank volume requirements.

To properly meter gas vapor stored in the EVAP canister back into the engine, the PCM verifies certain engine and vehicle operating conditions are being met. When the engine is warm and the vehicle is going down the road, the PCM then opens the canister purge valve allowing the vapors to be drawn from the storage canister into the intake manifold. The fuel vapors are then burned in the engine along with the air/fuel mixture. The PCM can monitor the amount of vapor being drawn into the engine by the oxygen sensor signal and make mixture corrections as necessary to limit excessive tail pipe emissions.