Mitsubishi Car 1999-05

Evaporative Emission Control (EEC) Systems

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Gasoline fuel is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated, from the refinery standpoint. However, the automobile still remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.

Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in warm area, such as a parking lot. As the fuel would warm, it would expand and should no provisions or area be provided for the expansion, the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition, the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.

However, this did not control the fuel vapor emission from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was stationary and the engine not operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor emissions would take place, increasing as the temperature increases.

To prevent the vapor emission from escaping into the atmosphere, the fuel systems were designed to trap the vapors while the vehicle is stationary, by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is not operating. When the engine is started, the storage system is then purged of the fuel vapors, which are drawn into the engine and burned with the air/fuel mixture.

Operation





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Fig. Typical evaporative emission control system schematic



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Fig. Evaporative emission canister commonly used on most models

Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air which escapes contains gasoline vapors.

The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through the EVAP emission valve, through vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold and burned during combustion.

A sealed, maintenance free evaporative canister is used. The canister is filled with granules of an activated carbon mixture. Fuel vapors entering the canister are absorbed by the charcoal granules. A vent cap is located on the top of the canister to provide fresh air to the canister when it is being purged. The vent cap opens to provide fresh air into the canister, which circulates through the charcoal, releasing trapped vapors and carrying them to the engine to be burned.

Fuel tank pressure vents fuel vapors into the canister. They are held in the canister until they can be drawn into the intake manifold. The canister purge valve allows the canister to be purged at a pre-determined time and engine operating conditions.

Vacuum to the canister is controlled by the canister purge valve. The valve is operated by the PCM. The PCM regulates the valve by switching the ground circuit on and off based on engine operating conditions. When energized, the valve prevents vacuum from reaching the canister. When not energized the valve allows vacuum to purge the vapors from the canister.

During warm up and for a specified time after hot starts, the PCM energizes (grounds) the valve preventing vacuum from reaching the canister. When the engine temperature reaches the operating level of about 120°F (49°C), the PCM removes the ground from the valve allowing vacuum to flow through the canister and purges vapors through the throttle body. During certain idle conditions, the purge valve may be grounded to control fuel mixture calibrations.

The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valve in the cap is a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.

Obd-II EVAP System Monitor

Some models have added system components due to the EVAP system monitor incorporated in the OBD-II engine control system. A pressure sensor is mounted on the fuel tank which measures pressure inside the tank, and a purge flow sensor measures the flow of the gases from the canister into the engine.

The PCM can store trouble codes for EVAP system performance, a list of the codes is provided later in this section. Normal testing procedures can be used, see EVAP System Component Testing in this Section.

Removal & Installation



Evaporative Emission Canister


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Fig. Remove the canister retaining bolts and remove the canister

  1. Disconnect the negative battery cable.
  2.  
  3. If necessary, raise and safely support the vehicle, remove the front passenger side wheel, then remove the splash shield.
  4.  
  5. Tag and disconnect all necessary vacuum lines.
  6.  
  7. Unfasten and retaining bolts and/or straps, then remove the canister from the vehicle
  8.  
  9. Installation is the reverse of the removal procedure.
  10.  

Solenoid Valves


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Fig. Evaporative emission purge solenoid location-2.0L DOHC engine



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Fig. Location of the evaporative emission purge solenoid-1990-93 2.0L DOHC turbo engine

  1. Disconnect the negative battery cable.
  2.  
  3. Label and remove the vacuum and electrical harness connections from the purge control solenoid.
  4.  
  5. Remove the solenoid and mounting bracket from the engine compartment.
  6.  
  7. Installation is the reverse of the removal procedure.
  8.  

Testing



Evaporative Emissions Purge Control Solenoid Valve
  1. Tag and disconnect the vacuum hoses from the solenoid valve.
  2.  
  3. Detach the harness connector.
  4.  
  5. Attach a hand-held vacuum pump to the nipple (A) of the solenoid valve, as shown in the accompanying figures.
  6.  



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Fig. Test connections for the evaporative purge solenoid-2.4L engine

  1. Check air tightness by applying a vacuum with voltage applied directly from the battery to the evaporative emission purge solenoid and without applying voltage. The desired results are as follows:
  2.  


    With battery voltage applied-vacuum should be maintained
     
    With battery voltage not applied-vacuum should leak
     

  3. Measure the resistance across the terminals of the solenoid. The standard values are as follows:
  4.  



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Fig. Measure the resistance across the terminal of the solenoid valve-2.4L engine shown, others similar

  1. 25-44 ohms when at 68°F (20°C).
  2.  

  1. If any of the test results differ from the specifications, replace the emission purge control solenoid.
  2.  

Purge Control System

NOTE
This test requires the use of a special purge flow indicator tool, MB991700, or equivalent.

  1. Disconnect the purge hose from the EVAP canister, then connect Purge Flow Indicator MB991700, or equivalent between the canister and the purge hose.
  2.  
  3. The engine should be warmed up to operating temperature, 170-203°F. (80-95°C), with all lights, fans and accessories off. The transaxle should be in Park for automatics or Neutral for manuals.
  4.  



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Fig. Purge control system check-2.4L engine

  1. Run the engine at idle for at least 3-4 minutes.
  2.  
  3. Check the purge flow volume when the brake is depressed suddenly a few times. The reading should be 2.5 SCFH (20cm/sec.)
  4.  
  5. If the volume is less than the standard value, check it again with the vacuum hose disconnected from the canister. If the purge flow volume is less than the standard, check for blockages in the vacuum port and vacuum hose, and also inspect the evaporative emission purge solenoid and purge control valve.
  6.  
  7. If the purge flow volume is at the standard value, replace the EVAP canister.
  8.  

Servicing


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Fig. The EVAP canister is typically located inside of the fender on most models covered by this manual

The evaporative canister requires no periodic servicing. However, a careful inspection of the canister and hoses should be made frequently. Replace damaged components as required.

The canister is typically located under one of the front fenders, however on some later models it may be under the rear of the vehicle, near the gas tank.

 
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