Volvo 240/740/760/780/940/960 1990-1998

Evaporative Emission Controls

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OPERATION



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 vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold.

The vapors are drawn into the engine at idle as well as at operating speeds. This system is called a Bi-level Purge System where there is a dual source of vacuum to remove fuel vapor from the canister. The source of vacuum at idle is a tee in the PCV system.

A sealed, maintenance free evaporative canister is used. The canister is mounted under the vehicle on either side behind the wheel well. The canister is filled with granules of an activated carbon mixture. Fuel vapors entering the canister are absorbed by the charcoal granules.

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 predetermined time and engine operating conditions.

Vacuum for the canister is controlled by the canister purge valve. The valve is operated by the ECM. The ECM 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 flow through to the canister.

During warm up and for a specified time after hot starts, the ECM 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 ECM 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 mix calibrations.

The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valves in the cap are 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.

During warm up and for a specified time after hot starts, the ECM 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 ECM 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 mix calibrations.

Some vehicles have added system components due to the EVAP system monitor incorporated in the OBD-II engine control system used on these years. Two, instead of one, EVAP canisters are used and they are mounted on the drivers side of the vehicle. The canister purge valve is located on the bracket with the canisters. A test port for pressurizing the EVAP system is included and located below the brake booster. The test port is used to pressurize the system with a special gas and serious precautions must be taken to avoid damage to the EVAP system and the fuel tank. This is a procedure best suited to a professional shop, due to the precautions and the equipment needed to test this system. The ECM can store trouble codes for EVAP system performance, a list of the codes is provided later in this section. Normal testing procedure can be used for any component listed in EVAP testing in this guide.

COMPONENT TESTING



The majority of the testing of the EVAP system is a visual inspection for damaged, leaking or missing components. Inspect the hoses to the throttle body, the canister, fuel tank, fuel filler pipe, and the filler cap. Any damaged components should be replaced.

Canister Purge Valve
  1. Remove the inlet hose to the valve.
  2.  
  3. Connect a hand-held vacuum pump to the fitting and pump up vacuum.
  4.  
  5. If vacuum holds, valve is ok. If vacuum falls off, valve diaphragm is leaking, replace the valve.
  6.  

REMOVAL & INSTALLATION



To relieve fuel tank pressure, the filler cap must be removed before disconnecting any fuel system component.

Evaporative (Carbon) Canister

See Figures 1, 2, 3 and 4

  1. Disconnect the negative battery cable.
  2.  
  3. Raise and support the vehicle.
  4.  
  5. Remove the retaining bolts from the canister mounting bracket.
  6.  
  7. Remove the canister.
  8.  
  9. Label and disconnect the hoses on the top of the canister.
  10.  
  11. Remove the canister from the mounting bracket.
  12.  

To install:
  1. Install the canister in the mounting bracket.
  2.  
  3. Install and tighten the canister brackets retaining bolts.
  4.  
  5. Install the hoses in their proper locations.
  6.  
  7. Lower the vehicle.
  8.  
  9. Connect the negative battery cable.
  10.  



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Fig. Fig. 1: Remove the bolts from the canister bracket and ...



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Fig. Fig. 2: ... lower the canister to access the hoses for removal



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Fig. Fig. 3: Exploded view of the EVAP canister and hoses



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Fig. Fig. 4: EVAP system hose connections on the intake manifold

Canister Purge Valve

The valve is located on the top of the canister. When the canister is removed, the valve can be removed from the canister by carefully pulling the valve off of the canister.

 
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