See Figures 1 through 8
All models sold nationwide are equipped with a fuel evaporative control system to prevent the evaporation of unburned gasoline. The 75 system consists of a sealed fuel tank filler cap, an expansion area at the top of the gas tank, a combination vapor separator and expansion tank assembly, a 3-way vapor control valve, a carbon canister located in the engine compartment which stores these vapors, and the hoses which connect this equipment.
The 76 and later system consists of a special vacuum/pressure relief filler cap, and expansion area at the top of the fuel tank, a foam-filled vapor separator mounted on top of the fuel tank, a carbon canister which stores fuel vapors and hoses which connect this equipment.
On carbureted systems, the carburetor fuel bowl vapors are retained within the fuel bowl until the engine is started, at which point they are internally vented into the engine for burning. The system functions as follows:
Changes in atmospheric temperature cause the gasoline in fuel tanks to expand or contract. If this expansion and consequent vaporization takes place in a conventional fuel tank, the fuel vapors escape through the filler cap or vent hose and pollute the atmosphere. The fuel evaporative emission control system prevents this by routing the gasoline vapors to the engine where they are burned.
As the gasoline in the fuel tank of a parked car begins to expand due to heat, the vapor that forms moves to the top of the fuel tank. The fuel tanks on all cars are enlarged so that there is an area representing 4-20% of the total fuel tank volume above the level of the fuel tank filler tube where these gases may collect. The vapors then travel upward into the vapor separator which prevents liquid gasoline from escaping from the fuel tank. The fuel vapor then travels through the vapor separator outlet hose, through the 3-way vapor control valve (75 only), and to the charcoal canister in the engine compartment. The vapor enters the canister, where it is absorbed and stored by the charcoal filter until engine operating conditions permit its consumption. Once the proper operating conditions are reached, fresh air is drawn through the canister filter and then the fresh air removes the vapors from the charcoal and carries them into the engine to be burned during the normal air/fuel combustion process.
When the engine is started, vacuum created from the engine air intake draws fresh air into the canister. As the fresh air passes through the charcoal in the canister, it picks up the hydrocarbons that were deposited there by the fuel vapors. This mixture of hydrocarbons and fresh air is then carried through a hose in the air cleaner. It combines with the incoming air/fuel mixture and enters the combustion chambers of the engine where it is burned.
On all systems, there still remains the problem of allowing air into the tank to replace the gasoline displaced during normal use and the problem of relieving excess pressure from the fuel tank should it reach a dangerous level. On 75 systems, the 3-way control valve accomplishes this. On 75 and later systems, the special filler cap performs this task. Under normal circumstances, the filler cap functions as a check valve, allowing air to enter the tank replacing the consumed fuel. At the same time it prevents vapors from escaping from the cap. In case of severe pressure within the tank, the filler cap valve opens, venting the pollutants to the atmosphere.
DIAGNOSIS & TESTING
Canister Purge Regulator Valve
- Disconnect the hoses at the purge regulator valve. Disconnect the electrical lead.
- Connect a vacuum pump to the vacuum source port.
- Apply 5 in. Hg (5 kPa) to the port. The valve should hold the vacuum. If not, replace it.
Canister Purge Valve
See Figure 9
- Apply vacuum to port A . The valve should hold vacuum. If not, replace it.
- Apply vacuum to port B. Valves E5VE-AA, E4VE-AA and E5E-AA should show a slight vacuum leak-down. All other valves should hold vacuum. If the valve doesn't operate properly, replace it.
- Apply 16 in. Hg (53 kPa) to port A and apply vacuum to port B . Air should pass. On valves E5VE-AA, E4VE-AA and E5E-AA, the flow should be greater than that noted in Step 2.
Never apply vacuum to port C. Doing so will damage the valve.
- If the valve fails to perform properly in any of these tests, replace it.
REMOVAL & INSTALLATION
The only service performed on the evaporative control system is the replacement of the charcoal (carbon) canister at the intervals listed in the maintenance schedule in Routine Maintenance . The procedure is as follows.
- Working under the vehicle, disconnect two hoses from the control valve. Remove the vent valve cover.
- Remove the attaching bolts and remove the valve from the crossmember at the rear of the gas tank.
- Position the valve to the crossmember and install two attaching bolts.
- Connect the two hoses to the valve assembly. Install the cover.
Loosen and remove the canister mounting bolts from the mounting bracket. Disconnect the purge hose from the air cleaner and the feed hose from the fuel tank. Discard the old canister and install a new unit. Make sure that the hoses are connected properly.