Evaporative Emissions System: Testing and Inspection

Fig. 1 Evaporative emission control (EEC) system:

Fig. 2 Relief valve:

505

This system, Fig. 1, consists of a non-vented fuel cap, charcoal canister, routing hoses, relief valve, thermo switch and an electro valve. The canister contains activated charcoal and is mounted on the right front side of the engine compartment. A hose connects the charcoal canister to port A of electro valve and another hose connects port B of electro valve to the throttle body.

When the engine is not operating, fuel vapors from the fuel tank are transmitted through a metered orifice in the relief valve, and flow to the charcoal canister where the fuel vapors are absorbed by the activated charcoal. In the event the relief valve metering orifice becomes obstructed, causing a vacuum within the fuel tank, fuel tank venting is accomplished through portion A of the relief valve, Fig. 2.

When the engine is cold the thermo switch, which is subjected to coolant temperature, does not allow the electro valve to operate, preventing fuel vapors from flowing to the throttle body.

When the engine is at normal operating temperature, the thermo switch opens the circuit to the electro valve allowing fuel vapor flow between electro valve ports A and B, Fig. 1. To prevent possible fuel leaks at the accumulator (AC), the tap located at the back of the accumulator is connected to the fuel return line going to the fuel tank.

Fig. 3 Thermo switch installation:

Diagnosis

1. Turn ignition ON.
2. Disconnect electrical connector wire No. 157 from thermo switch, Figs. 1 and 3.
3. Alternately touch the other thermo switch terminal wire No. M157 with the disconnected wire.
4. A ``clicking'' noise should be heard or felt coming from the thermo switch when touching the disconnected wire to the other thermo switch terminal.

System Service

If abnormal fuel odors are detected in the vehicle, check hoses for correct routing and hose clamps for tightness. Replace deteriorated or damaged hose(s). Pay particular attention to the non return valve, the electrovalve, and the canister areas.