With Tank Pressure Control Solenoid Vacuum (Tpc SV) Valve

Evaporative Emission (EVAP) Control System - Vehicles With (TPC SV) Valve:

SYSTEM OVERVIEW
The basic Evaporative Emission (EVAP) control system used on all vehicles is the EVAP canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) device (EVAP canister) to hold the vapors when the vehicle is not operating. When the engine is running the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

DESCRIPTION
An EVAP control system is used to prevent emission of fuel vapor.

When the pressure of the vapor generated in the fuel tank increases higher than specified pressure, it pushes open the Tank Pressure Control (TPC) valve and enters the EVAP canister.

The TPC valve is provided to keep pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow into the EVAP canister. When the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let the air flow into the fuel tank.

NOTE: In addtion to the Tank Pressure Control (TPC) valve described above, certain Geo Metro models are factory equipped with a Tank Pressure Control Solenoid Vacuum (TPC SV) valve. Vehicles equipped with a Tank Pressure Control Solenoid Vacuum valve have a three vapor hose fitting Tank Pressure Control valve and vehicles equipped without a Tank Pressure Control Solenoid Vacuum valve have a two vapor hose fitting Tank Pressure Control valve.

However, under the conditions that the engine is running, the fuel is lower than the specified level (about 8 gallons or 3O liters), the Tank Pressure Control Solenoid Vacuum (TPC SV) valve is turned "ON" by the Engine Control Module (ECM) and vacuum is applied to the diaphragm of the TPC valve. The possibility of liquid fuel flowing into the vapor line is little, thus the vapor passage of the TPC valve opens to allow continuity between the fuel tank and EVAP canister. As a result, under the above conditions, the vapors in the fuel tank flow into EVAP canister without being stopped by the TPC valve.

The EVAP canister purge valve is control by the ECM according to signals from various sensors. Only when the following conditions are satisfied will the ECM open the vacuum passage of EVAP canister purge valve.

OPERATION
1. When the engine coolant reaches operating temperatures or higher and the fuel control system is in "Closed Loop" operation, the ECM will control the "ON"/"OFF" switching of the EVAP canister purge valve at a constant rate of 10 times per second and the length of its "ON" time depending on the following factors so as to prevent the air/fuel rate from heing changed suddenly:

^ Engine speed.
^ Manifold absolute pressure.
^ Concentration of fuel vapor in the purge line (fuel vapor concentration is calculated based on how much the air/fuel rate changes when the flow rate in the purge line is changed).

Also, when the vapor concentration is lower than the specified valve, the ECM stops operation of the EVAP canister purge valve for a specified time.

2. When the engine is running under higher load conditions and/or at a higher speed then specified values, the ECM keeps the EVAP canister purge valve open as long as such driving conditions continue.

When the EVAP canister purge valve opens, fuel vapor in the EVAP canister is sucked into the intake manifold. At this time, the EVAP canister purge valve is purged or cleaned by the air drawn through the Air Cleaner (ACL) housing.