Evaporative Emissions System: Description and Operation

Turbo Purge Vacuum Diagram:

The Evaporative Emission Control System is designed to prevent fuel vapors from being emitted into the atmosphere. The components incorporated into this system are: a Two-Way Check Valve, a Pressure-Vacuum Filler Cap, a Charcoal canister, and connecting lines and hoses. The Turbo model also utilizes a Dual Control Purge Valve to allow canister purging during boost and vacuum conditions. Canister purging is controlled by the ECU through the Purge Control Solenoid Valve, based on inputs from the coolant sensor and the Air Flow Sensor.
The Pressure-Vacuum Filler Cap is designed to hold a slight pressure on the fuel which occurs from normal heat expansion. The evaporation rate is decreased under higher pressures, so the cap aids to reduce evaporative emissions. When vacuum in the tank exceeds specifications, the vacuum relief valve will allow atmospheric pressure to enter the tank. The 2-way check valve vents excess pressure and fuel vapors from the tank into the charcoal canister, when the pressure becomes positive and reaches its specified value. The Two-Way Check Valve is mounted on or near the fuel tank in the fuel vapor line to the canister. The fuel vapors are absorbed and suspended by the charcoal in the canister until the Purge Control Valve is opened.
The Turbo Purge Solenoid is spring loaded open and closes when grounded by the ECU. When the Purge Solenoid is open (ungrounded), manifold vacuum is bled to a port at the air-intake and the canister is not purged. When the ECU grounds the Purge Solenoid, manifold vacuum is redirected through a restricting vacuum tee to the vacuum chamber of the Dual Control Purge Valve. Vacuum pulls the valve open and the canister is purged by the depression of the air cleaner. Air cleaner depression is a form of vacuum, but much less than manifold vacuum. It is used on the Turbo Purge systems to allow the canister to be purged under a vacuum or a boost condition. The valve can be "pulled" open by a vacuum greater than 2 inches or "pushed" open by a boost pressure greater than 2 psi.

NOTE: As long as there is a pressure differential of more than 2 PSI, between the two chambers of the Purge Valve, the canister will be purged.

NORMAL DRIVING PURGE VALVE VACUUM/BOOST READINGS
(With the Purge Solenoid Grounded, ports blocked)

Light Load Vacuum Hose Boost Hose
(W/red stripe) (black)

15 Inches Vacuum 0 PSI Boost
10 Inches Vacuum 2 PSI Boost
7 Inches Vacuum 3 PSI Boost
5 Inches Vacuum 4 PSI Boost
3 Inches Vacuum 4 PSI Boost
0 Inches Vacuum 5 PSI Boost
3 PSI Boost 7 PSI Boost
6 PSI Boost 10 PSI Boost

Heavy Load 7 PSI Boost 11 PSI Boost
(Not at WOT.)

At WOT. 10 PSI Boost 11 PSI Boost
(or Solenoid ungrounded, pressures will be close to equal)

When the Turbocharger is in operation, the bleed port at the air intake becomes the boost feed nipple. If the Purge Solenoid is grounded (closed), boost pressure is directed to the pressure chamber of the Dual Control Purge Valve.
When the Purge Solenoid is open (ungrounded), the ports of the Purge Solenoid are connected, causing equal boost pressures (within 1 psi) to be applied to both nipples of the Dual Control Purge Valve. This causes the spring to push the diaphragm and the valve closed, shutting off the canister purge. The same thing happens at Wide Open Throttle or heavy loads.
The ECU will ground the purge control solenoid when the coolant sensor indicates higher than 149°F and the Air Flow Sensor indicates an intake air flow rate of 2000 rpm (100 Hz). Once these parameters have been met, the ECU will internally ground the purge control solenoid for 3 minutes of ON time and then Unground the solenoid for 20 seconds of OFF time. This will continue as long as the above parameters are met. During engine warm-up, the purge ON time is progressive. For example, after the coolant sensor indicates above 149°F, the first purge cycle may only be for one minute of purge, then 20 seconds of no purge. When the purge control valve opens, the fuel vapors in the canister are drawn into the intake manifold through purge lines. At the same time, fresh air is drawn through the filter at the bottom of the canister, purging the charcoal of fuel vapors.