Evaporative Emissions System: Description and Operation

Fuel Delivery / Canister Purge:

To control fuel tank hydrocarbon emissions, the tank is vented through a system of vapor pipes, valves and a vapor separator to a charcoal (absorption) canister in the left front wheel well. Fuel vapors stored in the canister are purged to the intake manifold under specified conditions when the engine is running.

Fuel tank fill tube limits fill level and allows for 10% fuel expansion. Tank venting is via tank evaporative flange to carbon canister. The large port vents the tank back to the filler pipe during refueling. The intermediate port is a pressure relief valve that vents beneath the vehicle via a tube. The third outlet incorporates a roll-over valve and connects to the charcoal canister through the in-line vacuum/pressure relief (Rochester) valve. Vapor flow to canister is controlled by Rochester valve. Valve holds 1 - 1.25 psi when engine is "OFF". Tank pressure is released to canister when engine is started.

System uses two safety pressure relief valves that vent to atmosphere. Evaporative flange has a 2 psi relief valve and fuel filler cap a 4 psi relief valve.

Carbon Canister Purge Control:

Canister is purged in to stages. Two purge valves are used and are vacuum controlled from left throttle body ports via thermal valve. The ports are positioned to allow no purge at idle. Progressive purge is obtained as throttle is opened. Thermal vacuum valve controls both vacuum circuits. Vapor flows to intake manifolds through crankcase breather pipe.

When engine coolant temperature is 95° F or above, thermal vacuum valve opens allowing canister purge. As throttle plate moves off idle, ports are exposed to vacuum in progression. A delay valve in second stage further delays canister purge as engine speed increases, preventing on over-rich condition.