Exhaust Gas Recirculation: Description and Operation

EXHAUST GAS RECIRRCULATION (EGR) SYSTEM

EGR Valve Operation.:

PURPOSE

The EGR system is used to lower oxides of nitrogen (NOx) emission levels caused by high combustion temperature. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. This decreases combustion temperature.

The main element of the system is the negative back pressure EGR valve operated by vacuum and mounted on the cylinder head of the DOHC (LLO) engine.

OPERATION

The EGR valve is opened by vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion will not occur. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. The EGR valve is open under the following conditions:

1. Coolant temperature above:

^ DOHC (LLO) = 40°C (104°F) - Automatic

^ DOHC (LLO) = 50°C (122°F) - Manual

2. Idle speed above:

^ DOHC (LLO) = 4% TPS

EXHAUST GAS RECIRCULATION (EGR) CONTROL

The PCM controls the EGR system by energizing and de-energizing the EGR solenoid by supplying a ground for the solenoid. When the solenoid is energized, ported vacuum is prevented from being applied to the EGR valve causing it to close. When the solenoid is de-energized, ported vacuum flows to the EGR valve, opening the valve. With the EGR valve opened, exhaust gases flow into the cylinder reducing combustion chamber temperature.

The amount of exhaust gas circulated is controlled by variations in vacuum and exhaust back pressure. The negative back pressure valve is normally closed. The valve varies the amount of exhaust gas flow into the manifold depending on manifold vacuum and variations in exhaust back pressure.

Engine vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with negative exhaust back pressure, the vacuum bleed hole opens and the EGR valve closes.