The EGR system allows cooled (inert) exhaust gases to re-enter the combustion chamber, which lowers combustion
temperatures and Oxides of Nitrogen (NOx) emissions.
EGR system control is based off an air system model to estimate the percentage of exhaust gas in the cylinder. The
PCM looks at engine temperature, intake pressure, exhaust pressure (EP), RPM, and engine load to determine the EGR flow rate. The ratio of MAP and EP is used by the PCM to estimate a desired EGR valve position. The desired position is compared to the actual and the duty cycle is adjusted to meet that desired position for the required EGR flow rate. If the rate is not achieved with EGR valve position, the intake throttle body closes to a desired position, reducing intake manifold pressure. Reducing the intake manifold pressure increases the pressure ratio allowing more exhaust to fill the intake manifold at a given EGR valve position. As more exhaust gas is introduced into the intake manifold the amount of air measured by the Mass Air Flow (MAF) sensor is decreased.
The 6.7L has a hot side EGR valve due to it being before the EGR cooler. Once past the EGR valve, the exhaust gas is
either directed through the EGR cooler or bypasses the EGR cooler. This is done by the PCM controlling the EGR cooler bypass solenoid which turns vacuum on or off to the actuator on the bypass door. The EGR outlet temperature (EGRT) sensor measures the temperature of the exhaust gas leaving the system for cooler effectiveness and bypass control.