Exhaust Gas Recirculation (EGR) System

WARNING: This page is about a different car, the 2021 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

The EGR system routes exhaust gases back through the engine to reduce emissions. When the exhaust gases exit the exhaust manifolds they enter the intake manifold through the EGR valve by either first passing through the EGR cooler or bypassing the EGR cooler depending on the position of the EGR cooler bypass valve.

Exhaust Gas Recirculation (EGR) Cooler System

The exhaust gases are directed through the EGR cooler to lower the exhaust gas temperature before entering the intake manifold. Engine coolant reduces the exhaust gas temperature when the gases are directed through the EGR cooler by closing the EGR cooler bypass valve.

The PCM controls the EGR cooler bypass valve by commanding the EGR cooler bypass solenoid duty cycle. When the EGR cooler bypass valve is commanded to 0% duty cycle, the exhaust gases pass through the EGR cooler to the intake manifold. When the EGR cooler bypass valve is commanded to 100% duty cycle, the exhaust gases pass directly to the intake manifold without passing through the EGR cooler.

For additional information on the EGR cooler system components, refer the component descriptions In this article.

Exhaust Gas Recirculation (EGR) Cooler System Monitor

The EGR cooler system, including the EGR cooler bypass valve, is monitored by comparing a modeled EGR gas temperature downstream of the EGR cooler to an actual temperature sensor value. Both under cooling and over cooling conditions are monitored.

The over cooling monitor occurs when the engine is in normal closed loop operation, the engine coolant temperature is between 60-90 deg.C (140-194 deg.F), and no air system DTCs are present.

The under cooling monitor occurs when the engine is in normal closed loop operation, the engine coolant temperature is above 70 deg.C (158 deg.F), and no air system DTCs are present. If the difference between the modeled EGR gas temperature downstream of the EGR cooler and the actual EGR gas temperature sensor value is greater than a calibrated amount a concern is detected.

The monitor detects an under cooling condition if the EGR cooler is inoperative or EGR cooler bypass valve is stuck open. The monitor detects an over cooling condition if the EGR cooler bypass valve is stuck closed.

Exhaust Gas Recirculation (EGR) Closed Loop Control Limits System Monitor

The control system is closed loop around EGR rate during normal operation and the limits of the control system are monitored by observing the control deviation calculated as the difference between the desired EGR rate setpoint (after dynamic correction) and the actual EGR rate determined from an air system model. The air system model EGR estimates the percent of EGR in the cylinder by modeling the induction system based on the MAF sensor reading and an estimate of the airflow over the throttle restriction. The EGR mass flow is then calculated as the difference between the cylinder charge from the speed density equation and this model of the MAF into the intake manifold. This model value of the actual EGR flow based on measured values is compared with the desired EGR rate setpoint. If the deviation between the controller setpoint EGR rate and the modeled EGR rate exceed the calibrated thresholds, a system control limit concern is indicated.

If no air system concerns are present, the control limit monitor functions continuously during closed loop operation once the engine coolant temperature is greater than 70 deg.C (158 deg.F). Separate threshold maps indicate the EGR system operation is at its control limits for both the high flow and low flow extremes. Control limit threshold maps are a function of engine torque and engine speed and are based upon the expected distribution of deviations from a statistical sampling. An up/down counter is used that counts upward when the control deviation exceeds the calibrated threshold and counts downward when the control deviation falls below the threshold for the case where the system operation is limited to EGR rates higher than the controller setpoint. Similarly, the up/down counter counts upward when the rate deviation falls below a calibrated threshold and counts downward when it rises above the threshold for the case where the system operation is limited to EGR rates lower than the controller setpoint. Once the counter exceeds a calibrated threshold for either case a concern is detected.

Exhaust Gas Recirculation (EGR) System Monitor

The EGR system is a closed loop EGR control system that uses an air system model EGR to estimate the percent of EGR in the cylinder. Airflow into the engine is modeled using a MAF and a model of airflow over the throttle restriction. The EGR mass flow is then calculated as the difference between the cylinder charge from the speed density equation and this model of the mass airflow after the throttle. This model value of the actual EGR flow based on measured values is compared with the estimated EGR rate computed by the EGR rate controller. The EGR rate controller calculates the mass flow based on an open area estimate at the EGR valve and a modeled value of mass flow per open area. The EGR open area estimate uses a combination of the area calculated from the modeled EGR rate and a calibrated curve of the expected open area based on EGR valve position.

Once the engine is warm and the system is in closed loop EGR control, the monitor awaits calibrated engine speed/load conditions selected for robust detection of high flow and low flow concerns. When the engine enters the operational regime for robust detection, the calculated rate deviation (modeled controller estimate) is compared to thresholds that are calibrated as a function of speed and load. An up/down counter is used that counts upward when the rate deviation exceeds the calibrated threshold and counts downward when the deviation falls below the threshold for the case of high flow. Similarly, the up/down counter counts upward when the rate deviation falls below a calibrated threshold and counts downward when it rises above the threshold for the case of low flow. Once the debounce counter exceeds a calibrated threshold for either high or low flow a concern is detected and DTC P0401 sets for a low flow concern while DTC P0402 sets for a high flow concern.

The typical EGR low flow monitor entry conditions include:

Engine torque is 100 - 600 Nm (74 - 443lb-ft)
Engine RPM is between 600 RPM and 2500 RPM
Engine coolant temperature is 70 - 100 deg.C (158 - 212 deg.F)
EGR valve position between 40% and 60%
EGR rate desired value between 0% and 100%

DTC P0401 sets when the delivered EGR is 28% less than expected for the operating conditions. EGR rates are specified as a function of engine RPM and indicated torque setpoint.

The typical EGR high flow monitor entry conditions include:

Engine torque between 100 Nm (74 lb-ft) and 600 Nm (443 lb-ft)
Engine RPM is between 600 RPM and 2500 RPM
Engine coolant temperature between 70 deg.C (158 deg.F) and 100 deg.C (212 deg.F)
EGR valve position between 0% and 6%
EGR rate desired value between 0% and 15%

DTC P0402 sets when the delivered EGR is 15% greater than expected for the operating conditions. EGR rates are specified as a function of engine RPM and indicated torque setpoint.