The EGR system lowers the formation of nitrogen oxides by reducing combustion temperature. This is done by introducing exhaust gases into the cylinders through an EGR valve. The PCM, in the control module system, controls the amount of EGR to meet emission control requirements and maintains good driveability through a EGR pulse width modulated solenoid and a EGR vent solenoid, which regulates the vacuum to the EGR valve. One main sensor input to the PCM is used to calculate the amount of EGR.
The EGR valve, installed on the intake manifold, introduces the exhaust gases to the incoming fresh air at the engine crossover.
A vacuum pump is required to provide a vacuum source to operate the EGR system.
EGR Solenoid
The EGR solenoid assembly is mounted at the top, left rear of the engine. The PCM controls the time the EGR solenoid to regulate the vacuum to the EGR valve. This regulates the amount of EGR. The PCM calculates the amount of EGR based on engine speed and accelerator pedal position. The PCM is programmed to vary the ON and OFF time of the EGR solenoid, based on these two inputs. To monitor the PCM control of EGR, an EGR control pressure/BARO sensor is used to measure the amount of absolute pressure in the EGR vacuum line. If a minor variation in calculated EGR and actual EGR as monitored by the EGR control pressure/BARO sensor, the PCM makes a correction. If the variation exceeds an amount in excess of what the PCM can correct for, an error is detected by the PCM and the system will go into default.
When the PCM recognizes the operating range for no EGR, the EGR vent solenoid operates to allow rapid venting of vacuum to the EGR valve.
Adaptive Learn Matrix (ALM)
The Adaptive Learn Matrix (ALM) is a matrix of cells arranged by RPM and engine load. ALM is used to adjust EGR vacuum control based on Mass Air Flow (MAF). The ALM cells may change as a result of back pressure increases over the life of the engine or other engine system variations. Too much EGR will cause ALM cells to go high, not enough EGR will cause the ALM cells to go low as shown below:
| • | If backpressure is excessive (too much EGR), the ALM cells will be high (above 128). This will cause the PCM to reduce vacuum to the EGR valve. This in turn will reduce the amount of EGR going into the engine. |
| • | If the EGR is not opening completely (not enough EGR), ALM cells will be low (below 128). This will cause the PCM to increase the amount of EGR entering the engine. |
As the engine operating conditions change, the PCM will switch from cell to cell to determine what factor to use. ALM is made up of sixteen cells numbered zero to fifteen in which each cell covers a range of engine speed (RPM) and load (mm3). A normal functioning EGR system ALM cells should display a value between 115 and 140 (the scaling of these cells are 0 to 256).
Resetting the ALM cells will allow the EGR system to quickly return to a normal function (reset ALM cells will display 128). If the ALM cells are not reset, the vehicle may experience black smoke and poor driveability complaints until the system is able to adjust (approximately 5 to 10 miles) or a DTC P0401 may set.
