Repair Info - Emissions Repair Explained
Overview | Component Testing - Doing It Right | EGR System TestingExhaust Gas Recirculation (EGR) System Testing
The purpose of the EGR system is to recirculate a small amount of burned exhaust gases back into the engine under certain operating conditions. This lowers the temperature of combustion and, in doing so, reduces the formation of Oxides of Nitrogen (NOx).
It is easy to see then that the EGR is a device that primarily affects NOx emissions. However, certain EGR defects can also cause excessive HC emissions.
The first thing the technician should check on every EGR system is the exhaust passage. Adequate exhaust gas must be available to the EGR valve for the maximum flow (highest load) conditions. In most cases they can test this by manually opening the valve at idle and stalling the engine. If the engine does not stall, it should drop by at least 300 rpm. Otherwise, there is probably a restriction in the exhaust passage or pipe.
To test the EGR valve and its related components, the technician must monitor the valve to see that it is opening and closing the proper amount under the proper conditions. In some cases this will involve removal of the valve.
Many different controls are used in various types of vehicles to control and monitor the operation of the EGR valve. In many cases, the technician will need to refer to the manufacturer's recommended procedure to troubleshoot the EGR controls. However, some general procedures the technician should apply are shown below.
- Pressure feedback sensors and modulators should be checked for any blockage or restriction in the backpressure sensing tube.
- Pressure feedback sensors should be checked electrically for proper reference voltage, ground, and signal return to the PCM.
- EGR valve position sensors should be checked electrically for proper reference voltage, ground, and signal return to the PCM.
- EGR temperature sensors should be checked for proper reference voltage and ground. Temp sensors should also be checked for correct resistance using the manufacturer's specifications.
- EGR vacuum control solenoids should be checked for the correct pulsed ground and voltage. Solenoids should also be checked for correct resistance using the manufacturer's specifications.
- Venturi vacuum amplifiers should be checked for correct vacuum input and output using the manufacturer's recommended procedures and specifications.
- Thermostatic vacuum switches should be checked to ensure they switch properly and allow vacuum to pass through only when the engine is warm (or vice-versa for some switches). Also, remember that a low coolant level may prevent correct operation of any temperature sensor or switch.
- All associated vacuum hoses and fittings should be checked for cracks, leakage, and secure connections.
- The exhaust system should be checked for modifications that may be adversely affecting exhaust backpressure and the control of the EGR.
