Exhaust gas recirculation (EGR) is a reliable and practice-proven method of emission control: As a result of exhaust gas, the oxygen content in the air-fuel mixture is reduced, and consequently the combustion temperature in the cylinders lowered. Since harmful nitrogen oxides (NOx ) are mainly produced at high temperatures and pressures, it is possible to reduce the NOx concentrates emitted to the environment by up to 50%. On diesel engines, the formation of soot particulates is lowered by approx. 10%.
Principle of exhaust gas recirculation (pneumatic)
1 Air fi lter
2 Air mass sensor
3 Throttle valve/control fl ap
5 EGR valve (in this case:pneumatic)
6 Electropneumatic pressure transducer
7 Lambda probe (Otto)
8 Catalytic converter
Exhaust gas recirculation is only activated at defi ned operating points. On petrol engines this is normally the case above idling and up to upper part load, on diesel engines up to approx. 3000 rpm and medium load.
Components of exhaust gas recirculation system (EGR)
The EGR valve meters the amount of recirculated exhaust gas. It is added either at the exhaust manifold or at the intake air system, or it is located in a heatresistant exhaust-gas pipe which connects the exhaust manifold to the intake air system. Pneumatic EGR valves are actuated by way of vacuum via electromagnetic valves: On basic systems with an electric switchover valve, the EGR valve only has an open-close function. On systems provided with an electro-pneumatic pressure transducer (EPW), the EGR valve is infi nitely adjustable. The vacuum is taken from the intake manifold or generated by a vacuum pump.
Electric or electromotive EGR valves are actuated direct by the control unit and no longer need any vacuum or solenoid valve.
EGR valves on diesel vehicles have large opening cross-sections because of their high return rates.
Left: Pneumatic EGR valve
Centre: Pneumatic EGR valve with position detection
Right: Electric EGR double-seat valve
On EGR valves in petrol engines, the cross-sections are considerably smaller.
Left: Electric EGR valve with connection to the coolant circuit
Centre: Pneumatic EGR valve
Right: Electric EGR valve
1: Pneumatic EGR valves are actuated with the aid of electro-pneumatic valves.
2: The air mass sensor is required on diesel engines, among other things for controlling the exhaust gas recirculation.
3: Since the pressure diff erence between exhaust and intake side is inadequate for the high exhaust gas recirculation rates on diesel vehicles, "regulating throttles" are fi tted into the intake manifold to generate the required vacuum.
Tips for troubleshooting
The most common cause for malfunctions in the EGR system are stuck or carbonised EGR valves. Besides gaseous pollutants, recirculated exhaust gas also contains soot particulates, in particular in the case of diesel vehicles. Due to oil in the intake air, carbon deposits or carbon fouling are caused which eventually the force of the valve can no longer cope with ñ the EGR valve can then no longer open, or it remains in its open position. This results in jerking, irregular idling or insuffi cient engine power. The causes of a high portion of oil in the intake or charge air may be malfunctions in the crankcase ventilation, worn bearings, a clogged oil return line on the turbocharger, worn valve stem seals or guides, the use of unsuitable engine oil qualities or an excessive engine oil level. Exceptionnally intense deposits may also be caused by faults in the injection system.
Though EGR valves are designed for the high temperatures in the exhaust branch system, heat damage to the valve may occasionally occur. Such damage can be caused by incorrect actuation, excessively high exhaust gas back pressure or a blow-off valve ("waste-gate valve") of the turbocharger which does not open. There may even be some manipulation ("tuning") to increase the boost pressure. On pneumatic EGR valves, one potential cause of malfunctions can be found in the entire section of vacuum control (vacuum pump, vacuum lines, solenoid valves). Electric EGR valves and solenoid valves can mostly be actuated by way of an actuator diagnosis by the engine tester. The switching of a functioning valve can easily be heard when the engine is at a standstill.
Whenever a new EGR valve is installed after a defect, and the vehicle behaves as if the valve had not have been replaced at all, the map data required for operation must fi rst be "re-learnt". This is achieved either in the course of a longer test drive or by using a specifi c program option in the engine tester, e.g. "Basic settings".
We strongly advise against cleaning the EGR components! If a component really is already defective, no improvement will be achieved by cleaning it. And if functioning components are treated in this way, they may be damaged as a result of cleaning.
A defective component should always be replaced by a new one.
1: Since EGR valves do not soot themselves, it is essential to search for the causes of soot formation.
2: Salt and dirt may damage the sensor of an air mass sensor – they will at least falsify the measurements, which in turn could aff ect exhaust gas recirculation.
3: Whether dealing with pneumatic EGR valves or an electro-pneumatic pressure transducer, as in this case: The function can easily be tested using a vacuum hand pump.
Troubleshooting in exhaust gas recirculation system:
Caused by EGR valve
Insuffi cient engine power
MIL lights up/error code set
Insuffi cient engine power in lower rpm range or in cold run (Otto)
Insuffi cient engine power in upper rpm range (diesel)
General: Coked/stuck EGR valve
Poor, inadequate combustion
Engine management fault
Frequent short-distance drives
Leaks in vacuum system
Check engine controls
Check software update of engine control unit
Avoid frequent short-distance drives
Defective solenoid valves
Malfunctions in vacuum system
Check function, electrical actuation and
tightness of vacuum system
See below: "Vacuum system"