Exhaust gas recirculation

Car manufacturers have been forced to lower exhaust emissions even further as a result of increasingly stringent legislation. This applies to both diesel and gasoline engines. The exhaust gas recirculation system helps to reduce nitrogen oxide emissions. On gasoline engines, the system also reduces fuel consumption when the engine is operated under partial load.

If the combustion temperature is high, nitrogen oxides are produced in the engine's combustion chamber. The combustion temperature in the combustion chamber is reduced by recirculating a portion of the exhaust emissions back to the fresh intake air. The lower combustion temperature results in less nitrogen oxide.
The exhaust gas recirculation rate for diesel and gasoline engines is shown in the following table:

A distinction must be made here between two types of exhaust gas recirculation: "internal" and "external" exhaust gas recirculation.

With the internal exhaust gas recirculation, the exhaust emissions and the fresh mixture are mixed inside the combustion chamber. On all four-stroke engines, this is achieved by the system-related overlap between the inlet and exhaust valves. The design means the exhaust gas recirculation rate is very low and can only be influenced to a limited extent. It is only since the development of variable valve control that it has been possible to actively influence the recirculation rate in a load dependent and rpm dependent manner.

External exhaust gas recirculation is carried out using an additional line between the exhaust manifold/exhaust pipe and the intake manifold as well as the EGR valve.

In order to influence the exhaust gas recirculation rate to a greater extent, pneumatic check valves and pressure limiting valves were installed, as were delay valves. Some systems also factor in the exhaust gas back pressure as a control pressure for the vacuum cell. In some operating states, the exhaust gas recirculation is switched off entirely. This is enabled by the installation of electrical change-over valves in the control line. Despite these possibilities for influencing the recirculation rate, the system was always dependent on the engine's load state and the associated induction pipe vacuum for controlling the vacuum cell.

The following procedure applies when checking vacuum-controlled EGR valves:

With the engine switched off, pull off the vacuum line and connect the manual vacuum pump. Generate a vacuum of around 300 mbar. If the valve is OK, the pressure must not fall within 5 minutes. Repeat the test with the engine running and at operating temperature. With a pressure differential of approx. 300 mbar, idle mode must worsen or the engine must switch off.

If the valve is equipped with a temperature sensor, the sensor can also be checked. To do so, remove the temperature sensor and measure the resistance. The following table lists approximate resistance values for individual temperatures:

Temperature Resistance
20°C > 1000 k Ω
70°C 60 - 280 k Ω
100°C 60 - 120 k Ω

Use a heat gun or hot water for heating up. Using the digital thermometer, check the temperature in order to compare the measured values with the reference values

Valves with vacuum connections that are offset to the side are opened by one connection only. These can be arranged above one another or offset to the side on one level. Valves where the vacuum connections are arranged above one another operate in two stages. The top connection partially opens the valve, and the bottom connection fully opens the valve. Valves with vacuum connections that are offset to the side are opened by one connection only. Colour coding is used for marking the connections.

The following combinations are possible here:

• Black and brown
• Red and brown
• Red and blue

The vacuum supply is to be connected to the red or black connection.

The leak tests are performed under the same conditions as for valves with a single diaphragm, but need to be performed at both vacuum connections. To check the vacuum supply to the valve, the manual vacuum pump can be used as a manometer. It is to be connected to the supply line of the EGR valve. With the engine running, the prevailing vacuum is displayed. On valves with connections arranged above one another, the manual vacuum pump must be connected to the line of the bottom connection; if the connections are offset to the side, it must be connected to the line of the red or black connection

Some EGR valves come with a potentiometer to report back the position of the valve. These EGR valves are checked as described above. When checking the potentiometer, proceed as follows:
Remove the 3-pin connector and measure the total resistance at pin 2 and pin 3 of the potentiometer using a multimeter. The measured value must be between 1,500 ohms and 2,500 ohms. To measure the resistance of the slideway, connect the multimeter to pin 1 and pin 2. Using the manual vacuum pump, slowly open the valve. The measured value should begin at around 700 ohms and increase to 2,500 ohms

During this check, the manual vacuum pump is not used to generate the vacuum, but instead is used as a manometer. Remove the vacuum hose from the pressure transducer to the EGR valve on the pressure transducer and connect the vacuum pump. Start the engine and slowly move the pressure transducer linkage. The display on the vacuum pump manometer must change accordingly.

Electric change-over valves have three vacuum connections. If only two connections are assigned, a non-sealing cap is fitted to the third connection.

To run the check, the manual vacuum pump can be used to perform a continuity test on the output lines of the change-over valve. To do so, connect the vacuum pump to an output line. If a vacuum can be generated, the change-over valve must be supplied with voltage.