Exhaust gas recirculation

Here you will find useful information and important tips relating to the exhaust gas recirculation system in vehicles.

The exhaust gas recirculation system (EGR) reduces NOx emissions in internal combustion engines. The system is subjected to very high loads, which can cause problems, particularly on vehicles with high mileages. This page provides a wealth of interesting information on the most common system variants and how they work. It also offers you numerous practical tips for troubleshooting and for checking and replacing components in the EGR system.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


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:

(direct-injection engine)
EGR rate (max.)50%20%Up to 50% (depending on engine operation, homogeneous or stratified charge)
Exhaust gas temperature when the EGR system is active450°C650°C450°C to 650°C
Why is an EGR system used?To reduce nitrogen oxide, soot particles, and noiseTo reduce nitrogen oxide and fuel consumptionTo reduce nitrogen oxide and fuel consumption



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.

Pneumatic EGR valve

The first systems were controlled by a poppet valve which was opened or closed by a vacuum cell (pneumatic drive). In this context, the induction pipe pressure served as a control variable for the vacuum cell. As such, the position of the poppet valve was dependent on the engine operating state.

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.

Electrical EGR valve

In order to meet the requirements placed on modern engines independently of the induction pipe vacuum, electrical drives for the exhaust gas recirculation valves were developed. Sensors for detecting the valve position were integrated at the same time. These developments have enabled precise control with short adjustment times. In addition to stepper motors, linear solenoids, and rotary solenoids, DC motors are now also used as electrical drives. The control valve itself has also changed over time. Alongside needle valves and poppet valves in different sizes and dimensions, rotary slide valves and flap valves are also used nowadays.


EGR valve (exhaust gas recirculation valve)

The EGR valve is the most important component in the system. It establishes a link between the exhaust pipe and the intake section. Depending on how it has been controlled, the EGR valve releases the valve opening and allows exhaust emissions to flow into the intake manifold. The EGR valve is available in different versions: Single or twin-diaphragm versions, with and without position feedback or temperature sensor, and – of course – electrically controlled. Position feedback means that a potentiometer is fitted on the EGR valve. This potentiometer sends the control unit signals regarding the valve position. This then enables the recirculated quantity of exhaust emissions to be accurately recorded across all load states. If a temperature sensor is fitted, it is used for running self-diagnostics on the EGR valve.

Pressure transducer

The purpose of pressure transducers is to control the vacuum required for the EGR valve. They adjust the vacuum to suit the respective engine load state so as to maintain a precisely defined recirculation rate. They are controlled either mechanically or electrically.


They perform a similar task to pressure transducers, but are temperature-dependent. Pressure transducers and thermovalves could also be combined.


Faults occurring and their causes

On account of the high loads it has to withstand, the EGR valve is doubtless the biggest source of faults. Oil mist and soot from the exhaust emissions impede the valve, and the cross section of the valve opening reduces over time until it completely closes up. This causes a steady reduction in the recirculated quantity of exhaust emissions, and this is reflected in the exhaust emission performance. The high thermal loads further facilitate this process. In many cases the hose system for the vacuum is also a cause of faults. The vacuum required for the EGR valve can be lost due to leaks, meaning the valve no longer opens. If the EGR valve is not working due to a lack of vacuum, this may, of course, also be caused by a faulty pressure transducer or a thermovalve that is not working properly.


There are several ways of checking the exhaust gas recirculation system. Which method you choose depends on whether or not the system is capable of running self-diagnostics. Systems which are not capable of running self-diagnostics can be checked using a multimeter, a manual vacuum pump, and a digital thermometer.


However, before you start running complex tests, conduct a visual inspection of all system-related components. This means checking the following:

  • Are all vacuum lines leak-tight, correctly connected, and routed without kinks?
  • Are all electrical connections on the pressure transducer and change-over switch correctly connected? Are the cables OK?
  • Are there any leaks on the EGR valve or the connected lines?


If no defects are found during the visual inspection, check the system using other tests and measurements.


Checking vacuum-controlled EGR valves on gasoline engines

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.

EGR valves on diesel engines

EGR valves on diesel engines can be checked in the same way as for gasoline engines: With the engine switched off, generate a vacuum of around 500 mbar using the manual vacuum pump. This vacuum must be maintained for 5 minutes and must not drop. You can also conduct a visual inspection. To do so, again generate a vacuum via the vacuum connection using the manual vacuum pump. Monitor the valve rod (the connection between the diaphragm and valve) through the openings. It must move smoothly when the manual vacuum pump is actuated.

EGR valves with a potentiometer

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.


Checking mechanical pressure transducers

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.

Checking electro-pneumatic pressure transducers

The manual vacuum pump is used as a manometer in this case as well. It is connected to the electro-pneumatic pressure transducer at the vacuum connection to the EGR valve again. Start the engine and remove the connector of the pressure transducer electrical connection. The vacuum displayed on the manometer must not exceed 60 mbar. Refit the connector and increase the engine speed. The value displayed on the manometer must increase at the same time.


To check the resistance of the pressure transducer winding, remove the electrical connector again and connect a multimeter to both connection pins. The resistance value should be between 4 ohms and 20 ohms.


To check the control of the pressure transducer, connect the multimeter to the connections on the connector and monitor the voltage value that is displayed. It should change in line with the changes to the engine speed.


Checking electrical pressure transducers

The method for checking electrical pressure transducers is identical to that for checking electric change-over valves.

Checking electric change-over valves

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.


If the polarity of the connections (+ and -) is specified at the change-over valve connection, they must not be mixed up. If voltage has been applied to the change-over valve, it must change over and the generated vacuum drops. Repeat the check for the other connection.

Checking thermovalves

To check the thermovalves, remove the vacuum hoses. Connect the manual vacuum pump to the centre connection. When the engine is cold, there must be no passage at the thermovalve. When the engine is at operating temperature, the valve must open the passage. In order to be independent of the engine temperature, the thermovalve can be removed and heated up in a water bath or using a heat gun. When doing so, constantly monitor the temperature to find out what the switching points are.


All the test values listed here are approximate values. Vehicle-specific connection diagrams and test values must be provided in order to obtain precise data.


EGR systems with diagnostic capability can be checked using a suitable diagnostic unit. Again, the decisive factor is the test depth of the device being used and the system being checked. Sometimes it is only possible to read out the fault memory, but in other cases you can also read out measured value blocks and carry out an actuator test.


The key aspect in this context is to also check components that only have an indirect influence on the EGR system. This could be the air-mass sensor or engine temperature sensor, for example. If the control unit receives an incorrect value from the air-mass sensor, then the quantity of exhaust emissions to be recirculated will also be calculated incorrectly. This can worsen the emission values and cause major problems with engine operation.


On electrical EGR valves, it is possible that no faults are displayed during diagnostics and the actuator test also does not indicate a problem. In this case, the valve may be highly contaminated, meaning the valve opening no longer provides the cross section required by the control unit. It is therefore advisable to remove the EGR valve and check it for contamination.


Diagnostics on the EGR system

Diagnostics on the EGR system incl. the mega macs 66 diagnostic unit


04:33 min


Replacing the exhaust gas recirculation valve

EGR valve replacement including removal and installation instructions


02:43 min

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