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Exhaust Gas Recirculation - Solenoid Valve Diagnostics

The purpose of Exhaust Gas Recirculation (EGR) is to recycle a small amount of the exhaust gas back into the induction process to reduce oxides of nitrogen (NOx). NOx is produced when combustion temperatures are high, often associated with lean burn engines. By recycling a small amount of exhaust gas the combustion charge temperature is lowered and there is a reduction in NOx. The EGR solenoid will be control by the Electronic Control Module (ECM) and will also work in association with other devices that monitors the amount of gas that is recycled. This set up will often be different for each manufacture and there is normally a combination of vacuum and electrical solenoid valves.

EGR operation will take place under very specific conditions. The ECM will control the earth path to the solenoid valve. The information that the ECM requires for this operation is; engine temperature, vehicle speed and engine load. With such precise data required it will only be possible to see the activation of the EGR solenoid valve whilst the vehicle is on the dyno.

Technical Information

The function of Exhaust Gas Recirculation (EGR) is to lower Oxides of Nitrogen (NOx) under certain circumstances. As the internal combustion temperature rises, the nitrogen within the Air/Fuel mixture will start to oxidise causing NOx to be produced. This burning of the nitrogen is both unacceptable and unavoidable as the Air/Fuel ratio is increased and a weaker mixture is ignited.

The NOx output is at a maximum when the engine has reached its normal operating temperature and the vehicle is subjected to light throttle or light load conditions.

The catalytic converter is designed to eradicate the majority of the NOx by neutralising it when it comes into contact with the precious metal Rhodium, but by reducing the NOx before it reaches the 'cat' ensures even lower outputs. The EGR valve will allow a small amount of the exhaust gasses to 'bleed' back into the inlet manifold to lower the combustion temperature and reduce the chances of the Nitrogen burning. The EGR valve is a small mechanical device that allows the passage of exhaust gas when it receives a vacuum supply.

This supply is governed by a vacuum switch which in turn is activated by a signal from the Electronic Control Module (ECM). NOx, like hydrocarbons, are measured in parts per million and the reading recorded in a workshop environment is significantly lower to that recorded when the vehicle is at cruise.

A diagram of an typical EGR set up, with the EGR valve in the closed position.

EGR taken to excess can affect combustion and increase hydrocarbons. It is therefore necessary to monitor the amount of exhaust gas that enters the inlet manifold. Different manufacturers perform this task in different ways, some general examples of this are described below.

Honda use an ECM with a programmed map contained within it. The map contains information on the correct amount of EGR according to factors such as; engine speed, road speed, temperature and load.

Under the right conditions for EGR to take place, the ECM earths the path of the solenoid valve and this allows a vacuum source to operate the EGR valve. The EGR valve also includes a lift sensor, this is a similar device to a throttle potentiometer. It will have a 5 volt supply, earth and variable signal back to the ECM depending on the position of the EGR valve. If the amount of exhaust gas passing through the valve exceeds the parameters within the ECM's map, the ECM will shut the solenoid valve by removing its earth path. This making and breaking, or 'pulsing', of the earth path allows fine adjustments to be made ensuring the precise amount of EGR takes place.

Vauxhall have a similar system, however, the solenoid valve, lift sensor and the EGR valve are all one unit. Fault finding is also made harder by the fact that EGR takes places inside the cylinder head through a passage connecting the exhaust and inlet manifolds.

Ford, as always, have some interesting names and acronyms for the components within their EGR system. To start with the control solenoid is referred to as an Electronic Vacuum Regulator (EVR) and their method of monitoring the amount of EGR is by a Differential Pressure Feedback Electronic System (DPFE).

The DPFE sensor measures the pressure inside of the EGR tube either side of a restriction (venturi).

This pressure difference is then converted into a voltage and sent to the ECM for reference. Again the ECM contains a map for the correct amount of EGR and if this differs the ECM adjusts the control of the EVR to trim the amount of gas passing to the inlet manifold.

Circuit diagrams for the Ford and Vauxhall