The EGR valve, inhale your own exhaust and cool down ...

The EGR valve used to lower combustion temperatures to control oxides of nitrogen. By reintroducing a measured amount of exhaust into the combustion process the ECM lowers the burn temperature, but how is this possible? Find out in this blog...


Theory of Operation

The EGR system came about during the 1970’s as an answer to the, very difficult to reduce, nitrogen oxide gases. This family of gases is commonly referred to as NOx. The x stands for any number in the molecular formula (NO1, NO2, NO3, etc) . NOx is an odorless invisible gas. It is also a mayor component of photochemical smog, which is produced in big cities with the help of sunlight.

Nitrogen is an abundant gas found throughout the atmosphere making about 75% of it. Under normal conditions Nitrogen would never mix with oxygen to form an oxide. However, nitrogen does readily combines with oxygen at high temperature, above 2500 º F. At such high temperatures the production of NOx inside the cylinder combustion chamber is so excessive that the catalyst reduction process inside the catalytic converter is not enough. The job of the EGR valve is precisely that, to reduce the combustion chamber temperature. By reintroducing a measured amount of exhaust gas into the intake manifold and back to the combustion chamber, the actual combustion is cooled down. The purpose of this reintroduction being that the exhaust gases, although very hot, actually contain no fuel or oxygen left. Therefore, the reintroduced exhaust gases would be neutral or inert with respect to the combustion process. Reintroduction of exhaust gases or EGR operation does not take effect during idle. Actual EGR operation takes place during high load condition, cruising, and light acceleration. It is the job of various sensors and actuators to make sure that the EGR system works properly at all times. OBD II makes extensive diagnosis of the EGR system through the EGR monitor. The EGR monitor is a small program that runs inside the ECM’s memory, which actually tests all related EGR components during normal driving.

NOTE: Some engines do not employ an EGR system. This is because the reintroduction of exhaust gases into the intake manifold is done in a different way. The actual exhaust re-enters the intake manifold due to the camshaft leaving the intake and exhaust valves (both) open at the same time, for a couple of degrees of crank rotation. This causes some of the exhaust gases to go back into the intake manifold and then into the combustion chamber.

NOTE: Such a design (without an EGR valve) has the drawback of always being in operation, even at idle, therefore making the vehicle less of a performer. It is important to note that EGR operation does cause performance or a lack of power effect. It is also very common to find an engine that is pinging due to a faulty EGR system. The actual EGR system is taken into consideration when the engineers design a particular engine. Such modern engines will ping heavily due to excessively high combustion temperatures and pre-detonation. In such cases the exhaust reintroduction not only reduces combustion temperatures but also has the effect of raising the mixture octane to reduce pinging (pre-combustion).

With newer computer controlled valve timing, the EGR has been totally eliminated. These newer systems still leave the intake and exhaust (both) valves open at the same time, but only when needed. At idle there is no actual re-introduction of any gases. Computer controlled valve timing has the benefits of NOx control without excessive loss of performance and idle quality, or actually the best of both worlds.

There are four basic types of EGR systems. These are exhaust backpressure sensing, EGR temperature sensing, EGR valve lift position sensing, and MAP/O2 sensor feedback sensing systems. They all accomplish the same thing, which is to reintroduce a measured amount of the exhaust gases into the combustion chamber to reduce the combustion temperature.


The exhaust backpressure sensor is a device that checks exhaust back pressure through a built in potentiometer. The exhaust enters the sensor though a sampling hose at the bottom the sensor itself. Every time the EGR opens a drop in exhaust pressure is registered by the backpressure sensor and sent to the ECM. The sensor is usually a piezoelectric device, which means that a (quartz) crystal is used to do the measuring. In other words, as exhaust pressure is applied to the small quartz crystal block inside the backpressure sensor, the resistance and therefore the voltage output signal will change. The means by which the exhaust is measured does not actually matter. The important thing is that the sensor samples the exhaust pressure and sends a signal (changing voltage) back to the ECM as an indication of exhaust pressure change.

FORD uses the exhaust back pressure sensor, however the company uses two different sensors and they call them PFE and DPFE sensors. The PFE sensor works with only one sampling hose. It actually follows the same operation as explained in the general explanation above (refer to fig 1) and it is used only in older systems. The DPFE sensor is more accurate and somewhat similar to the PFE sensor, but has two sampling hoses instead. It is also still in use today. One hose is connected directly to the exhaust (HI. SIGNAL hose) like the PFE sensor, while the other hose connects to a port further up on the exhaust EGR tube (REF. SIGNAL hose) right after the metering orifice.

The upper metering port (REF. SIGNAL) is located after a restriction or metering orifice in the EGR tube. The DPFE sensor actually takes a sample of the exhaust before (HI. SIGNAL) and after the restriction (REF. SIGNAL) to arrive at a more accurate exhaust back pressure reading. In other words, it takes the difference between the two sample ports and sends a differential signal to the ECM. Hence the name DPFE or Delta (differential) pressure feedback electronic EGR sensor. The port before the restriction is regular exhaust pressure and the port after the restriction is the actual exhaust pressure drop (flow) through the EGR valve. During exhaust flow (EGR commanded on) the REF. SIGNAL port (after the restriction) will read a lower pressure than the HI SIGNAL or exhaust port, due to all the gasses after the restriction being sucked into the intake manifold. On the other hand, when no exhaust gas is flowing both ports are equalized and no differential pressure output is registered. All exhaust back pressure sensors have the advantage of directly measuring EGR flow. An inoperative (clogged) EGR valve would be detected by this sensor, causing no-change in the sensor’s signal output during flow command. Two DPFE sensors are actually in use, the older 0.50 volts offset and the newer 0.90—1.00 volts offset. The older type is usually metal while the newer type is made out of plastic. Bellow is a typical signal voltage-to-pressure chart.


The EGR exhaust gas temperature sensor works like the coolant temperature sensor. It is a thermistor having a negative temperature coefficient. This of course means that as temperature goes up the internal resistance of the sensor and its output voltage goes down. The operation of the EGR temperature sensor is fairly simple. The sensor is placed right on the path of the EGR flow. As the EGR valve opens, the hot exhaust gases pass through the exhaust passages and heat up the sensor. This action makes the sensor internal resistance drop. A drop in its internal resistance also means a drop in its output voltage. The EGR temperature sensor has two leads, an ECM provided ground (in most cases) and a 5 volt reference voltage lead (signal lead). With no EGR flow there is high internal resistance at the sensor and also a high voltage drop across it.. As soon as the EGR vacuum solenoid is commanded to open by the ECM, the EGR diaphragm moves up and there is EGR exhaust gas flow, heating the EGR temperature sensor, lowering its resistance, and bringing the 5 volt (signal lead) reference closer to ground.

It is important to remember that all temperature sensors (2 leads) are always wired in series with a fixed ECM internal resistor. This is simple Ohms law. As the thermistor changes resistance, the voltage drop shifts from the thermistor to the fixed ECM internal resistor. With this resistor shorted (inside the ECM), it is possible to see a 5 volt reference voltage at the signal lead but no voltage change would ever take place, since there is no other resistor in series with it. In this case, the ECM would have to be replaced. The EGR temperature sensor has the advantage, like the exhaust back pressure sensor, in that it takes a direct sensing approach in monitoring the EGR system. This differs from the EGR valve position sensor, which senses EGR diaphragm or valve lift regardless of weather the EGR is operating properly or not (clogged).


The EGR valve position sensor is a simple variable resistor or potentiometer. It is actuated by the EGR diaphragm as it moves up and down. This sensor has three leads, the sensor ground, a 5 volt reference voltage, and the signal lead. As the sensor is actuated or moved up and down, it shifts position and gets closer to either the ground side or the 5 volt reference line. This signal is fed directly to the ECM, so that it can calculate the EGR position (amount of lift).

The EGR valve lift position sensor has the big disadvantage of indirect feedback. So that as long as the EGR moves up and down, the position signal would be fed to the ECM. This however can not account for any valve operational problems, as in case of a clogged EGR valve. This is precisely the reason why California and tough emission law States mandate the use of an EGR temperature sensor or any other direct measuring system, in addition to any other type of EGR sensor.


The other method of EGR flow monitoring is the use of the MAP and the O2 sensor to detect EGR flow. Sometimes, on systems with MAF sensors, a dedicated MAP sensor is used. This sensor is usually referred to as an EGR boost sensor. The actual detection of EGR flow happens as the EGR solenoid is commanded on. This action opens the EGR valve and exhaust gases start flowing. Any time there is EGR flow there is also a vacuum drop in the intake manifold, due to the induced vacuum leak (EGR flowing). This is readily picked up by the MAP or EGR boost sensor (as higher voltage), as well as the O2 sensor cycle shift. Exhaust gases are actually depleted of any oxygen content. This means that they will register as a high O2 sensor (rich) voltage level, even though there is not an excess of CO present. The ECM, therefore, also looks at the O2 sensor reading (high voltage) as a confirmation that the EGR has opened and the exhaust gases are flowing. This is also a direct method of EGR monitoring, since the measurement of the flow of exhaust gases is made directly.


Basically anything that affects the flow of exhaust gases or vacuum to the EGR diaphragm has negative effect on EGR operation. The most common ailment to the EGR system are clogged passages. Carbon buildup tends to clog the small passages, creating a plug for the exhaust gases.

The EGR system also needs a vacuum source. This vacuum source goes to the EGR solenoid, which is triggered by the ECM at the proper time to actuate the EGR valve. Lack of vacuum to the EGR solenoid, restricted solenoid or broken vacuum hoses anywhere in between renders the EGR system inoperative.

An electrical open or short circuit to the EGR solenoid would either prevent vacuum from reaching the EGR valve or keep it open all the time. It is also important to understand that the EGR valve is actuated at loaded and cruising conditions. The EGR valve is always closed during idle and WOT conditions.



• The first thing to look at, when testing an exhaust back pressure sensor, is the sensor signal output voltage at idle (EGR closed). This is the offset voltage, which can be monitored with a scan tool or voltmeter, and will ascertain whether the sensor’s calibration is skewed or not. If the sensor reading is off, then all measurements thereafter will also be wrong. Always determine proper calibration of the sensor beforehand.

• Apply vacuum to the EGR valve, using a hand pump, and observe the signal voltage change at the backpressure sensor. If necessary, compare the readings to the chart and determine the signal voltage-to-vacuum (IN Hg of vacuum) applied. This however may not be necessary, as simply taking an initial (EGR closed) reading will determine proper sensor calibration. If there is no signal change while pre-loading the engine, the EGR valve is either stuck closed or the exhaust passages are clogged.


• A similar test procedure can also be followed for the temperature sensor, as done for the backpressure sensor. Take an initial EGR temperature reading to ascertain that the sensor is within calibration. This sensor rarely goes off calibration. Do not be fooled by a stuck semi-open EGR valve. This will make the EGR always flow a small amount of gas and raise the EGR temperature sensor reading. Carbon deposits will keep the EGR valve partially open, if they become excessive.

• Apply vacuum with a hand vacuum pump and observe the temperature change, either on the scan tool or with a voltmeter. Lack of a signal change is an indication of something keeping the EGR gasses from flowing. A stuck closed EGR valve or clogged passages can be the problem. Also remember that many EGR systems have an EGR tube, which tends to break. This would cause an exhaust leak and cause the gases to escape to the atmosphere, never reaching the intake manifold.


The EGR valve position sensor has a shaft, which comes into contact with the EGR diaphragm. This diaphragm is also connected at the other side to the conic valve itself. As carbon deposits raise the level of the conic valve, the diaphragm also changes position and so does the sensor’s signal output. Make a preliminary signal voltage check with the EGR closed to determine proper calibration. A raised EGR diaphragm caused by carbon deposits are common. Clean the EGR valve if necessary and then recheck further.

• Using a hand vacuum pump, apply a steady increasing vacuum and observe for correct voltage change using an oscilloscope or graphing meter. Pay close attention to any sudden drops in signal voltage. The potentiometer inside the position sensor tends to get defective segments or blind spots over time from excessive wear and tear. Because these signal voltage drops are hard to detect, it is useful to unscrew and separate the position sensor from the EGR valve and actuate it with your finger. This allows better control of the sensor’s shaft.


Apply vacuum to the EGR valve. This can be done using the scan tool bi-directional control feature or a hand vacuum pump. Observe the change in MAP reading and O2 signal voltage, while the EGR is being activated. The EGR gasses will cause the O2 sensor to go rich (above 500 mV), because of the lack of Oxygen in it. Map sensor readings will go towards a low of vacuum or high voltage reading (high frequency in case of FORD), since an open EGR valve also creates an intake leak. Although this leak is controlled and compensated for by the ECM. Lack of O2 and/or MAP signal change is an indication of a clogged or stuck closed EGR valve.

The remainder of the testing procedures apply to all the systems mentioned here.

• Disconnect the EGR solenoid connector, connect a test light or small light bulb to the EGR solenoid connector (across both leads), pre-load the vehicle and look for the ECM pulsing the test light. On some vehicles, the vehicle speed sensor has to be outputting a signal for the ECM to pulse the solenoid. In such cases, raise one wheel (switch the traction control system off) and raise the vehicle speed to about 25 MPH or so. This should activate the EGR solenoid. (It’s also a good idea to lightly tap on the brakes while spinning the raised wheel. This creates a small load on the engine, which creates the right conditions for the ECM to actuate the EGR solenoid.). A Lack of EGR solenoid pulse indicates an electrical problem or that the ECM is seeing something that is causing the EGR solenoid not to be activated. Make sure that battery voltage or ground is reaching one lead of the solenoid connector. Consult the proper wiring diagram first. Most ECM controlled devices are ground controlled, with a steady battery voltage applied to one of the leads. If battery voltage or ground is available but no pulsation from the ECM, then determine if the ECM is at fault or the ECM is seeing something that prevents is from operating the EGR solenoid.

• A lack of vehicle speed sensor input, for example, will prevent the EGR solenoid from being activated and so is the wrong coolant signal, TPS not registering proper throttle opening, etc.

• Connect a vacuum gauge in place of the EGR valve. Make the ECM actuate the EGR solenoid by whatever means necessary and observe for a vacuum reading. Lack of vacuum points to a clogged EGR solenoid, if it was determined that the ECM is pulsating the solenoid. Either clean or replace the solenoid.

As previously said before in this book. The first rule of diagnostics is to know the particular system that it is being worked on. Generally speaking, the EGR system diagnostics can be split into three parts, the valve and sensors, vacuum hoses and solenoid, and the electrical wiring/connector/ECM part of the system. Proper knowledge of each of the particular components will lead to a correct diagnostics each and every time.


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