Automotive EGR System Testing

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Procedure for the EGR System Testing

EGR Engine Gas Recirculation Diagram

 
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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. 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.




(EGR) System Testing (Engine Gas Recirculation)

By Mandy Concepcion

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.

EGR system

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.

EXHAUST BACK PRESSURE SENSING (PFE – DPFE)

DFE DPFE EGR automotive back pressure sensor

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.

COMPONENT TESTING

EXHAUST BACK PRESSURE SENSOR

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 back pressure 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.





copyright by Mandy Concepcion, Automotive Diagnostics and Publishing