Introduction to On Board Diagnostics (II)

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EGR System Monitoring: During overrun and heavy load of the vehicle the peak combustion n temperature ofthe cylinders of the engine will increase to more than 3000 o F. A measured quantity of exhaust gas isintroduced into intake manifold via a pintle valve connecting the exhaust gas to the intake manifold. Bymixing a portion of the exhaust gas with fresh intake air/fuel mixture mthe oxygen content is reduced withoutreducing the mass of gas processed by the cylinder. The engine acts apartially like an external combustionengine in that the combustion process must impart energy to the inert exhaust gas as well as to the air charge.The net effect is to reduce the flame temperature at part load while wretaining the power of the engine. Thereduction of temperature reduces NOx emission produced by the engine.The OBD II diagnostic has to monitor the pintle valve, and the amount of exhaust gas delivered by the pintlevalve. The correct amount of exhaust gas is obtained from predefined engine RPM/load (MAP) table showingoptimum EGR valve openings & gas amount, engine coolant temperature, manifold absolute pressure (MAP)pressure, and engine RPM. During EGR operation, the fuel is cut off. The OBD II diagnostic consists of severalalgorithms to monitor all the functions listed above. EGR pintle valve position is monitored by the PCM forproper opening. The amount of exhaust gas ingested is monitored from the EGR pintle valve flow rate, andthe time of the valve opening. This amount is compared with the required amount obtained from the tablewith predefined values. If there is a significant difference between the actual and the needed values, the EGRmalfunction is detected. Engine coolant temperature is monitored for an increase in value during EGRoperation. MAP pressure is monitored for increase in pressure during dEGR operation. Finally the EngineRPM (900 - 1100) is monitored for a decrease of about 50 RPM during EGR (DTC for fault is P0401 for nodecrease in RPM when vehicle speed is 25 MPH with brakes applied) ) operation.
OBD(II)In addition the electrical characteristics of the pintle valve are checked, including the voltage, the current drawn bythe moving pintle, , and the circuit continuity including open circuit as well as short scircuit in the wiring. There aretwo methods used in verifying that EGR is functioning properly meaning mno sticking valve or clogged EGR passage.The first method is to intentionally open the EGR valve through a measured value during normal operation whenthere is no need for EGR and measure the response of critical system sparameters due to this perturbation namely,Engine RPM, coolant temperature, MAP pressure, pintle valve position, and closed-loop loop fuel system correction. Ifthe critical parameters do not conform to the desired values EGR malfunction is indicated. The second method is towait for the condition of the vehicle when the EGR is operated by the PCM as a consequence of engine overrun orhigh load. Then intentionally disable EGR operation for a small predefined amount of time and measure the criticalparameters. If the difference in critical parameter values do not t conform to the expected values then EGRmalfunction is indicated.A much simpler algorithm measures the increase in coolant temperature during EGR and if the increase in notwithin desired range EGR malfunction is indicated. In addition increase iin manifold absolute pressure (MAP)during EGR and if the increase is not within desired range EGR malfunction mis indicated.Due to uncertainties encountered in EGR monitoring, more than one e diagnostic is necessary before a fault code isstored and the MIL is illuminated. One method is to requires three successive tests, each revealing an EGR fault,before a fault code is stored. If a test reveals no fault , the next test is performed eleven minutes later. The predefinedoperating condition is deceleration which means that the test is performed during deceleration of the vehicle.Different frequencies of testing are also used in the diagnostic. . Another method requires eight tests to be performedwithin a two minute period before a fault code is stored when two o failures occur within that period. Currently aboutfifty percent of the manufacturers monitor the EGR passage temperature, twenty-five percent monitor the EGR valvesignal (position), and twenty-five percent use the intrusive perturbation method to detect EGR malfunction.The legal OBD II requirement is: The diagnostic system shall monitor the EGR system on vehicle for low and highflow rate malfunction.The hardware failure code of P1406 is set for out of range voltage signal from the pintle valve position sensor ofmore than 10% from commanded value.Another manufacturer monitors the exhaust gas pressures on both sides of an orifice in the passage to the EGRvalve. The pressure drop across the orifice is measured as the exhaust egas flows through the orifice. If the pressuredifferential is not within permissible limits, EGR fault code is set.Different DTCs are set for tests performed with similar EGR diagnostic objectives due to differences in test time, andcritical parameter values.EGR diagnostics diagnostics are described in more detail in a later section.
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Introduction to On Board Diagnostics (II)

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