In our previous page titled "EGR valve
" we described in general how the EGR system works, in this page we are going to focus on Ford EGR systems.
In modern Ford vehicles the EGR flow rate is determined by monitoring the pressure across a fixed metering orifice as exhaust gasses pass through it. This system is called Differential Pressure Feedback ( DPFE ) system. The pressure sensor monitors upstream ( before ) and downstream ( after ) exhaust backpressure. This backpressure coefficient is relayed to the PCM and the correct amount of EGR ( duty cycle ) is applied to the EGR vacuum regulator control ( EVR ). By calculating the difference between the two pressures, the PCM determines exactly the EGR flow rate at all driving conditions.
The DPFE is more accurate than early systems because the ECM doesn't have to guess at the upstream pressure coefficient to determine EGR flow rate as the engine drives through various road conditions such as hard acceleration, downshifting, engine misfire, poor fuel combustion etc. All of these conditions will cause the exhaust backpressure to vary and requires more strict and responsive EGR control to limit NOx emission levels. Note that in this particular system when the check engine light comes on due to EGR system malfunction, most of the time is due to a malfunctioning DPFE sensor.
This is the DPFE sensor, if you suspect that this item is malfunctioning proceed to perform the following tests: Check for reference voltage to the DPFE sensor with the key on ( engine not running ) check for voltage on the harness side of the electrical connector on terminal VREF. It should be between 4.0 and 6.0 volts. If the test results are incorrect, replace the DPFE sensor. To check the operation of the DPFE sensor check for signal voltage to the sensor, backprobe the correct terminals and check for voltage signal while the engine is running, first at cold temperature and then at warm operating temperature. With the engine cold there should be no EGR therefore the voltage should be approximately 0.20 to 0.70 volts. As the engine warms up and the EGR is signaled by the computer, the voltage values should increase to approximately 4.0 to 6.0 volts.
The OBD-II system can detect a variety of different EGR system problems and set codes to indicate the specific trouble area. Trouble codes P1400 through P1410 are designated EGR system trouble codes.
Too much EGR flow tends to weaken combustion, causing the engine to run rough or stop. When the EGR flow is excessive, the engine can stop after a cold start or at idle after deceleration, the vehicle can surge at cruising speeds or the idle may be rough. If the EGR valve remains constantly open, the engine may not idle at all.
Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This can cause spark knock ( detonation ), engine overheating or emission test failure.
The EGR valve is controlled by a normally open EGR vacuum regulator ( EVR ) which allows vacuum to pass when energized. The PCM energizes the EVR to turn on the EGR. The PCM controls the EGR when 3 conditions are present:
1- Engine coolant is above 113 degrees F, 2-the TPS is at part throttle and 3-The MAF sensor is in its mid range.
This is the EGR valve, to perform a leakage test, hook up a vacuum pump to the vacuum port on top of the EGR valve, apply 5 to 6 in-Hg of vacuum to the EGR valve. The vacuum pump should hold vacuum if the EGR valve is not leaking. If possible, position your finger tip under the vacuum diaphragm as you apply vacuum to the valve, you should feel movement of the EGR valve as you apply vacuum to it. If the valve is sluggish due to carbon build-up, you can clean it by first removing it from the vehicle, clean the inlet and outlet ports with a wire brush or a scraper, do not sandblast the valve or use solvents or gasoline to clean it, these liquids will destroy the EGR diaphragm.
This is the Electronic vacuum regulator ( EVR ), this item is used to control the amount the amount of exhaust gas through the EGR valve. The valve is normally open ( engine at operating temperature ) and the vacuum source is a ported signal. The PCM uses a controlled "pulse width" or electronic signal to turn the EGR on and off ( duty cycle ).
The duty cycle should be zero percent ( no EGR ) when the vehicle is in Park or eutral,when the TPS is below the specified value or when wide open throttle ( WOT ) is indicated. To check the EGR vacuum regulator ( EVR ) disconnect the electrical connector and turn the ignition ON, ( engine not running ) to check for battery voltage.
Next, use an ohmmeter to check the resistance of the EVR , it should be between 30 and 70 ohms.