Feature April 1, 2002 by
Julio Oyola -- Domestic Specialist
Ford Pressure Feedback EGR Systems
In an attempt to control vehicle exhaust emissions, Ford advanced their EGR systems with the introduction of the EGR position sensor. This gave the ECM some idea of how much exhaust gas was being recy...
In an attempt to control vehicle exhaust emissions, Ford advanced their EGR systems with the introduction of the EGR position sensor. This gave the ECM some idea of how much exhaust gas was being recycled. This system works pretty well when everything is new. However, as vehicle mileage increases, carbon buildup becomes a problem. This system has only one way to control and measure EGR flow, and this is based on the assumption that everything will work as expected. This leaves something to be desired. Ford improved upon this by enabling the ECM to control EGR as well as monitoring EGR flow. There are two styles of EGR control. The first is called Pressure Feedback EGR (PFE). This is a system where the ECM can determine EGR flow based on a pressure drop across the opening in the EGR passage by using a transducer. Based on this input, the ECM, using an EGR vacuum regulator, will control the vacuum signal to the EGR valve. By regulating the vacuum signal that controls the EGR, the PFE system enables the ECM to precisely compute and command proper EGR flow (see Diagram #1).
The second style of EGR control is called the Differential Pressure Feedback EGR (DPFE). In this system, the ECM computes and controls EGR flow in a similar manner as the PFE system, except that this system calculates the difference in pressure between the EGR opening and the exhaust system (see Diagram #2).
It is important to know that even though these two systems measure exhaust pressure, and are ECM controlled, there are physical and electrical differences that you should be aware of when performing diagnostics. Physically, the PFE sensor has one pressure input nipple. The DPFE sensor has two pressure input nipples.
Electrically, both the PFE and the DPFE sensors are three wire sensors. Each have a 5-volt reference from the ECM, a ground, and a sensor output. It is in the sensor output voltage where these two sensors differ. The Key On Engine Off voltage output for the PFE sensor is 3.25 volts, where the output for the DPFE sensor is 0.45 volts. Refer to the chart to check pressure versus sensor output. Pressure can be checked by tapping into the pressure line while performing a road test and verify the correct voltage output.
This system is pretty good at determining whether or not the system is functioning properly. If the ECM detects something is wrong, it will set a fault code. The most common problem is carbon clogging the ports. Even though the same problems plague the EGR system, these newer systems are better able to detect these problems before they become a major problem.