If the gasses are metered when they are not supposed to be, drivability complaints of poor idle quality and/or engine stall when coming to a stop are usually paired with an illuminated Malfunction Indicator Lamp (MIL) and a code for "excessive flow." When the gasses can't get to where they are supposed to be, NOx emissions will be up and the MIL will be on with a low flow code stored. Drivability complaints may not exist, or be limited to hearing spark knock on acceleration.
The EGR monitor is responsible for keeping an eye on the EGR system. This monitor is a non-continuous monitor, operating once per any given drive cycle. Most EGR related codes are two-trip codes, meaning that the monitor must see the same failure on two consecutive passes to turn on the MIL and record the related freeze frame information. Unlike continuous monitor codes, however, there is little diagnostic value in this stored information.How these gasses are metered, and the specifics of how the system is monitored by the Engine Control Module (ECM), is where the differences between manufacturers lie. To successfully repair these systems, it is important to understand how they do what they do. This month, we'll focus on Ford's three EGR systems.
DPFE System
The DPFE, or Delta Pressure Feedback EGR, system is the oldest of the three and the one you most likely are familiar with. The system is composed of a vacuum- controlled EGR valve connecting the exhaust manifold to the intake just downstream of the throttle plate. The valve receives vacuum from the EVR, or EGR Vacuum Regulator. The EVR, in turn, is controlled by a variable duty cycle signal from the ECM. The duty cycle applied controls the amount of EGR flow, and is based on inputs to the ECM from several sensors: engine coolant temperature (ECT), cylinder head temperature (CHT), intake air temperature (IAT), throttle position (TPS), mass air flow (MAF) and crankshaft position (CKP).The system gets its name from the sensor used to provide the feedback to the ECM, the DPFE. It is a pressure transducer that measures the differential pressure across a restrictive orifice located in the EGR tube itself. The line connecting the sensor to the tube closest to the exhaust manifold connection is the "HI SIGNAL," and the one closest to the EGR valve is the "REF SIGNAL." The orifice is in the tube between these two. The ECM calculates what this differential pressure drop should be for any given commanded EGR opening, and compares the actual reading to this calculation for feedback on how the system is working.
DPFE sensor failures are not uncommon, and there have been several TSBs and parts revisions for these sensors. Their exposure to high temperature exhaust gasses containing inert gas, moisture and oil blowby all contribute to high failure rates.ESM System
ESM stands for EGR System Module and is similar to the DPFE system, using most of the same components integrated into one part. It was introduced in the 2002 ½ model year and is more reliable than the older DPFE system, because the restrictive orifice position was moved from the connecting tube to the intake side of the EGR valve at the mount.
The downstream pressure signal of the DPFE sensor now measures manifold absolute pressure, and differential pressure is determined in the ECM by subtracting the MAP value from the P1 pressure measured at the EGR valve itself. Later versions of the ESM system incorporated a dedicated MAP signal to the ECM.
Control of the EGR valve is the same in this system as in the DPFE system, with the ECM sending a variable duty cycle signal to the EVR to control EGR flow through the valve.EEGR System
The Electric EGR system uses an air or water-cooled electric stepper motor to control an incorporated EGR valve opening. EGR opening is operated through a range of 52 steps, and the motor itself is controlled by the ECM, based on the same inputs as the others.
There is no direct position measurement sensor on the EGR valve like those used by other manufacturers. EGR flow and inferred valve position is monitored by the use of feedback from a dedicated MAP sensor. This actual MAP value is compared to an inferred value calculated by the ECM to monitor EGR flow rate and detect flow errors.
The DPFE and ESM systems are unique Ford designs, while the EEGR shares similarities with other manufacturers. All three, however, require your understanding of their function if you are to effectively troubleshoot faults when they occur.Diagnosing DPFE Failures
DPFE sensor failures fall into the "pattern" failure category, but before you condemn the sensor, verify it first.
Check the DPFE PID reading on your scan tool with KOEO (Key On, Engine Off). Typically, this voltage should be about 1.0 volt. Higher readings indicate an internal failure of the sensor. Next, attach your vacuum pump to the EGR valve and start the engine. Monitor the DPFE PID as you open the EGR valve manually. The voltage should increase, and you should hear the engine start to run rough or even stall, as EGR is added.
If no voltage change is seen and the valve is verified as working, suspect the sensor. However, if no change in engine performance is seen with vacuum applied, check the EGR valve for proper operation and the passages for blockage.A Few Other Tips
EGR system faults are usually related to flow when there shouldn't be any and not enough flow when there should.
The amount of EGR needed is a function of engine load and temperature. There is no need for EGR flow at idle. The presence of EGR at idle is characterized by rough, poor idle quality and even engine stall when coming back to idle. If the engine is equipped with an Intake Air Temperature (IAT) sensor in the intake manifold, monitor its reading to see if it gets hotter than normal - an indication of EGR flow. Or you can remove the valve and block the intake side port, restart the engine and see if the idle quality returns to normal. Check the EGR valve for debris holding the valve open.
EGR gasses are inert gasses composed of combustion by-products, moisture and oil blowby. These gasses enter the intake, either through a single passageway exiting behind the throttle plate or through individual passages exiting just upstream of the intake valves. When the hotter gasses meet that cooler air, the by-products can begin to solidify, restricting the passages until they are closed off. If the system uses a single passage, visually inspect for this blockage or try this.Attach a manifold vacuum gauge to an intake vacuum port. Bring the engine rpm up to about 2,500, let the gauge stabilize and record the reading. It should be between 17 and 21 in/Hg. Now open the EGR valve. Vacuum should drop 6 to 8 in/Hg if the passages are clean and the EGR valve is working.
Restrictions in systems using individual ports are often accompanied by drivability complaints like "stumbles on acceleration." This is caused by an improper air/fuel mixture in those cylinders not getting the amount of EGR the ECM was counting on.
Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com.
