DPFE systems are most easily identified by their multiple components and the two tubes connecting the sensor to the EGR feed
tube. (Photos: Pete Meier)
The use of exhaust gas recirculation to assist in emissions control is common to most manufacturers. These systems all cool
combustion chamber temperatures to reduce the emissions of oxides of nitrogen, or NOx. Their function is similar. Burned gasses exiting the combustion chamber are metered back to the intake, either through a
common passage or through individual, cylinder specific, passages.
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.
An ESM is serviced as one assembly.
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 system measures pressure drop across an orifice in the EGR feed tube to monitor EGR flow.
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.
The ESM system is essentially the same as the DPFE with two exceptions. One, all individual components are combined into one
assembly; and two, the differential pressure reading is taken on the intake side of the valve instead of the exhaust side.
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.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
