The exhaust backpressure sensor is a device that checks
exhaust back pressure through a built in potentiometer. The exhaust enters
the sensor though a sampling hose at the bottom the sensor itself. Every
time the EGR opens a drop in exhaust pressure is registered by the
backpressure sensor and sent to the ECM. The sensor is usually a
piezoelectric device, which means that a (quartz) crystal is used to do the
measuring. In other words, as exhaust pressure is applied to the small
quartz crystal block inside the backpressure sensor, the resistance and
therefore the voltage output signal will change. The means by which the
exhaust is measured does not actually matter. The important thing is that
the sensor samples the exhaust pressure and sends a signal (changing
voltage) back to the ECM as an indication of exhaust pressure change.
FORD uses the exhaust back pressure sensor, however the
company uses two different sensors and they call them PFE and DPFE sensors.
The PFE sensor works with only one sampling hose. It actually follows the
same operation as explained in the general explanation above (refer to
fig 1) and it is used only in older systems. The DPFE sensor is more
accurate and somewhat similar to the PFE sensor, but has two sampling hoses
instead. It is also still in use today. One hose is connected directly to
the exhaust (HI. SIGNAL hose) like the PFE sensor, while the other hose
connects to a port further up on the exhaust EGR tube (REF. SIGNAL hose)
right after the metering orifice.
The upper metering port (REF. SIGNAL) is located after a
restriction or metering orifice in the EGR tube. The DPFE sensor actually
takes a sample of the exhaust before (HI. SIGNAL) and after the restriction
(REF. SIGNAL) to arrive at a more accurate exhaust back pressure reading. In
other words, it takes the difference between the two sample ports and sends
a differential signal to the ECM. Hence the name DPFE or Delta
(differential) pressure feedback electronic EGR sensor. The port before the
restriction is regular exhaust pressure and the port after the restriction
is the actual exhaust pressure drop (flow) through the EGR valve. During
exhaust flow (EGR commanded on) the REF. SIGNAL port (after the restriction)
will read a lower pressure than the HI SIGNAL or exhaust port, due to all
the gasses after the restriction being sucked into the intake manifold. On
the other hand, when no exhaust gas is flowing both ports are equalized and
no differential pressure output is registered. All exhaust back pressure
sensors have the advantage of directly measuring EGR flow. An inoperative
(clogged) EGR valve would be detected by this sensor, causing no-change in
the sensor’s signal output during flow command. Two DPFE sensors are
actually in use, the older 0.50 volts offset and the newer 0.90—1.00 volts
offset. The older type is usually metal while the newer type is made out of
plastic. Bellow is a typical signal voltage-to-pressure chart.
EGR EXHAUST GAS TEMPERATURE SENSING
The EGR exhaust gas temperature sensor works like the
coolant temperature sensor. It is a thermistor having a negative temperature
coefficient. This of course means that as temperature goes up the internal
resistance of the sensor and its output voltage goes down. The operation of
the EGR temperature sensor is fairly simple. The sensor is placed right on
the path of the EGR flow. As the EGR valve opens, the hot exhaust gases pass
through the exhaust passages and heat up the sensor. This action makes the
sensor internal resistance drop. A drop in its internal resistance also
means a drop in its output voltage. The EGR temperature sensor has two
leads, an ECM provided ground (in most cases) and a 5 volt reference voltage
lead (signal lead). With no EGR flow there is high internal resistance at
the sensor and also a high voltage drop across it.. As soon as the EGR
vacuum solenoid is commanded to open by the ECM, the EGR diaphragm moves up
and there is EGR exhaust gas flow, heating the EGR temperature sensor,
lowering its resistance, and bringing the 5 volt (signal lead) reference
closer to ground.
It is important to remember that all temperature sensors (2
leads) are always wired in series with a fixed ECM internal resistor. This
is simple Ohms law. As the thermistor changes resistance, the voltage drop
shifts from the thermistor to the fixed ECM internal resistor. With this
resistor shorted (inside the ECM), it is possible to see a 5 volt reference
voltage at the signal lead but no voltage change would ever take place,
since there is no other resistor in series with it. In this case, the ECM
would have to be replaced. The EGR temperature sensor has the advantage,
like the exhaust back pressure sensor, in that it takes a direct sensing
approach in monitoring the EGR system. This differs from the EGR valve
position sensor, which senses EGR diaphragm or valve lift regardless of
weather the EGR is operating properly or not (clogged).
EGR VALVE POSITION (LIFT) SENSOR
The EGR valve position sensor is a simple variable resistor
or potentiometer. It is actuated by the EGR diaphragm as it moves up and
down. This sensor has three leads, the sensor ground, a 5 volt reference
voltage, and the signal lead. As the sensor is actuated or moved up and
down, it shifts position and gets closer to either the ground side or the 5
volt reference line. This signal is fed directly to the ECM, so that it can
calculate the EGR position (amount of lift).
The EGR valve lift position sensor has the big
disadvantage of indirect feedback. So that as long as the EGR moves up
and down, the position signal would be fed to the ECM. This however can not
account for any valve operational problems, as in case of a clogged EGR
valve. This is precisely the reason why California and tough emission law
States mandate the use of an EGR temperature sensor or any other direct
measuring system, in addition to any other type of EGR sensor.
EGR MAP/O2 SENSING (EGR BOOST SENSOR)
The other method of EGR flow monitoring is the use of the
MAP and the O2 sensor to detect EGR flow. Sometimes, on systems with MAF
sensors, a dedicated MAP sensor is used. This sensor is usually referred to
as an EGR boost sensor. The actual detection of EGR flow happens as
the EGR solenoid is commanded on. This action opens the EGR valve and
exhaust gases start flowing. Any time there is EGR flow there is also a
vacuum drop in the intake manifold, due to the induced vacuum leak (EGR
flowing). This is readily picked up by the MAP or EGR boost sensor (as
higher voltage), as well as the O2 sensor cycle shift. Exhaust gases are
actually depleted of any oxygen content. This means that they will register
as a high O2 sensor (rich) voltage level, even though there is not an excess
of CO present. The ECM, therefore, also looks at the O2 sensor reading (high
voltage) as a confirmation that the EGR has opened and the exhaust gases are
flowing. This is also a direct method of EGR monitoring, since the
measurement of the flow of exhaust gases is made directly.
CONDITIONS THAT AFFECT OPERATION
Basically anything that affects the flow of exhaust gases or
vacuum to the EGR diaphragm has negative effect on EGR operation. The most
common ailment to the EGR system are clogged passages. Carbon buildup tends
to clog the small passages, creating a plug for the exhaust gases.
The EGR system also needs a vacuum source. This vacuum
source goes to the EGR solenoid, which is triggered by the ECM at the proper
time to actuate the EGR valve. Lack of vacuum to the EGR solenoid,
restricted solenoid or broken vacuum hoses anywhere in between renders the
EGR system inoperative.
An electrical open or short circuit to the EGR solenoid
would either prevent vacuum from reaching the EGR valve or keep it open all
the time. It is also important to understand that the EGR valve is actuated
at loaded and cruising conditions. The EGR valve is always closed during
idle and WOT conditions.
COMPONENT TESTING
EXHAUST BACK PRESSURE SENSOR
• The first thing to look at, when testing an exhaust back
pressure sensor, is the sensor signal output voltage at idle (EGR closed).
This is the offset voltage, which can be monitored with a scan tool or
voltmeter, and will ascertain whether the sensor’s calibration is skewed or
not. If the sensor reading is off, then all measurements thereafter will
also be wrong. Always determine proper calibration of the sensor beforehand.
• Apply vacuum to the EGR valve, using a hand pump, and
observe the signal voltage change at the backpressure sensor. If necessary,
compare the readings to the chart and determine the signal voltage-to-vacuum
(IN Hg of vacuum) applied. This however may not be necessary, as simply
taking an initial (EGR closed) reading will determine proper sensor
calibration. If there is no signal change while pre-loading the engine, the
EGR valve is either stuck closed or the exhaust passages are clogged.
EGR TEMPERATURE SENSOR
• A similar test procedure can also be followed for the
temperature sensor, as done for the backpressure sensor. Take an initial EGR
temperature reading to ascertain that the sensor is within calibration.
This sensor rarely goes off calibration. Do not be fooled by a stuck
semi-open EGR valve. This will make the EGR always flow a small amount of
gas and raise the EGR temperature sensor reading. Carbon deposits will keep
the EGR valve partially open, if they become excessive.
• Apply vacuum with a hand vacuum pump and observe the
temperature change, either on the scan tool or with a voltmeter. Lack of a
signal change is an indication of something keeping the EGR gasses from
flowing. A stuck closed EGR valve or clogged passages can be the problem.
Also remember that many EGR systems have an EGR tube, which tends to break.
This would cause an exhaust leak and cause the gases to escape to the
atmosphere, never reaching the intake manifold.
EGR VALVE POSITION SENSOR
The EGR valve position sensor has a shaft, which comes into
contact with the EGR diaphragm. This diaphragm is also connected at the
other side to the conic valve itself. As carbon deposits raise the level of
the conic valve, the diaphragm also changes position and so does the
sensor’s signal output. Make a preliminary signal voltage check with the EGR
closed to determine proper calibration. A raised EGR diaphragm caused by
carbon deposits are common. Clean the EGR valve if necessary and then
recheck further.
• Using a hand vacuum pump, apply a steady increasing vacuum
and observe for correct voltage change using an oscilloscope or graphing
meter. Pay close attention to any sudden drops in signal voltage. The
potentiometer inside the position sensor tends to get defective segments or
blind spots over time from excessive wear and tear. Because these signal
voltage drops are hard to detect, it is useful to unscrew and separate the
position sensor from the EGR valve and actuate it with your finger. This
allows better control of the sensor’s shaft.
MAP/O2 SENSOR
Apply vacuum to the EGR valve. This can be done using the
scan tool bi-directional control feature or a hand vacuum pump. Observe the
change in MAP reading and O2 signal voltage, while the EGR is being
activated. The EGR gasses will cause the O2 sensor to go rich (above 500
mV), because of the lack of Oxygen in it. Map sensor readings will go
towards a low of vacuum or high voltage reading (high frequency in case of
FORD), since an open EGR valve also creates an intake leak. Although this
leak is controlled and compensated for by the ECM. Lack of O2 and/or MAP
signal change is an indication of a clogged or stuck closed EGR valve.
The remainder of the testing procedures apply to all the
systems mentioned here.
• Disconnect the EGR solenoid connector, connect a test
light or small light bulb to the EGR solenoid connector (across both leads),
pre-load the vehicle and look for the ECM pulsing the test light. On some
vehicles, the vehicle speed sensor has to be outputting a signal for the ECM
to pulse the solenoid. In such cases, raise one wheel (switch the traction
control system off) and raise the vehicle speed to about 25 MPH or so. This
should activate the EGR solenoid. (It’s also a good idea to lightly tap
on the brakes while spinning the raised wheel. This creates a small load on
the engine, which creates the right conditions for the ECM to actuate the
EGR solenoid.). A Lack of EGR solenoid pulse indicates an electrical
problem or that the ECM is seeing something that is causing the EGR solenoid
not to be activated. Make sure that battery voltage or ground is reaching
one lead of the solenoid connector. Consult the proper wiring diagram first.
Most ECM controlled devices are ground controlled, with a steady battery
voltage applied to one of the leads. If battery voltage or ground is
available but no pulsation from the ECM, then determine if the ECM is at
fault or the ECM is seeing something that prevents is from operating the EGR
solenoid.
• A lack of vehicle speed sensor input, for example, will
prevent the EGR solenoid from being activated and so is the wrong coolant
signal, TPS not registering proper throttle opening, etc.
• Connect a vacuum gauge in place of the EGR valve. Make the
ECM actuate the EGR solenoid by whatever means necessary and observe for a
vacuum reading. Lack of vacuum points to a clogged EGR solenoid, if it was
determined that the ECM is pulsating the solenoid. Either clean or replace
the solenoid.
As previously said before in this book. The first rule of
diagnostics is to know the particular system that it is being worked on.
Generally speaking, the EGR system diagnostics can be split into three
parts, the valve and sensors, vacuum hoses and solenoid, and the electrical
wiring/connector/ECM part of the system. Proper knowledge of each of the
particular components will lead to a correct diagnostics each and every
time.
