Tip & How-To about Mercedes-Benz E-Class
1.7) DFPE - Differential Feedback Pressure
Exhaust (sensor)
What is it? This is an electrical device that
measures the differential pressure between two ports on the pipe that supplies
the exhaust gases to the EGR.
Where is it located? The DFPE is located near to both the EGR and
the exhaust feed-pipe to the EGR. THE
DFPE makes two piped connections to the exhaust feed-pipe in front of the EGR
valve
How does it work? The two pipes on the exhaust feed-pipe are
connected at two separate sites. When
exhaust flows through the feed-pipe it creates a differential in pressure between
the two ports and the DPFE detects this and responds by altering a voltage
output to the ECU. The ECU is able to
detect for how long and by how much the EGR is open by monitoring the DFPE output. Until the engine has warmed the DFPE signal
should indicate no EGR activity as the ECU disables the EGR from opening when coolant
(engine) temperature readings are low.
The EGR is kept closed by either the ECU not sending voltage to the EGR
actuator (in cases of direct motor/solenoid control of the EGR) or by not
charging the vacuum control solenoid valve on the vacuum line to the EGR
(in cases where vacuum is used to actuate the EGR).
Symptoms of faulty DFPE
The DFPE may misread the differential pressure
leading to the ECU opening or closing the EGR valve at the wrong time. The symptoms mimic very much the effects of a
faulty EGR namely:-
Note: Sometimes
a DPFE fault can be misdiagnosed:
1) If the coolant
temperature sensor is at fault and sends an incorrect 'the engine is warm'
signal then the ECU responds by opening the EGR valve prematurely causing the
engine to stumble and stall. This
pattern looks very much like a fault from the DPFE when in fact the DPFE might
be working perfectly.
2) If the vacuum
control valve fails to open when instructed. Apply 5 volts between the socket pins on the
valve to check for valve opening.
How to Check? Applying pressure to one or the
other ports on the DPFE should result in a change in electrical output as
detected by a volt meter. No change in voltage
output indicates the DFPE is faulty.
How to Fix? Make sure the pipes to the DFPE
are free of leaks and clear of carbon deposits.
If the DFPE is itself faulty then it must be replaced.
NEXT 1.8) PCV Positive Crankcase Vent valve
EGR Vacuum Regulator Solenoid
The EGR vacuum regulator solenoid (Figure 88) is an electromagnetic device which is used to regulate the vacuum supply to the EGR valve. The solenoid contains a coil which magnetically controls the position of a disc to regulate the vacuum. As the duty cycle to the coil increases, the vacuum signal passed through the solenoid to the EGR valve also increases. Vacuum not directed to the EGR valve is vented through the solenoid vent to atmosphere. Note that at 0% duty cycle (no electrical signal applied), the EGR vacuum regulator solenoid allows some vacuum to pass, but not enough to open the EGR valve. 
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