Common rail Euro-III engine controls fuel pressure with one control valve which controls inlet or outlet of fuel line. Inlet control type has disadvantage of activating fast increase of fuel pressure when initial starting or sudden acceleration because the procedure to get high fuel pressure takes many steps like low pressure pump ? control valve ? high pressure pump ? common rail. However, outlet control type has advantage of quick increase of fuel pressure but there are weak point such as loss of engine power and fuel temperatures up. Therefore, CRDi Euro-IV has been adopted "Dual Pressure Control type" so that precise and quick increase of fuel pressure can be easily established with various engine condition. Dual Pressure Control" type means that CRDi Euro-IV engine has the inlet pressure from high pressure pump and outlet pressure of common rail simultaneously.
Fuel pressure gets high if fuel pressure control valve(P-PRV, Pump Pressure Regulating Valve) installed between low pressure pump and high pressure pump(inlet side) closes the fuel line with increased duty values controlled by ECM. With rail pressure control valve(PRV, Pressure Regulator Valve), the higher duty values from ECM the higher rail pressure is set because it closes outlet of common rail.
ECM controls current for Both P-PRV installed on high pressure pump and PRV installed on common rail after sensing RPS installed on common rail so that CRDi engine gets optimum rail pressure control corresponding with current engine RPM and load condition. If the condition that is out of target rail pressure is occurred due to mechanical or electrical problem, ECM controls fuel amount to minimum limit to get limp home and set DTC. The relevant to this DTC is to indirectly diagnose mechanical operating condition of feed line of low pressure, high pressure pump, P-PRV and PRV etc. through out the current value of P-PRV, PRV and RPS output value.
DTC P0252 is set when high temperature is detected in P-PRV control monitoring circuit of ECM internal for 0.22sec. It is retrieved when it is malfunction of ECM internal circuit.
Item
Detecting Condition
Possible Cause
DTC Strategy
Voltage monitoring
P-PRV circuit
P-PRV
ECM
Enable Conditions
IG key ON
Threshold Value
High temperature is detected because it is short to battery in P-PRV monitoring circuit of ECM internal.
Diagnostic Time
0.22 sec.
Fail Safe
Fuel cut
NO
-
EGR Off
Fuel Limit
YES
Check Lamp
ON
P-PRV resistance
Operating frequency
2.9 ~ 3.15? (20?)
172 Hz
Fig.1) Signal waveform of fuel pressure regulater(pump) at idle. The duty is approx. 31%
Fig.2) Signal waveform of fuel pressure regulater(pump) at acceleration.
The duty is approx. 40% when a engine load increases.
(Rail pressure and fuel pressure regulator duty increases in proportion to acceleration)