As inputted rail pressure sensor signal, ECM of Commom rail diesel engine controls fuel metering unit(MPROP-intergrated with high press. pump) and rail pressure control valve(PCV-integrated with common rail) in order to maintain optimum rail pressure according to current engine rpm and load. However when the problem that leads rail pressure to out of target value intended by ECM occurs due to mechanical or electronical reason, ECM limits engine performance and sets DTC by limiting fuel (stops injector operation) in order to prevent engine from being controlled abnormally. "rail pressure monitoring error" is the DTC which diagnose 1).supplying state of low pressure fuel and 2).mechanical operating conditions of high pressure pump and 3). RPCV indirectly based on RPS output voltage and RPS duty. thus, repair relavant to this DTC requires mechanics the total understand of fuel system.

P0088 is set if rail pressure is above the upper limit value. This code is due to 1)fuel more than target value supplied to common rail or 2)poor return of fuel supplied to common rail or 3)short to high voltage line in rail press. sensor.

1.

Monitoring rail pressure data

(1)

Connect Scantool to Data Link Connector (DLC).

(2)

Warm engine up to normal operating temperature.

(3)

Turn "OFF" electrical devices and A/C.

(4)

Monitor "FUEL PRESSURE MEASURED", "FUEL PRESS. S/POINT", "RAIL PRESS. REGULATOR1", "INJ. PUMP REGULATOR" parameter on the Scantool.

Specification : FUEL PRESSURE MEASURED similar to "FUEL PRESSURE-TARGET"

FUEL PRESS. S/POINT : 28.5 ± 5 Mpa

RAIL PRESS. REGULATOR1 : 17 ± 5%

INJ. PUMP REGULATOR : 35 ± 5%

Check if "FUEL PRESSURE MEASURED"data is similar to "FUEL PRESS. S/POINT". Not only former two data but also "RAIL PRESS. REGULATOR1" and "INJ. PUMP REGULATOR"should be monitored carefully. Although "FUEL PRESSURE MEASURED" is similar to "FUEL PRESS. S/POINT", if "RAIL PRESS. REGULATOR1" and "INJ. PUMP REGULATOR" is out of specification, it means wear, leakage, stuck of fuel system.

2.

Monitoring rail pressure data at acceleration(loading condition).

(1)

Connect Scantool to Data Link Connector (DLC).

(2)

Warm engine up to normal operating temperature.

(3)

Turn "OFF" electrical devices and A/C.

(4)

Monitor "FUEL PRESSURE MEASURED", "INJ. PUMP REGULATOR","RAIL PRESS. REGULATOR1" parameter on the Scantool.

Specification :

	Idle(without load)	Accelerating(stall test)	Diagnosis
FUEL PRESSURE MEASURED	28 ± 5 Mpa	above 75 Mpa	press. increases
INJ. PUMP REGULATOR	35 ± 5%	38 ± 5%	duty increases
RAIL PRESS. REGULATOR1	17 ± 5%	45 ± 5%	duty increases

note

The waveform of fuel pressure regulator valve installed at high pressure pump(fuel detecting MPROP) shows 35 ± 5% duty at idle, duty rises to approx. 40% at acceleration to raise rail pressure.

▶ Fuel amonut delivered from injection pump to rail is increases if the duty of "Fuel pressure regulator" increases

The waveform of rail pressure regulator valve installed at common rail shows 17 ± 5% duty at idle, duty rises to approx. 45% at acceleration to raise rail pressure.

▶ If the current increases, pressure in common rail will increase. because fuel return amount delivered to commom rail will decrease.

After a repair, it is essential to verify that the fault is corrected.

5.

Are any DTCs recorded ?

▶ Go to the DTC guide of recorded NO. in Scantool.

▶ System operates within specification.