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 shuts engine down 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 measured rail pressure is higher than target rail pressure by more than 200bar when rail pressure is regulated by rail pressure control valve(PCV) or rail pressure is above the max limiting 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 fuel 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 PRESSURE-TARGET", "RAIL PRESS. REGULATOR1", "INJ. PUMP REGULATOR" parameter on the Scantool.

Specification :

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

FUEL PRESSURE-TARGET : 28 ± 5 Mpa

RAIL PRESS. REGULATOR1 : 20 ± 5%

INJ. PUMP REGULATOR : 40 ± 5%

Check if "FUEL PRESSURE MEASURED"data is similar to "FUEL PRESSURE-TARGET". 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 PRESSURE-TARGET", 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", "RAIL PRESS. REGULATOR1", "INJ. PUMP REGULATOR" parameter on the Scantool.

Specification :

	Idle(without load)	Accelerating(stall test)	Diagnosis
INJ. PUMP REGULATOR	38 ± 5%	32 ± 5%	duty decreases
FUEL PRESSURE MEASURED	28.5 ± 5 Mpa	145 ± 10 Mpa	press. increases
RAIL PRESS. REGULATOR1	19 ± 5%	48 ± 5%	duty increases

note

The waveform of fuel metering unit installed at high pressure pump(fuel detecting MPROP) shows 38% duty at idle, duty drops to approx. 32% at acceleration to raise rail pressure. Duty drop means the decrease of current.

→ Fuel delivered to common rail increases as current drops.

The waveform of rail pressure control valve installed at common rail shows 19% duty at idle, duty rises to approx. 48% at acceleration to raise rail pressure. Duty rise means the increase of current.

→ If current rises, the returning quantity of fuel deliverd to common rail decreases and common rail pressure rises.

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.