PACCAR MX (2004 Emissions) Fault Code: P1091 Common Rail Fuel Pressure – Data Erratic, Intermittent or Incorrect

PACCAR MX (2004 Emissions) Fault Code: P1091 Common Rail Fuel Pressure – Data Erratic, Intermittent or Incorrect

SET CONDITIONS:
The PCI ECU (D420) detects that the controlled common rail pressure release valve current differs too much from the learned current during an autonomous test.

DIAGNOSTIC CONDITION:
This diagnostic runs every 30 minutes of engine operation when the following conditions are met for longer than 5 seconds:
1. The engine has been running for more than 30 minutes and
2. During a fuel injection cutoff event and
3. Engine speed is between 1200 and 1600 rpm

IMPORTANT: If the autonomous test passes it is repeated every 30 minutes. If the test fails it is repeated every 5 minutes. If the test fails 3 times subsequently this DTC becomes active.

CORRESPONDING OBD READINESS MONITOR:
Fuel system monitor

RESET CONDITION:
This DTC changes to inactive after the ignition is keyed off for at least 15 seconds and keyed on again.

REPAIR VERIFICATION:
1. Clear all DTCs.
2. Test drive or operate the vehicle to meet the following conditions:
– Corresponding readiness monitor displays ‘Ready’
– Diagnostic conditions are met
– Set conditions are met
3. Did the DTC come back?
– No, repair successful
– Yes, repair unsuccessful

POSSIBLE CAUSES:
1. Common rail pressure release valve leakage

2. Only if a new or different common rail pressure release valve has been installed recently: the “Reset common rail pressure release valve” procedure with the diagnostic service tool was not performed. Investigate whether the reset was performed in the past before performing it again. Resetting the common rail pressure release valve without further investigation of the DTC can lead to recurrence of the DTC.

ADDITIONAL INFORMATION:
1. For this diagnostic the common rail pressure sensor (F854) is checked via an autonomous test. During this test, the rail pressure operating state is: RELEASE VALVE ONLY. The rail pressure is controlled by the common rail pressure release valve (L094) during a fuel cutoff event. The PCI ECU controls the common rail pressure release valve with the learned current to reach a rail pressure of 1500 bar. This is compared with the common rail pressure sensor reading.

2. The current to control the common rail pressure release valve (L094) is learned in the PCI ECU memory. This learned current is determined shortly after the vehicle has left the factory or when the common rail pressure release valve is reset by the diagnostic service tool. During the learning process the diagnostics for the DTC is disabled. The status of learning process and the current cannot be read with the diagnostic service tool.

3. It is possible that the engine does not run anymore when this DTC is active.

4. When this DTC is active:
– The engine torque is reduced.
– The engine runs in protection mode.
– The maximum rail pressure is limited to 1300 bar.