GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2005 🢒 Isuzu MD F-Series 🢒 Engine 🢒 Engine Controls - 7.8L 🢒 DTC P1088

Circuit Description

The engine control module (ECM) monitors fuel rail pressure (FRP) using the FRP sensor. If the sensor indicates a pressure more than the commanded rail pressure plus a possible transitional overshoot, the ECM will set DTC P1088 for FRP too high.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1088 Fuel Rail Pressure (FRP) Trim Too High

Conditions for Running the DTC
    • The ignition is ON.
    • The battery voltage is between 10-16 volts.

Conditions for Setting the DTC

Fuel rail pressure is more than 234 mPa (33,900 psi) anytime during the ignition cycle. This indicates the fuel pressure is too high and pressure limiter most likely did not activate.

Action Taken When the DTC Sets
    • The ECM illuminates the malfunction indicator lamp (MIL) and reduced engine power indicator lamp, if fitted, when the diagnostic runs and fails.
    • The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.
    • The ECM limits fuel injection quantity for fail-safe operation.
    • The ECM limits fuel part pressure for fail-safe operator.
    • The ECM closes the exhaust gas recirculation (EGR) valve and stops EGR control.
    • The ECM inhibits cruise control.

Condition for Clearing the DTC
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

If an intermittent condition is suspected, the following may cause an intermittent:

    • Poor connections
    • Misrouted harness
    • Rubbed through wire insulation
    • Broken wire inside the insulation

Inspect for the following conditions:

    • Poor connection at ECM--Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
    • Damaged harness--Inspect the wiring harness for damage. If the harness appears to be OK, observe the DTC P1088 display on the scan tool while moving connectors and wiring harness related to the sensor. A change in the display will indicate the location of the fault.

Important: The Actual Fuel Rail Pressure on scan tool will only update with the engine running or one time after the key is cycled when the ignition has been turned OFF for longer than 10 seconds.

    • An intermittently sticking suction control valve (SCV) may have allowed the fuel pressure to become high enough to open the pressure limiter valve.
    • Normal Actual Fuel Rail Pressure readings on the scan with the engine running in Park or Neutral at idle are around 20-35 mPa (3,650-5,050 psi) and around 60-130 mPa (8,700-18,850 psi) in Park or Neutral at wide open throttle (WOT) (accelerator pedal full travel).
    • A skewed FRP sensor value (shifted to a higher pressure) can set this DTC. The Actual Fuel Rail Pressure on the scan tool should read 0 mPa (0 psi) with the key ON and engine OFF after the engine has stopped for a minimum of 2 minutes.
    • Resistance in the FRP sensor low reference circuit can set this DTC. Ensure that the sensor low reference circuit has no resistance and the connectors are tight and free of corrosion.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Observe the Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Diagnostic Aids

3

Review the DTC.

  1. Using the scan tool, check the DTC.
  2. Clear the DTC.
  3. Bleed the air from the fuel system.
  4. Perform test run and review the DTC.

Is DTC detected?

--

Go to Step 4

System OK

4
  1. Perform the fuel system check.
  2. Check for a poor connection at the fuel rail pressure sensor.
  3. If a problem is found, repair as necessary.

Was a problem found?

--

System OK

Go to Step 5

5
  1. Inspect the attachment state of a supply pump.
  2. If a problem is found, adjust as necessary.

Was a problem found?

--

System OK

Go to Step 6

6

Inspect the fuel pipe.

  1. Stop the engine.
  2. Check the fuel line for damage and leakage.
  3. If a problem is found, repair or replace as necessary.

Was a problem found?

--

System OK

Go to Step 7

7

Inspect the fuel injection state of a injector.

  1. Connect the scan tool.
  2. Turn ON the ignition.
  3. Clear the DTC.
  4. Perform the actuator test.
  5. Check the engine noise.

Change the engine noise?

--

Go to Step 8

Go to Step 9

8

Replace the injector. Refer to Fuel Injector Replacement .

Is the action complete?

--

Go to Step 20

--

9
  1. Test the signal circuit between the engine control module (ECM) and the fuel rail pressure sensor for the following conditions:
    2. • An open or short to ground circuit
    • An open or short to battery voltage
  2. Repair the circuits as necessary.

Did you find and correct the condition?

--

Go to Step 19

Go to Step 10

10

Using the scan tool, check the fuel pressure.

  1. Turn ON the ignition.
  2. Check the engine data on display.

Is the fuel pressure value within the specified range?

0 mPa

Go to Step 11

Go to Step 13

11

Using the scan tool, check the fuel pressure.

  1. Bleed the air from the fuel system.
  2. Engine start
  3. Check the engine data on display.

Is the fuel pressure value within the specified range?

25-40 MPa (3,626-5,801 psi

Go to Step 12

Go to Step 13

12

Check the fuel pressure in fuel rail.

  1. Bleed the air from the system.
  2. Recheck the DTC.
  3. Check the fuel pressure.

Was a problem found?

--

Go to Step 13

Go to Step 11

13

Check the fuel rail pressure sensor.

Was a problem found?

--

Go to Step 17

Go to Step 14

14
  1. Turn ON the ignition.
  2. Check the fuel leakage on the fuel line.
  3. Turn OFF the ignition.
  4. Check the flow damper noise.

Was a problem found?

--

Go to Step 15

Go to Step 16

15

Check the fuel pipe.

  1. Engine is stop
  2. If the fuel leakage is found, repair or replace as necessary.

Was a problem found?

--

Go to Step 20

Go to Step 16

16
  1. Bleed the air from the fuel system.
  2. Start the engine.
  3. Compare the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure parameters with a scan tool.
  4. Operate the accelerator pedal slowly while observing the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure parameters.

Did the actual fuel rail pressure follow closely to desired fuel rail pressure through the entire operating range?

--

Go to Step 17

Go to Step 18

17

Replace the fuel rail system assembly.

Is the action complete?

--

Go to Step 20

--

18

Review the DTC.

  1. Using the scan tool, check the DTC.
  2. Clear the DTC.
  3. Bleed the air from the fuel system.
  4. Perform test run and review the DTC.

Is DTC detected?

--

Go to Step 19

Go to Step 20

19

Important: The replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module Programming and Setup , Engine Control Module Replacement/Restore Fuel Rate Data , and Fuel Injector Flow Rate Programming .

Did you complete the replacement?

--

Go to Step 20

--

20
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 21

21

Observe the stored information. Capture Info with a scan tool.

Are there any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK