GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Express 🢒 Engine 🢒 Engine Electrical 🢒 DTC P0251

Circuit Description

The optical sensor provides a pump cam signal to the powertrain control module (PCM) by counting the pulses from the sensor that monitors slots on the data track disk that is located inside the injection pump. The pump cam is one of the most important PCM inputs for fuel control and timing. This test monitors the number of crankshaft position pulses that have occurred since the last cam pulse. The physical one-to-one correspondence between the pump cam and the crankshaft implies that if more crank pulses than cam pulses are detected, the cam pulses have been missed.

Conditions for Running the DTC

The engine is operating.

Conditions for Setting the DTC

Eight consecutive cam pulses are missing.

or

An average of eight cam pulses are missing.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Intermittent DTCs P0251, P0370, and P1216 may be caused by air entering the fuel system when fuel levels are less than 1/8 of a tank while the driver is performing hard acceleration or turning maneuvers. A DTC P0251, P0370, and P1216 may set if the vehicle has run out of fuel. Verify the customer's driving habits to determine if the vehicle has been performing in these manners. If the vehicle has been performing in these conditions, bleed the fuel system of all air and test drive the vehicle.

When the PCM is in backup fuel, fast idle and faulty performance problems will exist.

An intermittent condition may be caused by the following conditions:

    • Faulty connections
    • Rubbed-through wire insulation
    • Broken wire inside the insulation

Review Freeze Frame/Failure Records. If code P0251 is set at 0 RPM, the code is false and should be ignored.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines if this is a hard or intermittent DTC.

    2.

  2. This step determines if there is a 5-volt reference.

    3.

  3. This step verifies whether the sensor is sending a signal back to the PCM.

    4.

  4. The PCM supplies 5 volts on the signal circuit. This step determines if that voltage is present, not present, or if too much voltage is present.

    5.

  5. This step determines if the signal circuit is shorted to 5 volt. A normal CAM signal circuit will have 3-5 mA. Any reading over 35 mA indicates a short to 5 volt.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame/Failure Records for reference. The data in the control module is deleted once the Clear Info function is used.

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Is DTC P0370 set also?

--

Go to DTC P0370

Go to Step 3

3

  1. Start and idle the engine.
  2. With the throttle closed, observe the CAM Ref Missed display on the scan tool.

Does scan tool display the specified value?

8

Go to Step 5

Go to Step 4

4

The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs are stored, refer to the applicable DTC tables first.

Are any additional DTCs stored?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Diagnostic Aids

5

  1. Turn OFF the ignition.
  2. Disconnect the optical/fuel temperature sensor electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Use a DMM in order to measure the voltage between the optical sensor 5-volt reference circuit at the harness connector and chassis ground .

Does the voltage measure within the specified range?

4.8-5.2 V

Go to Step 6

Go to Step 8

6

Probe the sensor ground circuit with an J 35616-200 Unpowered Test Light connected to B+ at the harness connector.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 9

7

  1. Reconnect the optical/fuel temperature sensor electrical connector.
  2. Start and idle the engine.
  3. With a scan tool, command the engine to 900 RPM.
  4. With a DMM set on the Hertz (Hz) scale, backprobe the pump CAM signal circuit at the PCM.

Is the Hz reading within the specified range?

57-63 Hz

Go to Step 15

Go to Step 11

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM and check the optical sensor 5-volt reference circuit for the following conditions:
    3. • An open
    • A short to ground
    • A short to the sensor ground circuit
  3. If the optical sensor 5-volt reference circuit is open or shorted to ground, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 21

Go to Step 10

9
  1. Check for an open in the sensor ground circuit.
  2. If you find a problem, repair the problem as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 21

Go to Step 15

10

Check the optical/fuel temperature 5-volt reference circuit for a faulty connection at the PCM and replace the terminal if necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 21

Go to Step 20

11

  1. Turn ON the ignition, with the engine OFF.
  2. Disconnect the optical/fuel temperature sensor electrical connector.
  3. With a DMM connected to a ground, probe the CAM signal circuit.

Does the voltage measure within the specified range?

4.8-5.2 V

Go to Step 16

Go to Step 12

12

Does the voltage measure more than the specified value?

5.2 V

Go to Step 13

Go to Step 14

13

Repair the short to battery/ignition voltage on the CAM signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 21

--

14
  1. Turn OFF the ignition.
  2. Check the CAM signal circuit for an open or short to ground.
  3. Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 21

Go to Step 15

15

Check for a proper connection at the PCM harness terminal and replace the terminal if necessary. Refer to Wiring Repairs in Wiring Systems.

Did you perform a repair?

--

Go to Step 21

Go to Step 20

16

With a DMM set to the mA scale, measure the current between the CAM signal circuit and the ground circuit at the optical/fuel temperature sensor electrical connector.

Does the current measure less than the specified value?

35 mA

Go to Step 18

Go to Step 17

17

Repair the short to reference voltage on the CAM signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 21

--

18
  1. Check for a faulty connection at the injection pump.
  2. If you find a problem, repair the problem as necessary.

Did you perform a repair?

--

Go to Step 21

Go to Step 19

19

Important: The new injection pump must be timed. Refer to Fuel Injection Pump Timing Adjustment .

Replace the injection pump. Refer to Fuel Injection Pump Replacement .

Did you complete the replacement?

--

Go to Step 21

--

20

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 21

--

21
  1. Use a scan tool in order to clear the DTCs.
  2. Start the engine.
  3. Allow the engine to idle until the normal operating temperature is reached.
  4. Select DTC and the Specific DTC function.
  5. Enter the DTC number that was set.
  6. Operate the vehicle within the Conditions for Setting this DTC until the scan tool indicates the diagnostic ran.

Does the scan tool indicate the diagnostic passed?

--

Go to Step 22

Go to Step 2

22

Does the scan tool display any additional undiagnosed DTCs?

--

Go to Diagnostic Trouble Code (DTC) List

System OK