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

Steps 1-10:

Steps 11-22:

The numbers below refer to the step numbers on the diagnostic table.
3. This step determines if this is a hard or intermittent DTC.
5. This step determines if there is a 5-volt reference.
7. This step verifies whether the sensor is sending a signal back to the PCM.
11. 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.
16. 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.