P0251

CIRCUIT DESCRIPTION
The optical sensor provides a pump cam signal to the Powertrain Control Module (PCM) by counting pulses on the sensor disk located in the injection pump. The pump cam is one of the most important inputs by the PCM 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 if more crank pulses are detected than cam pulses, cam pulses have been missed.

CONDITIONS FOR RUNNING THE DTC
^ The PCM performs this Diagnostic Trouble Code (DTC) diagnostic continuously.
^ The engine is operating.

CONDITIONS FOR SETTING THE DTC
^ The number of consecutive missing cam pulses is more than or equal to 8.
^ All diagnostic set conditions are met for 2 seconds.

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.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
Intermittent DTCs P0251, P0370, and P1216 may be caused by air entering the fuel system when fuel levels get below 1/8 of a tank while performing hard acceleration or turning maneuvers. It is also possible that a P0251, P0370 and P1216 will set the if vehicle has run out of fuel. Customer driving habits should be checked 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 poor performance problems will exist.

An intermittent can be caused by the following conditions:
^ Poor connections
^ Rubbed through wire insulation
^ Broken wire inside the insulation

TEST DESCRIPTION

Steps 1-12:

Steps 13-22:

The numbers below refer to the step numbers on the diagnostic table.
3. This step will determine if this is a hard or intermittent DTC.
5. This step will determine if there is a 5-volt reference.
7. This step will check to see if 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 too much voltage is present.
16. This step determines if the signal circuit is shorted to 5 volts. A normal CAM signal circuit will have 3-5 mA. Any reading over 50 mA indicates a short to 5 volts.