P0251

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION
The optical sensor provides a pump cam signal to the PCM by counting the pulses on the sensor which is disk located inside of 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 that it more crank pulses are detected than cam pulses, the cam pulses have been missed. This is a type A DTC.

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
Backup fuel.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn the Malfunction Indicator Lamp (MIL) off after three consecutive trips without a fault condition.
^ A History DTC will clear after forty consecutive warm-up cycles during which the diagnostic does not fail (the coolant temperature has risen 5°C (40°F) from the start up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle).
^ Use of a Scan Tool will clear the DTC codes

DIAGNOSTIC AIDS
When the PCM is in backup fuel, fast idle and poor performance problems will exist. A P0251 and P0370 may possibly set if the vehicle has run out of fuel. An intermittent condition may be caused by the following:
^ Poor connections.
^ A rubbed through wire insulation.
^ A broken wire inside of the insulation.

TEST DESCRIPTION
The number(s) below refer to the Step number(s) on the Diagnostic Table.
2. This Step will determine if this is a hard or intermittent DTC.
4. This Step will determine if there is a 5 volt reference.
6. This Step will check to see if the sensor is sending a signal back to the PCM.

***This is TSB update 67-65-39, dated January 1997****

Subject:
Intermittent Malfunction Indicator Lamp (MIL) with DTCs P0251, P0370, P1216 and Stumble, Stall, Hard Start (Check Fuel Tank Level)

Condition
Some owners may experience an intermittent Malfunction indicator Lamp (MIL) associated with history DTC's P0251, P0370, P1216. Also, owners may experience a stumble, hard start or stall. It is possible that the technician may not be able to duplicate the driveability concern but will notice history DTCs on the Tech 2 scanner.

Cause
Air entering the fuel system when fuel levels get below 1/8 of a tank during hard acceleration or turning maneuvers.

Correction
Check fuel tank level, if necessary fill tank to at least 1/2 full and purge air from fuel system using procedure in Service Manual. Erase DTCs on Tech 2 scanner. Additionally, inform customer of conditions associated with low fuel levels indicated in the Diesel Supplement Owner's Manual.