Powertrain Controls - ECM/PCM

DTC 55 Chart Fuel Lean Monitor:

Circuit Description:

The ECMs internal circuitry can identify if the vehicle's fuel system is capable of supplying adequate amounts of fuel during heavy acceleration (power enrichment). When a power enrichment mode of operation is requested during "Closed Loop" operation (by heavy acceleration), the ECM will provide more fuel to the engine. Under these conditions, the ECM should detect a "rich" condition. If this "rich" exhaust is not detected at this time, a DTC 55 will set. A plugged fuel filter or restricted fuel line can prevent adequate amounts of fuel from being supplied during power enrichment mode.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 55 will set if:

^ The ECM detects lean oxygen sensor voltage for 8 seconds during power enrichment modes of operation.

^ Wrap a shop towel around the fuel pressure connector to absorb any small amount of fuel leakage that may occur when installing the gage. Ignition "ON," pump pressure should be 284-325 kPa (41-47 psi). This pressure is controlled by spring pressure within the regular assembly. Pressure below 284 kPa (41 psi) may cause a lean condition and may set a DTC 44 or 64. It could also cause hard starting cold and poor driveability. Low enough pressure will cause the engine not to run at all.

^ Restricted flow may allow the engine to run at idle, or low speeds, but may cause a surge and stall when more fuel is required, as when accelerating or driving at high speeds. Low fuel pressure under heavy acceleration conditions may set a DTC 55.

2. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With a scanner enable the fuel pump, pressure should rise above 325 kPa (47 psi) as the valve in the return line is partially closed.

Diagnostic Aids:

A restricted fuel filter can supply adequate amounts of fuel at idle but may not be able to supply enough fuel during heavy acceleration. A vapor lock condition can cause a DTC 55.