For 1990-2009 cars

Checks | Action
--- | ---
DEFINITION: The engine power output varies under a steady throttle or cruise. The vehicle seems to speed up and slow down with no change in the accelerator pedal position.
Preliminary Checks |

• | Refer to Diagnostic System Check - Engine Controls .

• | Refer to Symptoms - Engine Controls .

• | Educate the driver about normal A/C compressor operation.

• | Use a scan tool in order to verify that the reading of vehicle speed sensor (VSS) matches the reading of the vehicle speedometer. Vehicles with electronic transmissions are excluded because some variation between VSS readings and speedometer readings are considered normal.

Sensor Check |

• | Check the performance of the mass air flow (MAF) sensor. Refer to DTC P0101 .

• | Inspect the performance of the heated oxygen sensors (HO2S) 1. Refer to:

- | DTC P0131

- | DTC P0132

- | DTC P0133

- | DTC P0134

• | Inspect for a shifted or fixed engine coolant temperature (ECT) sensor. Test the ECT sensor using the scan tool. Compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is more than 5°C (9°F) more or less than the ambient air temperature, check for a high resistance in the coolant sensor circuit or the sensor itself. Refer to Temperature Versus Resistance .

• | Check the CMP sensor engine reference signal with a scan tool. Observe the Engine Speed parameter while operating the engine at the speed of the concern. The scan tool should indicate steady and linear RPM readings while running. If the erratic values, such as sudden spikes or drops in engine speed are displayed, the engine reference signal is not stable enough for the engine to run properly. Inspect the camshaft position (CMP) sensor signal rotor and housing for worn bushings or drive coupling. Inspect for electromagnetic interference (EMI) of the reference signal.

Fuel System Checks |

• | Determine whether a rich or lean fueling condition can cause the concern. Drive the vehicle at the speed of the concern. Monitor the Fuel Trim parameters on a scan tool in order to identify the fuel system status.

- | Lean -- The long term fuel trim will be approaching 20 percent. Refer to DTC P0171 Diagnostic Aids.

- | Rich -- The long term fuel trim will be approaching -20 percent. Refer to DTC P0172 Diagnostic Aids.

• | Check the fuel injectors. Perform the Fuel Injector Coil Test Procedure and the Fuel Injector Balance Test Procedure. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Fuel Injector Balance Test with Special Tool .

• | Check the fuel pressure while the condition exists. Refer to Fuel System Diagnosis .

• | Check for contaminated fuel or inferior quality fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Ignition System Checks |

• | Check for adequate secondary ignition voltage with a J 26792 spark tester or an equivalent. Refer to Electronic Ignition (EI) System Diagnosis .

• | Check the spark plugs for correct operation and performance. Refer to Spark Plug Inspection .

• | Check for the correct ignition timing. Refer to Ignition System Timing Adjustment .

Additional Checks |

• | Verify the PCM grounds are clean, tight, and in the proper locations. Refer to Engine Controls Schematics Power and Ground.

• | Check the exhaust system for a restriction. Refer to Restricted Exhaust in Engine Exhaust.

• | Check the generator output voltage.

• | Check the vacuum lines and hoses for kinks, leaks, and correct routing.

• | Check the Service Bulletins for any updates.