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 .

• | Check for normal A/C compressor operation. Refer to Symptoms - HVAC Systems - Manual in HVAC Systems.

• | 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 or P0151

- | DTC P0132 or P0152

- | DTC P0133 or P0153

- | DTC P0134 or P0154

• | Inspect for a shifted or fixed engine coolant temperature (ECT) sensor. With the scan tool, compare the engine coolant temperature of a cold engine with the ambient air temperature. A difference of more than 5°C (9°F) in temperatures may indicate high resistance in the coolant sensor circuit or the sensor. Refer to Temperature Versus Resistance .

• | Check the camshaft position (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 |

• | Use the scan tool to check for an overly rich or lean fuel mixture. Drive the vehicle at the speed of the concern. Monitor the Fuel Trim parameters on a scan tool in order to identify the fuel trim status.

- | Lean -- The long term fuel trim will be approaching 20 percent. Refer to Diagnostic Aids in DTC P0171 and DTC P0174 .

- | Rich -- The long term fuel trim will be approaching -20 percent. Refer to Diagnostic Aids in DTC P0172 and DTC P0175 .

• | Check the fuel injectors for correct operation and performance. 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 for damaged or worn spark plugs. Refer to Spark Plug Inspection .

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

Additional Checks |

• | Verify the powertrain control module (PCM) grounds are clean, tight, and in the proper locations. Refer to Engine Controls Schematics .

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

• | Check the generator output voltage. Lower than normal system voltage can affect the ability of the engine contol module to operate the ignition and fuel delivery systems.

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

• | Check the operation of the AC system. Excessive AC system pressure or erattic AC compressor operation can affect engine performance. Refer to Symptoms - HVAC Systems - Manual in HVAC Systems.

• | Check the Service Bulletins for any updates.