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 Powertrain On Board Diagnostic (OBD) System Check .

• | Refer to Symptoms .

• | 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 Mass Air Flow (MAF) Sensor Performance .

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

- | DTC P0131 HO2S Circuit Low Voltage Sensor 1

- | DTC P0132 HO2S Circuit High Voltage Sensor 1

- | DTC P0133 HO2S Slow Response Sensor 1

- | DTC P0134 HO2S Circuit Insufficient Activity Sensor 1

• | 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 Fuel Trim System Lean Diagnostic Aids.

- | Rich -- The long term fuel trim will be approaching -20 percent. Refer to DTC P0172 Fuel Trim System Rich 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 .

• | 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.

• | Check the spark plugs for any of the following conditions:

- | Wet plugs

- | A terminal screw that is bent or loose

- | A ceramic insulator that has cracks, carbon tracking or deposits

- | A tip insulator that has cracks, carbon tracking, deposits, or is loose

- | A center electrode that is loose

- | An incorrect spark plug gap that is too large or too small

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

Additional Checks |

• | Verify the PCM grounds are clean, tight, and in the proper locations. Refer to

#### Power and Ground

Engine Controls Components
MIL and DLC
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
G100, G101 and G102
H/L, HTR, IG and LAMP Fuses
HAZ, D/L, TAIL, STOP and DOME Fuses
BATT, ABS, FI and ACC Fuses
.

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

• | Check the generator output voltage.

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

• | Check the Service Bulletins for any updates.