For 1990-2009 cars

Checks | Action
--- | ---
DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.
Preliminary |

• | Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .

• | Search for bulletins.

• | Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations. Refer to Engine Controls Schematics .

• | Verify the owner's driving habits, by asking the following questions:

- | Is the A/C or the Defroster mode ON full time?

- | Are the tires at the correct pressure?

- | Is there excessively heavy loads being carried?

- | Is the acceleration rate too much, too often?

- | Is the vehicle used for towing?

• | Remove the air filter element and inspect for dirt or for being restricted. Refer to Air Cleaner Element Replacement .

Fuel System |

• | Inspect the type, quality, and alcohol content of the fuel. Oxygenated fuels have lower energy and may deliver reduced fuel economy. Refer to Fuel System Specifications and Alcohol/Contaminants-in-Fuel Diagnosis .

• | Test the fuel injectors. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .

• | Inspect the fuel system. Refer to Fuel System Diagnosis .

• | Test for incorrect fuel pressure.

• | Test for a restricted fuel filter.

• | Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | Confirm that each injector harness is connected to the correct injector/cylinder. Relocate the injector harnesses as necessary.

• | Inspect for foreign material accumulation in the throttle bore, coking on the throttle valve, or on the throttle shaft. Refer to Throttle Body Inspection and Cleaning . Also inspect for throttle body tampering.

• | Inspect for items which can cause an engine to run rich (long term fuel trim near -13 percent). For a rich condition, refer to DTC P0172 or P0175 .

Sensor/System |

• | Inspect the air intake system and the crankcase for air leaks.

• | Test the crankcase ventilation system for proper operation.

• | Test for proper calibration of the speedometer. Incorrect tire size or axle ratios can affect speedometer calibration and vehicle mileage.

• | Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Detonation/Spark Knock .

• | Inspect the Secondary Air Injection (AIR) system for proper operation.

Ignition System |

• | Test for proper ignition voltage output with a J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis .

• | Test for secondary ignition system leakage. Perform the procedure in the Diagnostic Aids portion of the Electronic Ignition (EI) System Diagnosis .

• | Remove the spark plugs and inspect for the following conditions:

- | Correct heat range

- | Wet plugs

- | Cracks

- | Wear

- | Improper gap

- | Burned electrodes

- | Heavy deposits

• | An improper spark plug gap may cause a driveability concern. Gap the spark plugs using a wire gauge gap tool. Refer to Spark Plug Replacement .

• | Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas, coolant or oil fouled. Refer to Spark Plug Inspection . Once you determine the cause of the fouling, refer to one of the following procedures:

- | For diagnosis of a rich condition, refer to DTC P0172 or P0175 .

- | For diagnosis of coolant contaminated spark plugs, refer to Loss of Coolant in Engine Cooling.

- | For diagnosis of oil fouled spark plugs, refer to Oil Consumption Diagnosis in Engine Mechanical-3.0L (L81).

Engine Cooling System |

• | Inspect the engine coolant level for being low. Refer to Cooling System Draining and Filling in Engine Cooling.

• | Test the engine thermostat for proper operation and correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical |

• | Inspect engine mechanical for the following:

- | Test compression

- | Sticking or leaking valves

- | Worn camshaft lobes

- | Camshaft timing

- | Broken valve springs

- | Excessive oil in combustion chamber-Leaking valve seals

• | For more information, refer to one or more of the following procedures in Engine Mechanical-3.0L (L81):

- | Engine Compression Test

- | Timing Belt Timing Adjustment

- | Oil Consumption Diagnosis

• | Inspect the following components for incorrect basic engine parts:

- | Camshafts

- | Cylinder heads

- | Pistons, etc.

Refer to Engine Mechanical-3.0L (L81) for the proper diagnosis procedures.

Additional Checks |

• | Visually/physically inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on vehicle emission control information label. Refer to Emission Hose Routing Diagram .

• | Inspect for air leaks in the following areas:

- | The throttle body mounting area

- | The mass air flow (MAF) sensor

- | The duct between the MAF and the throttle body

• | Test the transmission torque converter clutch (TCC) for proper operation. The scan tool should indicate an RPM drop, when the system commands the TCC ON. Refer to Torque Converter Diagnosis in Automatic Transmission-4L30-E.

• | Inspect the exhaust system components for possible restrictions:

- | The exhaust system for physical damage

- | The mufflers for heat distress or possible internal failure

- | The catalytic converters for possible plugged pipes by comparing the exhaust system back pressure on each side of engine. Refer to Restricted Exhaust in Engine Exhaust.

• | Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine speed. A sudden increase in engine speed with little change in actual engine speed, indicates that the EMI is present. Inspect for high voltage components near ignition control circuits if a problem exists.

• | Test PNP switch circuit . Refer to DTC P0705 or DTC P0706 .

• | Inspect the intake manifold and the exhaust manifold passages for casting flash.

• | Inspect the brake system, including the parking brake, for dragging or improper operation. Ensure that the vehicle operator does not drive with a foot on the brake pedal.