GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Grand Prix 🢒 Engine 🢒 Engine Controls - 3.1L 🢒 Poor Fuel Economy

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

Preliminary
    • Refer to Intermittent Conditions .
    •  Search for bulletins.
    •  Check owners driving habits.
       -  Is the A/C ON full time (Defroster mode ON)?
       -  Are tires at correct pressure?
       -  Are excessively heavy loads being carried?
       -  Is acceleration too much, too often?

Fuel System
    • Check the fuel pressure. Refer to Fuel System Pressure Test .
    •  Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    •  Ensure each injector harness is connected to the correct injector/cylinder.
    •  Check for fuel in the pressure regulator vacuum hose.

Sensor/System
    • Check for 24X Crankshaft position and Camshaft position sensors on scan tool. If both are not responding, check the sensor feed circuit (Both sensors use a common feed circuit).
    •  Check Air intake system and crankcase for air leaks. Refer to Air Intake System Description and Crankcase Ventilation System Description .
    •  Check for proper calibration of speedometer. Connect J 33431-B Signal Generator to VSS electrical connector. Ignition on, tester on and monitor the speedometer.

Ignition System
    • Check for proper ignition voltage output using the following steps:
       -  Clip the J 26792 spark tester to engine ground.
       -  Connect one end of a spark plug wire to the J 26792 spark tester; leave the other end of the spark plug wire to the coil being tested.
       -  Connect one end of another spark plug wire to the other coil tower, connect the other end of the spark plug wire to ground.
       -  Crank the engine while observing the J 26792 spark tester; a spark should be observed.
       -  Repeat the above steps for each coil.
    •  If adequate spark is not present at the coil(s), check for the following conditions:
       -  Coil(s)--Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

Coil Resistance
5000-7000 ohms (5K-9K ohms).

       -  Spark Plug wires--Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
600ohms per foot (1 968ohms per meter).

    Important: :  Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. Ignition components will arc to ground when a secondary component is faulty.

       -  Defective module.
       -  Ignition System Wiring--Loose ignition module feed or ground connection, or damaged system wiring.
    •  Remove spark plugs and check for the following:
       -  Fouled plugs
       -  Cracks
       -  Wear
       -  Improper gap
       -  Burned or damaged electrodes
       -  Improper heat range or reach
    •  If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Cooling System
    • Check engine coolant level for being.
    •  Check for incorrect or faulty engine thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Check engine mechanical for the following:
       -  Check compression.
       -  Sticking or leaking valves.
       -  Worn camshaft lobes.
       -  Valve timing.
       -  Bent push rods.
       -  Worn rocker arms.
       -  Broken valve springs.
       -  Excessive oil in combustion chamber-Leaking valve seals.
    •  For incorrect basic engine parts. Inspect for the following:
       -  Camshaft
       -  Cylinder heads
       -  Pistons, etc.
    •  Refer to Base Engine Misfire Diagnosis .

Additional Checks
    •  Check TCC operation. Refer to Torque Converter Clutch in Transaxle Diagnosis.
    •  Check exhaust system for possible restriction. Check for the following:
       -  Inspect exhaust system for damaged or collapsed pipes.
       -  Inspect mufflers for heat distress or possible internal failure.
       -  For possible plugged three-way catalytic converter. Refer to Restricted Exhaust System Check and Exhaust Leakage - Not OBD II .