GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Monte Carlo 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 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
    • Search for bulletins.
    • Check owners driving habits.
       - Is 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 the fuel injectors using the following procedures:
      •  Fuel Injector Balance 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 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-C (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 spark tester J 26792 to engine ground.
       - Connect one end of a spark plug wire to the 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 spark tester; a spark should be observed.
       - Repeat the above steps for each coil.
    • If spark is not present at the plugs(s), check for the following conditions:
       - Inspect the coil(s) for cracks, carbon tracking/arcing, or a secondary resistance value outside the specified range.

Coil Resistance
5000 - 8000 ohms (5K - 8K ohms)

       - Inspect the spark plug wires for 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
600 ohms per ft.

   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 malfunctioning.

       - Inspect for a malfunctioning ignition control module.
       - Inspect the ignition system wiring for loose ignition module feed or ground connection, or damaged system wiring.
    • Remove spark plugs and check for the following:
       - Fouled plugs
       - Cracks

   Important:: If carbon tracking is detected on any component, replace that component and it's mate.

       - Carbon tracking on the porcelain and pitting on the plug wire contact
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach. Refer to Maintenance Items .
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Check for the following intermittent ignition system conditions:
       - CKP Sync or CKP 18X Signal.
       - Ignition Feed circuit or sensor ground circuit to the crankshaft position sensor or camshaft position sensor.
       - 3X reference signal.
       - camshaft position signal,

Engine Cooling System
    • Check engine coolant level for being low.
    • Check for incorrect or faulty engine thermostat.

Engine Mechanical
    • Check engine mechanical for the following:
       - Check compression.
       - Sticking or leaking valves.
       - Worn camshaft lobe(s).
       - 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.

Additional Checks
    • Check TCC operation.
    • 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.