GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1999 🢒 Shelby Series 1 🢒 Engine 🢒 Engine Controls - 4.0L 🢒 Poor Fuel Economy

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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 .
    • Search for bulletins.
    • Inspect the PCM grounds for being clean, tight, and in the proper locations. Refer to Engine Controls Schematics .
    • How are the owner driving habits.
       - 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?
    • Remove the air filter element and inspect for dirt or for being restricted. Refer to Air Filter Element Service . Replace as necessary.

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 for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Test for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Test for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect that each injector harness is connected to the correct injector or cylinder according to the firing order. 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. For a rich condition, refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 and DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1 .

Sensor/System
    • Inspect the air intake system and the crankcase for air leaks.
    • Test the crankcase ventilation valve for proper operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Test for proper calibration of the speedometer. Connect the J 33431-B signal generator to the vehicle speed sensor (VSS) electrical connector. Turn ON the ignition with the tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph). If not, refer to Gauge System Check.
    • Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Test for proper ignition voltage output with the J 26792 spark tester.
    • 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 causes a driveability concern. Gap the spark plugs using a wire gauge gap tool.
    • Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas, coolant or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 or DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition. Refer to Spark Plug Visual Diagnosis for diagnosis of coolant or oil fouled spark plugs.
    • Visually and physically inspect the secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
       - Ignition coils for cracks or carbon tracking
    • Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply water.
    • Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Cooling System
    • Inspect the engine coolant level for being low.
    • Test the engine thermostat for proper operation and correct heat range.

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Test 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 the following components:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Visually and 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 the following exhaust system components for possible restrictions:
       - The exhaust system for physical damage
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
       - The three-way catalytic converters (TWC) for possible plugged pipes by comparing the exhaust system back pressure on each side of engine
    • 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 change, indicates that the EMI is present. Inspect for high voltage components near ignition control circuits if a problem exists.
    • Inspect for faulty engine mounts.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.
    • Inspect the brake system for dragging or improper operation. Ensure that the vehicle operator does not drive with a foot on the brake pedal.