GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 2000 🢒 Intrigue 🢒 Engine 🢒 Engine Controls - 3.5L 🢒 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 Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Verify that the PCM grounds are clean, tight, and in the correct locations.
    • Check the owner's driving habits.
       - Is the A/C on full time (Defroster mode on)?
       - Are the tires at the correct pressure?
       - Are any excessively heavy loads being carried?
       - Is the acceleration too much, too often?
    • Inspect the air cleaner element for dirt and restrictions. Replace the element as necessary.

Fuel System
    • Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .
    • Check the fuel pressure. Refer to Fuel System Pressure Test .
    • Check for a restricted fuel filter.
    • Check for contaminated fuel. Refer to Fuel Quality Diagnosis .
    • Check for an incorrect minimum idle speed.
    • Verify that each fuel injector harness is connected to the correct injector and cylinder according to the firing order. Relocate any fuel injector harnesses as necessary.
    • Check for fuel injectors that are shorted internally. Compare the injector resistances.
    • Check for fuel in the pressure regulator vacuum hose. If any fuel is present replace the regulator assembly.
    • Check for any foreign material accumulation in the throttle bore, and any coking on the throttle valve or on the throttle shaft. Also inspect for any throttle body tampering.
    • Check the items that can cause an engine to run rich.
    • Check the items that can cause an engine to run lean.

Sensor and System
    • Check the air intake system and the crankcase for air leaks.
    • Check the crankcase ventilation valve for the correct operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Check for the correct calibration of the speedometer. Connect a J 33431-B Signal Generator to the vehicle speed sensor (VSS) electrical connector. With the ignition on and the tester on, monitor the speedometer. The speedometer should indicate the speed being generated.
    • Using a scan tool, monitor the throttle position (TP) sensor voltage. The TP sensor voltage should be less than 0.9 volts at rest. If the TP sensor voltage is more than 0.9 volt, inspect the throttle body and the accelerator cables for binding.
    • Using a scan tool, monitor the knock sensor (KS) system for any excessive spark retard activity.

Ignition System
    • Remove the spark plugs and check for the following:
       - Any wet plugs
       - Any cracks
       - Any wear
       - An improper gap
       - Any burned electrodes
       - Any heavy deposits
    • An improper spark plug gap will cause a driveability problem. The spark plug gapping should be done with a wire gauge gap tool. If the spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Check for any loose ignition control module grounds.
    • Check for the correct ignition voltage output. Refer to Secondary Ignition Diagnosis .

Engine Cooling System
    • Check for a low engine coolant level.
    • Check for the correct engine thermostat operation and heat range. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

Engine Mechanical

Check for the following conditions:

    • A low compression
    • Any sticking or leaking valves
    • Any worn camshaft lobes
    • An incorrect valve timing
    • Any worn rocker arms
    • Any broken valve springs
    • Any excessive oil in the combustion chamber may indicate leaking valve seals
    • Check for any incorrect basic engine parts. Inspect for the following:
       - The camshafts
       - The cylinder heads
       - The pistons, etc.
    • Refer to Base Engine Misfire Diagnosis in Engine Mechanical for diagnostic procedures.

Additional Checks
    • Visually and physically check the vacuum hoses for splits, kinks, and the correct connections and routing as shown on the Vehicle Emission Control Information label.
    • Check the torque converter clutch (TCC) operation. A scan tool should indicate an RPM drop when the TCC is commanded on. Refer to Diagnostic Starting Point - Automatic Transmission in Automatic Transaxle.
    • Check the exhaust system for any restriction. Refer to Restricted Exhaust System Check .
    •  Check for any improper harness routing. An engine miss condition can be caused by electromagnetic interference (EMI) on some engine controls circuits. Use the scan tool to check for any erratic sensor values or excessive changes in the indicated RPM with little change in the actual engine RPM. This may indicate EMI is present. If a problem exists, check for any improper routing of the harness near the high voltage components or the high current components.
    •  Check the charging system. A bad diode in the alternator can allow A/C noise to enter the electrical system. This may affect the engine controls system operation. Refer to Diagnostic Starting Point .
    • Check the transaxle range switch circuit. Verify that the scan tool indicates the vehicle is in drive with the gear selector in Drive or in Overdrive.