GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1999 🢒 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.
    • Check the PCM grounds for being clean, tight and in the proper locations.
    • Check the owners driving habits.
       - Is A/C on full time (Defroster mode on)?
       - Are tires at the correct pressure?
       - Are excessively heavy loads being carried?
       - Is acceleration too much, too often?
    • Remove and check the air filter element for dirt, or for being restricted. Replace as necessary.

Fuel System
    • Perform the injector balance test. Refer to Fuel Injector Balance Test .
    • Check 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 incorrect minimum idle speed.
    • Check that each injector harness is connected to the correct injector/cylinder according to the firing order. Relocate injector harnesses as necessary.
    • Check for injectors that are shorted internally. Compare injector resistance.
    • Check for fuel in pressure regulator vacuum hose. If fuel is present, replace the regulator assembly.
    • Check for foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft. Also inspect for throttle body tampering.
    • Check items which can cause an engine to run rich (long term fuel trim near -99%).
    • Check items that can cause an engine to run lean (long term fuel trim near 100%).

Sensor/System
    • Check the air intake system and crankcase for air leaks.
    • Check 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.
    • Check for proper calibration of the speedometer. Connect J 33431-B (Signal Generator) to the 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 TP sensor voltage. The TP sensor voltage should be less then 0.9 volts at rest. If the TP sensor voltage is greater than 0.9 volt, inspect the throttle body and accelerator cables for binding.
    • Using a scan tool, monitor KS system for excessive spark retard activity.

Ignition System
    • Remove the spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability problem. The spark plug gapping should be done using a wire gauge gap tool (J 41319). If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Check for loose ignition control module grounds.
    • Check for proper ignition voltage output with spark tester J 26792. Refer to Secondary Ignition Diagnosis .

Engine Cooling System
    • Check engine coolant level for being low.
    • Check for proper engine thermostat operation or for the wrong heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical

Check for the following conditions:

    • Low compression
    • Sticking or leaking valves
    • Worn camshaft lobe(s)
    • Incorrect valve timing
    • Worn rocker arms
    • Broken valve springs
    • Excessive oil in the combustion chamber may indicate leaking valve seals
    • Check for incorrect basic engine parts. Inspect for the following:
       - Camshafts
       - Cylinder heads
       - Pistons, etc.
    • Refer to Base Engine Misfire Diagnosis for diagnostic procedures.

Additional Checks
    • Visually / physically check: Vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Check TCC operation. Using a scan tool should indicate an RPM drop, when the TCC is commanded on. Refer to Flexplate/Torque Converter Vibration Test in Automatic Transmission -- 4T65-E.
    • Check the exhaust system for a possible restriction. Refer to Restricted Exhaust System Check .
    •  Check for 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 erratic sensor values or excessive changes in indicated RPM with little change in actual engine RPM. This may indicate EMI is present. If a problem exists, check for improper routing of the harness near high voltage components (Ignition Components) or high current components (alternator, motors, solenoids etc.).
    •  Check the charging system. a bad diode in the alternator can allow AC noise to enter the electrical system. This may affect the engine controls system operation. Refer to Charging System Check .
    • Check the Trans Range switch circuit and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive.