GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1998 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.0L and 4.6L 🢒 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 .
    • Search for bulletins.
    • Check PCM grounds for being clean, tight and in the proper locations.
    • 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?
    • Remove and check air filter element for dirt, or for being restricted. Replace as necessary.

Fuel System
    • Perform injector balance test. Refer to Fuel Injector Balance Test .
    • Check fuel pressure. Refer to Fuel System Pressure Test .
    • Check for restricted fuel filter.
    • Check for contaminated fuel. Refer to Fuel Quality Diagnosis .
    • Check for incorrect minimum idle speed.
    • Check each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2). 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 regulator assembly. Refer to Fuel Pressure Regulator Replacement .
    • 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 Intake Air system and crankcase for air leaks.
    • Check Crankcase ventilation valve for proper operation. Place finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.
    • Check for proper calibration of speedometer. Connect J 33431-B (Signal Generator) to VSS electrical connector. Ignition ON, tester ON and 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
    • Check for proper ignition voltage output with spark tester J 26792. Refer to Fuel System Pressure Test .
    • Remove 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.
    • Visually/Physically inspect Secondary ignition for the following:
       - Ignition wires for cross firing.
       - 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/listen for arcing or misfiring as water is applied.
        Check for loose ignition control module ground.

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

Engine Mechanical
    • Check engine mechanical for the following:
       - Check compression.
       - Sticking or leaking valves.
       - Worn camshaft lobe(s).
       - Valve timing.
       - Worn rocker arms.
       - Broken valve springs.
       - Excessive oil in combustion chamber -- Leaking valve seals.
       - Low cylinder compression.
    • For incorrect basic engine parts. Inspect for the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

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.
    • 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 converters by comparing exhaust system back pressure on each side of engine.
    • An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits).
        Check PNP switch circuit and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive.