GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Chevy P12 Chassis 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Hesitation, Sag, Stumble

Checks

Action

Definition: Momentary lack of response as the accelerator is pushed down. Can occur at any vehicle speed. Usually more pronounced when first trying to make the vehicle move, as from a stop sign. May cause the engine to stall if severe enough.

Preliminary
    • Refer to Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight, and in their proper locations. Refer to PCM wiring diagrams in Electrical Diagnosis.

Sensor/System

Check the MAP sensor operation.

Fuel System
    • Check for a low fuel pressure condition. Refer to Fuel System Diagnosis .
    • Check for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Check for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check both injector fuses for being open. An open injector fuse causes four ignition coils and four injectors not to operate. Replace the fuse. Inspect the ignition coil circuits and the injector circuits for an intermittent short to ground.
    • Perform the injector balance test. Refer to Sequential Multiport Fuel Injector Balance Test for procedure.
    • Check the items which can cause an engine to run rich (long term fuel trim near -13%). For a rich condition, refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 .
    • Check the items that can cause an engine to run lean (long term fuel trim near 23%). For a Lean condition, refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 and DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1 .

Ignition System
    • Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as you apply water.
    •  Check for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis for the procedure.
    • 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. Refer to Engine Electrical/Spark Plugs . Do the spark plug gapping using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement. Determine the cause of the fouling before replacing the spark plugs if spark plugs are gas or oil fouled. Refer to Fuel System Diagnosis for diagnosis the rich condition or Engine/Engine Mechanical for diagnosis of the oil fouling condition.
    • Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .
    • Check the spark plugs for proper heat range. Refer to Spark Plug Visual Diagnosis .

Engine Cooling System

Check the engine thermostat for proper operation and for proper heat range. Refer to Engine Overheating .

Additional Checks
    • Check the generator output voltage. Refer to Engine/Engine Electrical for procedure. Repair the charging system if the generator output voltage is less than 9 volts or more than 16 volts.
    • Check the EGR Valve operation. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .