GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

DEFINITION: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary Checks
    • Refer to Symptoms .
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight and in their proper locations. Refer to Wiring Repairs in Wiring Systems.
    • Remove and check the air filter element for dirt, or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Perform the injector balance test. Refer to Sequential Multiport Fuel Injector Balance Test for procedure.
    • 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 that 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 the injector resistance. Injector resistance should be within one ohm of each other. Refer to Fuel Injector Solenoid Coil Test .
    • Check 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 and DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1 .
    • Check 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 .

Sensor/System
    • Check the crankcase ventilation valve for proper operation. Place a finger over inlet hole of the valve end several times. The valve should snap back. If not, replace the valve.
    • Check for silicon contamination from fuel or improperly used sealant. The HO2 Sensor will have a white powdery coating. The HO2 Sensor will result in a high but false signal voltage (rich exhaust indication). The PCM will reduce the amount of fuel delivered to the engine causing a severe driveability concern.
    •  Use a scan tool in order to monitor KS system for excessive spark retard activity.

Ignition System
    • Check for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis for procedure.
    • 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. 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 or diagnosis of the rich condition or for diagnosis of the oil fouling condition.
    • Visually/Physically inspect Secondary ignition for the following:
       - Ignition wires for cross firing
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • 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 loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

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

Refer to Engine/Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check the exhaust system for possible restrictions. Check for the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the exhaust manifold for a collapsed inner wall.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged catalytic converters by comparing exhaust system back pressure on each side of engine. Check the back pressure by removing Heated Oxygen Sensor (HO2S) near exhaust manifolds. Refer to Restricted Exhaust System Check and Exhaust System.
    • Electromagnetic Interference (EMI) on the reference circuit can cause an engine miss condition A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change, this indicates that EMI is present. If a problem exists, check routing of secondary ignition wires or high voltage components near ignition control circuits.
    • Check the PNP switch circuit. Refer to Park/Neutral Position Switch Diagnosis .
    • Check for faulty motor mounts.
    • Check the intake manifold and the exhaust manifold passages for casting flash. Refer to Engine/Engine Mechanical.
    • Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle.
    •  Refer to DTC P0507 Idle Speed High .
    • Check for the EGR being ON while idling which will cause roughness, stalling, and hard to start. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
    • Check the battery cables and ground straps. They should be clean and secure.