GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2007 🢒 Isuzu MD F-Series 🢒 General Information 🢒 Vibration Diagnosis and Correction 🢒 Symptoms - Engine Controls

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

Symptoms Description

Symptoms covers conditions that are not covered by DTCs. Certain conditions can cause multiple symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause specific symptoms are listed separately under Additional Symptoms Testing. Perform the Symptoms Testing before using the Additional Symptoms Testing.

Symptoms Definition

Cuts Out, Misses: A steady pulsation or jerking that follows engine speed, which is usually more pronounced as the engine load increases. This condition is not normally felt above 1500 RPM or 48 km/h (30 mph). The exhaust has a steady spitting sound at idle or at low speed.

Hard Start: The engine cranks OK, but does not start for a long time. The vehicle does eventually run, or may start but immediately stall.

Hesitation, Sag, Stumble: A momentary lack of response as the accelerator is pushed down. This condition can occur at any vehicle speed. This condition is usually more pronounced when first trying to make the vehicle move, as from a stop. This condition may cause the engine to stall in severe conditions.

Lack of Power, Sluggishness, or Sponginess: The engine delivers less than expected power. Little or no increase in vehicle speed when the accelerator pedal is pushed down part way.

Poor Fuel Economy:  Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.

Rough, Unstable, or Incorrect Idle and Stalling: The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle speed may vary. Either condition may be severe enough to stall the engine.

Surges/Chuggles: An engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator pedal position.

Fuel Knock/Combustion Noise: A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that change with the throttle opening.

Excessive Smoke (Black): Black smoke under load, idle or start up, hot or cold.

Excessive Smoke (White): White smoke under load, idle or start up, hot or cold.

Excessive Smoke (Gray or Blue): Gray or Blue smoke under load, idle or start up, hot or cold.

Symptoms Verification

Before using the Symptom tables, perform the following inspections:

    • Ensure that the engine control module (ECM) and malfunction indicator lamp (MIL) are operating correctly.
    • Ensure that there are no DTCs that are stored.
    • Ensure that the scan tool data is within a normal operating range. Refer to Scan Tool Data List .
    • Verify the customer concern.
    • Perform the Visual/Physical Inspection in this section. The visual/physical inspection is extremely important, and can lead to correcting a condition without additional testing. It may also help reveal the cause of an intermittent condition.

Identifying Intermittent Conditions

Many intermittent conditions occur with harness or connector movement due to engine torque, rough pavement, vibration or physical movements of a component. Refer to the following for a list of issues that may cause an intermittent condition:

    • Moisture and water intrusion in connectors, terminals, and components
    • Incomplete connector mating
    • Poor terminal contact
    • High circuit or component resistance--High resistance can include any resistance, regardless of the amount, which can interrupt the operation of the component.
    • Harness that is too short or tight
    • Wire insulation that is chaffed or cut
    • High or low ambient temperature
    • High or low engine coolant temperatures
    • High underhood temperatures
    • Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load
    • High or low system voltage
    • High vehicle load conditions
    • Rough road surfaces
    • Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge
    • Incorrect installation of aftermarket, add on accessories

Visual/Physical Check

    • Ensure that the control module grounds are clean, tight, and correctly located. Refer to Power and Grounding Connector End Views .
    • Ensure that the air filter is clean and free from restrictions.
    • Ensure that there is no water intrusion in connectors terminals and components.
    • Inspect the air intake ducts for the following conditions:
       - Collapsed
       - Damaged areas
       - Looseness
       - Incorrect installation
       - Leaking
    • Inspect for air leaks at the intake AIR valve, the intake AIR heater, the MAF sensor and intake manifold sealing surfaces.
    • Inspect the wiring harness for the following conditions:
       - Poor connections
       - Pinches
       - Cuts
    • Inspect for loose, damaged, unseated, or missing sensors/components.
    • Inspect the terminals for corrosion and correct contact.

Symptoms Testing

Cuts Out/Misses, Hard Start, Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy, Rough, Unstable, or Incorrect Idle and Stalling, Fuel Knock/Combustion Noise, or Surges/Chuggles Excessive Smoke (Black), Excessive Smoke (White), Excessive Smoke (Gray or Blue)

  1. Test for the following conditions:
    2. • The sensor systems for the following:
       - Inspect the mass air flow (MAF) sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement .
       - Test for an intermittent FRP sensor signal by wiggling the harness between the sensor and the ECM with the ignition ON and the engine OFF, while monitoring the parameter with a scan tool.
       - Idle the engine and observe the Actual Fuel Rail Pressure and the Desired Fuel Rail Pressure parameters. If the pressure difference is more than 2 MPa, a fuel control issue may exist.
       - Inspect for an intermittent crankshaft position (CKP) sensor signal. Refer to Testing for Intermittent Conditions and Poor Connections .
       - Inspect the CKP sensor reluctor wheel. Remove the CKP sensor and attempt to move the reluctor wheel front to back or side to side with a probe. If there is any movement, the reluctor wheel retaining bolts are loose.
       - Inspect the engine coolant temperature (ECT) sensor. Use the scan tool in order to compare the ECT with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more or less than the ambient air temperature on a cold engine, inspect for a high resistance in the coolant sensor circuit or the sensor itself.
    • The Fuel System for the following:
       - Inspect the fuel pressure regulator 12-volt circuit for an intermittent short to ground. Observe the FRP regulator command percent while cranking. A range of 85-95 percent indicates a possible short to ground condition.
       - Inspect the fuel supply to the fuel injection pump.
       - Inspect the fuel system for a plugged fuel filter, high vacuum, air in the fuel system, etc.
       - Inspect for air in the fuel system.
       - Inspect for external fuel leaks.
       - Inspect for low fuel pressure.
       - Inspect the fuel supply system vacuum while the problem exists.
       - Inspect the fuel quality.
       - Inspect the engine speed signal circuit for high resistance.
       - If the ECM or a fuel injector has recently been replaced, it may be necessary to confirm the fuel injector flow rate values.
       - Inspect for restricted or plugged fuel injectors.
       - Inspect the fuel tank cap vent for proper operation.
    • Engine mechanical for the following.
       - Improper valve timing
       - Bent pushrods
       - Worn rocker arms
       - Low engine compression
       - Broken or weak valve springs
       - Worn camshaft lobes
       - Excessive oil in the combustion chamber--Leaking valve seals
  2. If the above conditions do not address the symptom, refer to the additional symptoms tests.

Additional Symptoms Tests

Hard Start

    • Verify that the ECT has not shifted in value.
    • Inspect for a slow cranking speed. Refer to Symptoms - Engine Electrical .
    • Verify the engine electrical system for correct operation. Refer to Symptoms - Engine Electrical .
    • Verify that the driver is using the correct starting procedure as described in the owners manual.
    • Inspect for a restriction in turbocharger inlet duct.
    • Inspect for a restriction or a leak in the intake manifolds.
    • Inspect the exhaust system for a possible restriction. Refer to Restricted Exhaust .
    • Inspect for excessive crankshaft endplay that will cause the CKP sensor reluctor wheel to move out of alignment with the CKP sensor.

Hesitation, Sag, Stumble

    • Test the generator. Refer to Symptoms - Engine Electrical . Repair the charging system if the generator output voltage is less than 9 volts or more than 16 volts.
    • Inspect for excessive crankshaft endplay that will cause the CKP sensor reluctor wheel to move out of alignment with the CKP sensor.

Poor Fuel Economy

    • Heavy loads being carried or towed
    • Acceleration rate too much or too often
    • Inspect the brake system for brake drag.
    • Inspect the tires for the correct air pressure.
    • Inspect for incorrect operation of the speedometer.
    • Inspect for an air leak in the charge air cooler.
    • Inspect the air conditioning system for proper operation.
    • Suggest to the owner to fill the fuel tank and inspect the fuel economy.
    • Suggest to the driver to read the Important Facts on Fuel Economy in the Owner Manual.
    • Verify that the ECT has not shifted in value.

Lack of Power, Hesitation, or Cut Out

    • Inspect the engine electrical system for correct operation. Refer to Symptoms - Engine Electrical .
    • Inspect for auxiliary fuel filters. Aftermarket fuel filters may restrict fuel flow.
    • Compare the vehicle with a similar unit. Ensure the vehicle has an actual problem.
    • Inspect for a proper transmission operation.
    • Inspect the engine oil level and quality.
    • Inspect for an air leak in the charge air cooler.
    • Inspect for a worn or damaged turbo charger turbine wheel, shaft or compressor wheel.
    • Inspect for a restriction in the charge air cooler.
    • Inspect for a skewed mass air flow (MAF) sensor.
    • Inspect that the exhaust gas recirculation (EGR) valve is fully closing. Remove the EGR valve and visually inspect that there is no light seen between the valves and the valve seats.
    • Ensure that the EGR valve does not bind when manually moving the valve.
    • Inspect the torque converter clutch (TCC) operation.
    • Inspect the air conditioning system for proper operation.
    • Inspect the exhaust system for a possible restriction.

Rough, Unstable, or Incorrect Idle and Stalling

    • Inspect the engine mounts.
    • Inspect the intake and exhaust manifolds for casting flash.
    • The exhaust system for damaged or collapsed pipes
    • The exhaust manifold for a collapsed inner wall
    • The mufflers for heat distress or possible internal failure
    • 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, indicates that EMI is present. If a problem exists, inspect routing of high voltage components, such as fuel injector wiring, near the sensor circuits.
    • Inspect the park neutral position (PNP) switch circuit.

Surges/Chuggles

    • Ensure the driver understands the torque converter clutch (TCC) operation.
    • Ensure the driver understands the A/C compressor operation.
    • Use the scan tool to ensure the vehicle speed sensor (VSS) reading matches the speedometer. This excludes vehicles with electronic transmissions where some variation between VSS and the speedometer is normal.

Fuel Knock/Combustion Noise

    • Inspect for obvious overheating problems.
    • Inspect for a low engine coolant level.
    • Inspect for any restricted air flow through the radiator.
    • Inspect for a malfunctioning or incorrect thermostat.
    • Inspect for a correct coolant solution. The solution should be a 50/50 mix of anti-freeze and water.

Excessive Smoke (Black)

    • The DPF is damaged and must be replaced. Inspect the following possible causes that may have damaged the DPF.
    • Inspect for an air leak in the charge air cooler or the air ducts between the turbocharger and the intake manifold.
    • Inspect for a restriction in the turbocharger charge air cooler.
    • Inspect for a restriction in the intake manifold.
    • Inspect for excessive oil entering the combustion chamber.
    • Inspect for a stuck open EGR valve.
    • Ensure that the EGR valve does not bind when manually moving the valve.

Excessive Smoke (White)

    • Check the coolant level in the reservoir. White coolant smoke may be mistaken for blue/gray smoke. If the coolant level is low, refer to Loss of Coolant .
    • If excessive smoke is present, inspect for a stuck open fuel injector by performing the following procedure:
        1. Disconnect the CKP sensor.
        2. Remove the glow plugs.

        Important: Do not stand in front of the glow plug holes while cranking the engine.

        3. Crank the engine while observing the glow plug holes for fuel spray.
        4. Replace the fuel injectors for the cylinders that spray fuel.
        5. Inspect for contaminated oil or high oil level.
    • If the ECM glow plug control module (GPCM) or a fuel injector has recently been replaced, it may be necessary to confirm the fuel injector flow rate values.
    • Inspect the fuel injectors.