GM Service Manual Online
For 1990-2009 cars only

Inspection/Test

Action

DEFINITION: Steady pulsation or jerking that follows engine speed, usually more pronounced as the engine load increases.

Preliminary

    â€¢ Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    â€¢ Search for bulletins.
    â€¢ Inspect the intake air system for the following conditions:
       - Dirty or damaged air filter element
       - Restrictions
       - Damaged or disconnected intake air ducting

Sensor/System

    â€¢ Inspect mass air flow (MAF) sensor and connections.
    â€¢ Test the engine coolant temperature (ECT) sensor for being shifted in value. Connect the scan tool and compare the engine coolant temperature (IAT) with the intake air temperature on a cold engine. The ECT and IAT should be within ±3°C (5°F) of each other. Check the resistance of the ECT sensor if the temperature is out of range with the IAT sensor. Refer to Temperature Versus Resistance for specifications.
    â€¢ Inspect throttle position (TP) sensor and connections. Refer to DTC P0121 .
    â€¢ Inspect for throttle body tampering, excessive deposits, or damage.
    â€¢ Test the exhaust gas recirculation (EGR) system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description .

Fuel System

    â€¢ Test fuel pressure. Refer to Fuel System Diagnosis .
    â€¢ Test fuel injectors and related wiring. Refer to Fuel Injector Solenoid Coil Test , Fuel Injector Balance Test with Special Tool and Fuel Injector Balance Test with Tech 2 .
    â€¢ Inspect for fuel in the fuel pressure regulator.
    â€¢ Test for contaminants in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Inspect the following that may cause the engine to run rich:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

    â€¢ Any water intrusion in the HO2S connector.
    â€¢ Engine oil contaminated by fuel.
    â€¢ An EVAP canister purge condition.
    â€¢ Incorrect fuel pressure--Refer to Fuel System Diagnosis .
    â€¢ A leaking fuel pressure regulator.
    â€¢ Any rich fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 and Fuel Injector Balance Test with Special Tool .
    â€¢ Inaccurate MAF sensor.
    â€¢ Blockage on the inlet screen of the MAF sensor.
    â€¢ Vacuum hoses for splits, kinks and proper connections.
    â€¢ The air intake duct for being collapsed or restricted.
    â€¢ The air filter for being dirty or restricted--Refer to Air Cleaner Element Replacement .

Inspect the following that may cause the engine to run lean:

Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.

    â€¢ Any water intrusion into the HO2S connector.
    â€¢ An exhaust leak between the HO2S and the engine--Refer to Exhaust Leakage in Engine Exhaust.
    â€¢ Any vacuum leaks.
    â€¢ Incorrect fuel pressure--Refer to Fuel System Diagnosis .
    â€¢ Any lean fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 and Fuel Injector Balance Test with Special Tool .
    â€¢ An inaccurate mass air flow (MAF) sensor.
    â€¢ Fuel Contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    â€¢ Vacuum hoses for splits, kinks and proper connections.

Ignition System

    â€¢ Test crankshaft position (CKP) sensor  A and B for proper operation.
    â€¢ Inspect the ignition control (IC) circuits at each ignition module assembly for poor connections, damage, misrouting.
    â€¢ Inspect the ignition coils for damage.
    â€¢ Inspect the spark plug boots for misalignment, and damage.
    â€¢ Inspect for a damaged or missing ground spring at each ignition module assembly.
    â€¢ Inspect the spark plugs. Refer to Spark Plug Inspection .

Engine Cooling

    â€¢ Inspect the engine coolant level for being too low. Refer to Loss of Coolant in Engine Cooling.
    â€¢ Inspect for an incorrect, damaged, or malfunctioning thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical

    â€¢ Inspect the following:
       - Engine compression--Refer to Engine Compression Test in Engine Mechanical
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Worn rocker arms-- Refer to Valve Rocker Arms Cleaning and Inspection in Engine Mechanical.
       - Broken valve springs--Refer to Valve Spring Inspection and Measurement in Engine Mechanical.
       - Excessive oil in the combustion chamber, leaking valve seals.
    â€¢ Inspect the following for incorrect basic engine parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional

    â€¢ Inspect the vacuum hoses for splits, kinks, proper routing and connections, and/or damage. Refer to the Vehicle Emission Control Information label.
    â€¢ Inspect the exhaust system for possible restrictions. Refer to Restricted Exhaust in Engine Exhaust.
    â€¢ An engine miss condition can be caused by electromagnetic interference (EMI). EMI can usually be detected by monitoring the engine RPM with a scan tool. A sudden increase in RPM with little change in the actual engine RPM, indicates EMI is present.