Possible Causes & Corrections

Check the following items:

• Start engine.
• Allow engine to stabilize.
• Disconnect Idle Air Control (IAC) motor.
• Do not perform this test for more than 2 minutes to prevent damaging catalytic converter. Perform Cylinder Power Balance Test with a scan tool.
• If there is an RPM drop on all cylinders, equal to within 50 RPM, see ROUGH, UNSTABLE OR INCORRECT IDLE & STALLING .
• Reconnect IAC motor with ignition off.
• If there is no RPM drop on one or more of the cylinders, or excessive variation in drop, check for spark on the suspected cylinders with Spark Tester (J-26792).
• If there is no spark, see Distributor Ignition System Diagnosis in DISTRIBUTOR IGNITION SYSTEM under IGNITION SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• If there is spark, remove spark plugs in affected cylinders and check for insulation cracks, wear, improper gap, burned electrodes or heavy deposits. Check spark plug wire resistance. The resistance should not exceed 1000 ohms per foot. Check distributor rotor and cap. If previous check did not find the problem, visually check ignition system for moisture, dust, cracks, burns, etc. With the engine running, spray the plug wires with a fine water mist to check for shorts. Perform the Fuel Injector Balance Test. See SYSTEM & COMPONENT TESTING - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Perform cylinder compression check. See CYLINDER COMPRESSION PRESSURE under MECHANICAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Check engine for improper valve timing, bent pushrods, worn rocker arms, worn camshaft lobes or broken/weak valve springs.
• Perform Fuel Injector Balance Test. See SYSTEM & COMPONENT TESTING - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Check intake and exhaust manifold passages for casting flash.
• Check fuel pressure. See FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Inspect for water contamination in fuel.
• Perform fuel injector coil test. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Perform fuel injector balance test. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 4.3L ASTRO, BLAZER, JIMMY CANADIAN, SAFARI, SONOMA & S10 PICKUP article.
• Check for Electromagnetic Interference (EMI).
• Check routing of secondary wires and ground circuit.