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Rough, Unstable, Incorrect Idle Or Stalling

  1. Check ignition timing. Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH DIAGNOSIS chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
  2. Check injector for leaks. See DIESELING, RUN-ON. Check fuel pressure. See FUEL SYSTEM PRESSURE TEST. If idle is rough only when engine is hot, perform the following checks:
    • Check for vacuum leaks by installing Plug (J-330247) in idle air passage. If closed throttle engine speed is above 650 RPM, locate and correct vacuum leak.
    • Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH DIAGNOSIS chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
    • Check for sticking throttle shaft or binding linkage that may cause a high TPS voltage (open throttle indication). If binding occurs, the ECM will not control idle. Check TPS voltage. Reading should be less than 1.2 volts with throttle closed.
    • Check EGR valve operation. See appropriate EGR SYSTEM CHECK chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
    • Check battery terminals, cables, and grounds straps for loose or corroded connections. Erratic voltage will cause IAC motor to change its position, resulting in poor idle quality. IAC valve will not move if voltage is below 9 or greater than 17.8 volts.
    • On 2.5L engine, check power steering pressure switch. See POWER STEERING PRESSURE SWITCH chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section. ECM should compensate for power steering loads.
    • Check MAP sensor operation. Start and allow engine to idle. Disconnect MAP sensor electrical connector. If idle improves, substitute a known good sensor and retest.
    • Check A/C for high refrigerant pressure. Check A/C compressor or relay. If inoperative, see appropriate A/C CLUTCH CONTROL DIAGNOSIS chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
    • Inspect oxygen sensor for silicon contamination from fuel or improper use of RTV sealant. Sensor may have a White, powdery coating, and will result in a high but false signal voltage (rich exhaust indication).
    • Check PCV valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace PCV valve. Perform compression test.
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