Rough, Unstable, or Incorrect Idle, Stalling

Definition: The engine runs unevenly at idle. If bad enough, the car may shake. Also, the idle may vary in RPM (called "hunting" or "surging"). Either condition may be bad enough to cause stalling. Engine idles at incorrect speed.

^ Perform careful visual check as described at start of Symptoms. - Visual/Physical Underhood Inspection
^ Observe ISC operation in output cycling. Manipulate the four terminal ISC connector while watching for ISC sticking or stalling.
^ With engine running, observe parameter ED01, TPS. Watch for erratic behavior, jumping or skips. parameter ED01 should increase smoothly with throttle opening. Tap gently on the TPS with a pen or pencil, watch parameter ED01 for erratic behavior.
^ Check throttle linkage including throttle, cruise control and TV cables for binding or sticking.
^ Check ignition secondary pattern on a scope. Look for cylinders with erratic spark lines or crossfiring. Check spark plugs and wires on suspect cylinders for arcing. Check plug wire routing, distributor cap and rotor if crossfiring.
^ Check for proper brake switch part number and adjustment. A misadjusted brake switch will allow Viscous Converter Clutch (VCC) to stay engaged when applying the brakes.
^ Remove plugs. Check for proper heat range and type, for cracks, fuel fouling (sooted) or oil fouling. Replace plugs and investigate cause of oil fouling if applicable.
^ Check power steering pressure sensor for proper reading. ECM parameter ED19 should read between 20 and 100 at no power steering load and between 750 and 999 at full lock position.
^ Check ignition timing. See Emission Control Information Label
^ Check fuel pressure. See Chart A-4B, "Dynamic Fuel Pressure Diagnosis". A-4B Dynamic Fuel Pressure Check
^ Check for dripping or leaking injectors, intermittent injector operation. See Chart A-5, "Injector System Diagnosis." Chart A-5 Injector System Diagnosis
^ Check EGR system. EGR valve should be closed at idle.
^ Check EECS operation.
^ Check PCV valve for proper separation by placing finger over inlet hole in valve several times. Valve should snap back. If not, replace valve.
^ Using input test EI72, check for proper operation of the throttle switch. Repair faults (if found) using EI72 trouble tree in ECM Trouble COdes. Testing and Inspection
^ Check P/N switch circuit. See Input EI74, also check for intermittents. E174
^ Check A/C clutch for erratic operation or rapid cycling.
^ Check for loose ECM ground at the engine block.
^ Check distributor gear for wear or loose gear to shaft fit. Idle surge complaint due to worn distributor gear may also be accompanied by hard or intermittent code EO30. Replace distributor gear or shaft as necessary. If the gear is worn check cam gear, distributor bore in engine block for metal shavings, for excessive oil pump drive effort (the distributor gear drives the oil pump). See Engine. Engine
^ Check distributor shaft for wear, side play, runout, alignment in distributor housing.
^ Check for timing chain loose or timing gear wear. Repair as necessary. See Engine. Engine
^ Remove rocker covers. Check for bent pushrods, worn rocker arms, broken or weak valve springs, sticking valves, worn cam lobes. Repair as necessary. See Engine. Engine
^ Air leaks at throttle body mounting and intake manifold.
^ Check for foreign material deposits on throttle blades. Clean as necessary.
^ Remove carbon with top engine cleaner. Follow instructions on can.
^ Run a cylinder compression check.