Rough, Unstable, or Incorrect Idle, Stalling

SYMPTOM DESCRIPTION:
The engine runs unevenly at idle. If bad enough, the vehicle may shake. Also, the idle may vary in rpm (called "hunting"). Either condition may be severe enough to cause stalling. Engine idles at incorrect speed.




SYSTEM OR COMPONENT TO CHECK ACTION

COMPUTER INPUTS/OUTPUTS CHECK: ^ OXYGEN SENSOR (O2) - Inspect sensor for
silicon contamination from fuel, or use of
improper RTV sealant. The sensor will have
a white, powdery coating, and will result
in a high but false signal voltage (rich
exhaust indication). The ECM will then
reduce the amount of fuel delivered to the
engine, causing a severe driveability
problem.
^ THROTTLE POSTION SENSOR (TPS) - If a
sticking throttle shaft or binding linkage
causes a high TPS voltage.
A voltmeter should read
less than 1.25 volts with throttle closed.
^ COOLANT TEMPERATURE SENSOR (CTS) - Using
"Diagnostic Mode" compare coolant temperature
with ambient temperature on a cold engine.
If coolant temperature reads 5 degrees
greater than or less than ambient air
temperature. Check for high resistance
in coolant sensor circuit or sensor itself.
^ MANIFOLD ABSOLUTE PRESSURE (MAP)
Sensor response and accuracy.
^ ECM grounds for being clean,
tight, and in their proper location.
^ ISC operation - See DIAGNOSTIC CHARTS/
ECM CODES.
^ P/N switch circuit. See DIAGNOSTIC CHARTS
/ECM CODES, or use "Diagnostic Mode" and be
sure parameter indicates vehicle is in drive
with gear selector in drive or overdrive.
NOTE: ^ Use the "Diagnostic Mode" to determine if ECM
is receiving A/C request signal.
Whenever A/C is selected, refer to DIAGNOSTIC
CHARTS/APPROPRIATE BCM CODE CHART.
If problem exists with A/C "ON," check A/C
system operation.







FUEL NOTE: To determine if the condition is caused by
a lean system, the vehicle should be driven at the
speed of the complaint. If closed loop is achieved,
the fuel system has to checked for rich, lean or
improper fuel control. The DFI system has ECM
parameter ED19 "Integrator" which monitors the
oxygen sensor rich lean status and drives the fuel
system richer if the 02 sensor is lean or
leaner if the sensor is rich.
To help identify the problem
Refer to DFI SYSTEM CHECK. Or codes E44, EO44
for lean conditions and E45, EO45 for rich
NOTES ON INTERMITTENTS
CHECK: ^ FUEL PRESSURE: use CHART A-4A in
DIAGNOSTIC CHARTS/A-CHARTS
^ FUEL INJECTOR BALANCE TEST





IGNITION CHECK: ^ SECONDARY AND PRIMARY SYSTEM.
^ IGNITION TIMING.





EMISSIONS CHECK: ^ EVAPORATIVE SYSTEM use CHART C-3 in
DIAGNOSTIC CHARTS/COMPONENT TESTING/C-CHARTS.
^ EGR SYSTEM "ON," while idling, will cause
roughness, stalling and hard starting.
^ PCV VALVE for proper operation by placing finger
over inlet hole in valve end several times.
Valve should snap back. If not, replace valve.
^ AIR MANAGEMENT SYSTEM.
NOTE: ^ Refer to EMISSION CONTROLS
for complete testing procedures.




ENGINE CHECK: ^ VACUUM LEAKS can cause higher
than normal idle and low ISC counts.
^ BATTERY cables and ground straps should be
clean and secure.
Erratic voltage will cause ISC to change its
position, resulting in poor idle quality.
ISC valve will not move, if system voltage
is below 9 or greater than 16 volts.
^ A/C refrigerant pressure too high,
overcharge or faulty high pressure switch.
^ MOTOR MOUNTS.
^ CYLINDER COMPRESSION.