Rough/Unstable Idle or Stalls

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 ECU/MCU will then
reduce the amount of fuel delivered to the
engine, causing a severe driveability
problem.
- THROTTLE POSITION SENSOR (TPS) - If a
sticking throttle shaft or binding linkage
causes a high TPS voltage.
A "Scan" tool and/or voltmeter should read
less than 1.25 volts with throttle closed.
- COOLANT TEMPERATURE SENSOR (CTS) - Using
a "Scan" tool 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.
Compare resistance value to the chart in
COMPONENT TESTS.
- MANIFOLD ABSOLUTE PRESSURE (MAP)
Sensor response and accuracy. Refer to
COMPONENT TEST.
- MCU grounds for being clean,
tight, and in their proper location.
- ISA, Idle speed motor.
- PARK/NEUTRAL switch circuit. Use
"Scan" tool, and be sure tool indicates
vehicle is in drive with gear selector in
drive or overdrive.







FUEL NOTE: - To determine if the condition is caused
by a rich or lean system, the vehicle should be
driven at the speed of the complaint.
CHECK: - FUEL PRESSURE
- FUEL INJECTOR BALANCE TEST





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





EMISSIONS CHECK: - EVAPORATIVE SYSTEM
- 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.




ENGINE CHECK: - VACUUM LEAKS can cause higher
than normal idle and low IAC counts.
- BATTERY cables and ground straps should be
clean and secure.
Erratic voltage will cause ISA to change its
position, resulting in poor idle quality.
ISA 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.