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,
coating, and will result in a high but
false signal voltage (rich exhaust).
The Control Unit will then reduce the
amount of fuel being delivered to the
engine, causing a driveability problem.
Refer to COMPONENT TEST PROCEDURES.
- THROTTLE POSITION SENSOR (TPS) - If a
sticking throttle shaft or binding linkage
causes a high TPS output voltage.
Refer to COMPONENT TEST PROCEDURES.
- COOLANT TEMPERATURE SENSOR (CTS) -
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.
Refer to COMPONENT TEST PROCEDURES.
- MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR -
Test sensor for response and accuracy.
Refer to COMPONENT TEST PROCEDURES.
- CONTROL UNIT -
Check grounds for being clean,
tight, and in their proper location.
- IDLE SPEED CONTROLS
Check operation of Stepper Motor, Actuator,
or Sole-Vac Throttle Positioner.
Refer to COMPONENT TEST PROCEDURES.
- A/C SELECT SWITCH
Check to see that the Control Unit
is receiving an input signal indicating
A/C system activation.
Refer to COMPONENT TEST PROCEDURES.
If problem exists with A/C ON, check
A/C System operation.







FUEL CHECK: - FUEL PRESSURE
- FUEL INJECTOR BALANCE TEST





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





EMISSIONS CHECK: - EVAPORATIVE SYSTEM
- EGR SYSTEM
- PCV VALVE
- AIR MANAGEMENT SYSTEM.

Refer to Powertrain Management/Emission Control Systems/Testing and Inspection/Procedures.





ENGINE CHECK: - VACUUM LEAKS
Can cause higher than normal idle speeds.
- BATTERY
Cables and ground straps should be
clean and secure.
Erratic voltage will cause poor idle quality.
- A/C SYSTEM
Refrigerant pressure too high,
overcharge or faulty high pressure switch.
- MOTOR MOUNTS.
- CYLINDER COMPRESSION.