Checks
| Action
|
DEFINITION: The
engine runs unevenly at idle. If severe enough, the engine or vehicle may
shake. The engine idle speed may vary in RPM. Either condition may
be severe enough to stall the engine.
|
Preliminary Check
| Refer to
Symptoms - Engine Controls
.
|
Sensor Checks
|
| • | Check the heated oxygen sensor (HO2S). Check for silicon contamination
from fuel or improperly used sealant. The sensor will have a white powdery
coating. The sensor will result in a high but false signal voltage,
or rich exhaust indication. The control module will reduce the
amount of fuel delivered to the engine causing a severe driveability
problem. |
| • | Check the engine coolant temperature (ECT) sensor using the scan
tool in order to compare the engine coolant temperature with the ambient air
temperature on a cold engine. If the coolant temperature reading
is more than 5 degrees more than or less than the ambient
air temperature on a cold engine, check for a high resistance in
the coolant sensor circuit or the sensor itself. |
| • | Check the manifold absolute pressure (MAP) sensor response and
accuracy. |
|
Fuel
System Checks
|
| • | Check to determine if a rich or lean system causes the condition.
Drive the vehicle at the speed of the complaint. Monitoring the fuel trim
will help identify the problem. |
| - | Lean--The long term fuel trim is more than 150. |
| - | Rich--The long term fuel trim is less than 115. |
| • | Check the fuel injector driver circuit. |
| 1. | Disconnect the injector harness connector at the injectors. |
| 2. | Connect an injector test lamp between the terminals of each injector
connector and note the lamp while cranking. |
| 3. | If the test lamp fails to blink at any connector, it is a faulty
injector drive circuit harness, connector, or terminal. |
|
Ignition
System Checks
|
| • | Check the overall ignition system |
| • | Check for the following conditions: |
|
Additional
Checks
|
| • | Check for vacuum leaks. Vacuum leaks can cause a higher than normal
idle and low idle air control (IAC) counts. |
| • | Check the control module grounds for being clean, tight, and in
their proper locations. |
| • | Check the battery cables and ground straps for being clean and
secure. |
| • | Check the crankcase ventilation valve for proper operation by
placing a finger over the inlet hole in the valve end several times. The valve
should snap back. If not, replace the valve. Refer to
Crankcase Ventilation System Inspection/Description
in Engine Mechanical. |
|
Engine
Mechanical Check
| Check for the following:
| • | Broken or weak valve springs |
|
