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
.
|
sensor Checks
|
| • | Inspect the heated oxygen sensor (HO2S). |
| | Inspect 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, rich exhaust indication. The control
module will reduce the amount of fuel delivered to the engine causing
a severe driveability problem. |
| • | Inspect the throttle position (TP) sensor. If a sticking throttle
shaft or binding linkage causes a high TP sensor open throttle indication,
the control module will not control the idle. Monitor the TP sensor
voltage. A scan tool or voltmeter should read less than 0.85 volts
with the throttle closed. |
| • | Inspect the engine coolant temperature (ECT) sensor using the
scan tool 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. |
| • | Inspect 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 FT is more than 150. |
| - | Rich--The Long Term FT is less than 115. |
| • | Inspect the fuel injector driver circuit. |
| 1. | Disconnect the injector harness connector at the injectors. |
| 2. | Connect an injector test light between the terminals of each injector
connector and note the light while cranking. |
| 3. | If the test light fails to blink at any connector, check for a
damaged injector drive circuit harness, connector, or terminal. |
|
Ignition System Checks
|
| • | Inspect for the following conditions: |
| • | Inspect the spark plug wires by connecting a DMM to the ends
of each wire in question. If the meter reads over 30,000 ohms, then
replace the wires. |
|
Additional Checks
|
| • | Inspect for vacuum leaks. Vacuum leaks can cause a higher than
normal idle and low idle air control (IAC) counts. |
| • | Inspect the battery cables and ground straps. They should be clean
and secure. |
| • | Inspect 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 Diagnosis
. |
|
Engine
Mechanical Check
| Inspect for the following:
| • | Broken or weak valve springs. |
|
