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
|
| • | 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. The control module will reduce
the amount of fuel delivered to the engine causing a severe driveability
problem. |
| • | Check the throttle position (TP) sensor. If a sticking throttle
shaft or a 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 and/or voltmeter should read less than 0.85 volts
with the throttle closed. |
| • | Check 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 in the sensor. |
| • | 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, check for a
damaged injector drive circuit harness, connector, or terminal. |
|
Ignition System Checks
|
| • | Check for the following conditions: |
| - | Any blistered insulators |
| • | Check the spark plug wires by connecting a digital multimeter
(DMM) to the ends of each wire in question. If the meter reads more than
30,000 ohms, replace the wire. |
|
Additional Checks
|
| • | Check for vacuum leaks. Vacuum leaks can cause a higher than normal
idle and low idle air control (IAC) counts. |
| • | Verify the control module grounds are clean, tight, and in the
proper locations. Refer to Engine Controls Component Location. |
| • | Check the battery cables and ground straps. The cables and straps
should be 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 Diagnosis
. |
|
Engine Mechanical Check
| Check for the following:
| • | Any broken motor mounts |
| • | An improper valve timing |
| • | Any broken or weak valve springs. |
| • | Any worn camshaft lobes |
|
