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 (rich exhaust indication). 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 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 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 FT is more than 150. |
| - | Rich - The Long Term FT is less than 115. |
| • | Check 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, it is a damaged
injector drive circuit harness, connector, or terminal. |
| • | Perform the fuel injector coil test. Refer to Fuel Injector Coil Test
. |
| • | Check the Evaporative Emission (EVAP) control system. Refer to Evaporative Emission (EVAP) Control System Diagnosis
. |
|
Ignition System Checks
|
| • | Check for the following conditions: |
| • | Check 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
|
| • | 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 scan tool to determine if the control module is receiving
an A/C signal. Refer to A/C Compressor Control Circuit
Diagnosis
. If a problem exists with the A/C ON, check
the A/C system operation. |
| • | Check for the EGR being ON while idling which will cause roughness,
stalling, and hard starting. |
| • | Check the battery cables and ground straps. They 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:
| • | Broken or weak valve springs. |
|
