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, which indicates rich exhaust. 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
|
| • | Inspect 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. |
| • | Inspect 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 light while cranking. |
| 3. | If the test lamp fails to blink at any connector, it is a damaged
injector drive circuit harness, connector, or terminal. |
| • | Inspect for fuel in the pressure regulator vacuum hose. |
|
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 control module grounds for being clean, tight, and
in their proper locations. |
| • | 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 Description
. |
|
Engine Mechanical Check
| Inspect for the following:
| • | Broken or weak valve springs |
|
