The idle speed and fuel mixture are controlled by the engine's Engine Control Module (ECM). The ECM relies on a variety of sensors to monitor the engine's operating parameters, and adjusts the fuel mixture and idle speed as necessary. If a problem or malfunction is detected by the ECM, a fault is stored in the module's fault memory. Depending on the severity or frequency of the fault, a warning light could be activated.
Beginning with model year 1996, all passenger vehicles sold in the United States must conform to the On Board Diagnostics version II (OBD II) standards. These standards included generic terminology and acronyms for the components used by the ECM to monitor and adjust the engine's fuel and emissions management system. The OBD II standards also mandate that a 16-pin diagnostic port be provided for connection of a Data Scan Tool (DST). The DST is used to interrogate the ECM's fault memory and clear fault codes if present.
Should a condition occur that effects idle speed or fuel mixture, check the ECM sensor wiring and electrical connections. Also check for possible sources of an engine vacuum leak possibly caused by a deteriorated hose, gasket of seal. Having access to a suitable DST allows for quick and easy reading of any stored faults. On OBD II compliant vehicles, should the Check Engine light flash or operate continuously, the ECM should be interrogated immediately to determine the source of the stored fault. The Check Engine light operates when performance is compromised and there exists the possibility of damaging the catalytic converter.
The advantage of an OBD II compliant vehicle is that with a suitable DST scan tool, all of the ECM sensors can be monitored dynamically during both the warm up cycle and actual driving conditions without having to probe the wires or electrical connections for each individual sensor. Because the idle speed and idle mixture cannot be adjusted, if present, avoid adjusting any screws on the throttle body.
Idle speed is maintained by the Powertrain Control Module (PCM). No adjustment is necessary or possible.
Idle Speed Stop
- Loosen the idle speed adjustment stop lock nut and loosen the screw 1/10 of a turn in the counterclockwise direction.
- Close the throttle plates and press the throttle lever against the stop screw. Tighten the nut.
- Install a dial indicator on the throttle body with the tip on the deepest point of the throttle plate. Preload the tip and zero the dial.
- Loosen the nut and turn in the screw to move the throttle plate 0.004-0.006 inch (0.10-0.15mm). This will be approximately 1/10 of a turn. Tighten the locknut and mark with a paint dot.
Throttle Linkage Adjustment
- Before servicing the vehicle, refer to the Precautions section.
- On the injection system throttle body, loosen the 2 locknuts at the end of the throttle cable and adjust the cable until there is a play of 0.010-0.030 inches (0.254-0.762mm).
- Loosen the locknut and lower the kickdown stop under the accelerator pedal. Have someone depress the accelerator pedal until the transmission detent can be felt. Then, back the kickdown stop back out until it just touches the pedal.
- Check that the distance from the seal at the throttle body end of the cable housing is at least 1.732 inches (43.9mm) from the rear end of the threaded sleeve and tighten the locknuts.