On carbureted engines, idle speed is adjusted periodically to compensate for engine wear and to ensure smooth operation. Idle mixture adjustments are not required as a matter of routine, but only when major carburetor work is required.
Fuel injected engines employ an idle speed control which compensates for all normal variations in idle speed. This requires adjustment only if a major fuel system part is replaced. For further information, see Fuel System in this guide.
See Figures 1, 2 and 3
The throttle valve adjusting screw should not be tampered with unless the carburetor has been rebuilt. This screw is preset and determines the relationship between the throttle valve and the free lever, and has been accurately set at the factory. If this setting is disturbed, other service adjustments cannot be done accurately. Also the improper setting (throttle valve opening) will increase the exhaust gas temperature during deceleration, which in turn will reduce the life of the catalyst greatly and deteriorate the exhaust gas cleaning performance. It will also effect fuel consumption and engine braking.
- Turn all lights and accessories to the off position.
- Connect a tachometer and timing light to the engine.
- Place the transmission in Neutral and disconnect the electric cooling fan.
- Start and warm up the engine at idle speed for a few minutes. On 1987-89 vehicles, press the gas pedal once to disengage the fast idle.
- Check the ignition timing and adjust as necessary.
- Check the idle speed. If it does not meet specifications, adjust the idle speed using the idle speed adjusting screw, which is located closest to the primary throttle valve shaft and labeled SAS 1 in the illustration.
- After the idle speed is set, stop the engine and reconnect the cooling fan connector. Disconnect the tachometer and the timing light.
FUEL INJECTED ENGINES
These vehicles are equipped with an Idle Speed Control (ISC) system which controls the idle speed automatically, making adjustment unnecessary. For more information on the ISC system, see Fuel System . To check the curb idle speed, proceed as follows:
- Turn all lights and accessories to the off position and place the transmission in Neutral (manual) or Park (automatic). Place the steering wheel in the straight ahead position.
- Start the engine and warm it up for a few minutes.
- Locate the engine speed detection connector between the noise filter and the primary side of the ignition coil. Insert a paper clip through the harness side to short it and connect a tachometer to the paper clip. Refer to the ignition timing procedure for details.
- Run the engine at idle speed and check the ignition timing. If not within specification, adjust it.
- Race the engine at 2,000-3,000 rpm for five seconds then allow the engine to run at idle for two minutes.
- Read the tachometer and check the idle speed.
If idle speed is not within the specified limits, check the ISC system.
- Stop the engine and disconnect the tachometer and the timing light.
See Figure 4
Use of a diesel (magnetic) tachometer is required. A conventional tachometer as used on gasoline engines will not work. When connecting the diesel tachometer, follow the manufacturer's instructions carefully.
- Make certain that the lights and all electrical accessories are turned off.
- Set the parking brake and place the shift selector in Neutral.
- Connect the tachometer.
- Run the engine until the coolant warms to normal operating temperature.
- Once the engine is completely warmed up, run the engine at 2000-3000 rpm for at least 5 seconds.
- Allow the engine to run at idle for 2 minutes.
- Read the idle speed on the tachometer. If the idle is not within specifications, adjust the idle speed screw. Loosen the locknut, then turn the adjusting screw with a small screwdriver.
Do not disturb other screws or linkages.
- When the proper idle speed is achieved, hold the screw in position and tighten the locknut.
- Stop the engine and remove the tachometer.
See Figure 5
This procedure is only for Pick-up trucks equipped with air conditioning.
When doing this procedure, the following conditions should be met:
- Make sure that the curb idle speed is within the specified speed and reset it by turning the idle speed adjusting screw, if necessary.
- Loosen nuts 1 and 2. Turn the adjuster to set the length of the vacuum actuator rod so that the end of the rod is positioned as shown in the illustration in relation to the accelerator lever. Retighten the two nuts.
- Turn the air conditioner switch on and off several times, then set it to the off position and confirm that the rod and accelerator lever are not in contact with each other.
- Loosen the nut which secures the idle-up adjusting screw, adjust the engine speed to the specified speed by using the idle-up adjusting screw, and then retighten the nut. The idle-up engine speed should be 850-900 rpm.
- Turn the air conditioner on and off several times to check the vacuum actuator for operation (lever goes up and down).