On all vehicles equipped with adjustable ignition timing and an adjustable idle speed the two processes of adjusting the idle speed and the ignition timing are very closely related and affect each other closely. When performing either of these adjustments, it is usually necessary to adjust each in a back and forth manner since they affect each other. For example, if the ignition timing is adjusted the idle speed could increase past what is recommended. This in turn could affect actual ignition timing during normal use of the vehicle, which would affect the vehicle's driveability. Therefore, the idle speed would have to be adjusted, which could in turn affect the ignition timing. Eventually, after adjusting each back and forth a few times to reach the desired specifications, they will meet both specifications simultaneously. Most often they only need to be adjusted a few times before both are correct.
See Figures 1 and 2
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. Adjustment of this system is usually unnecessary-refer to Emission Controls .
- Stop the engine and disconnect the tachometer and the timing light.
See Figure 3
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.