This section contains only adjustments as they normally apply to engine tune-up. Descriptions of the carburetor or fuel injection systems and other procedures can be found in Fuel System in this repair guide.
When the engine in your Toyota is running, air/fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the downward movement of the pistons on the intake stroke of the 4-stroke cycle of the engine. The amount of air/fuel mixture that enters the engine is controlled by throttle plates in the bottom of the carburetor. When the engine is not running the throttle plates are closed, blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected, through the throttle linkage, to the gas pedal in the passenger compartment of the car. After you start the engine and put the transmission in gear, you depress the gas pedal to start the car moving. What you actually are doing when you depress the gas pedal is opening the throttle plate in the carburetor to admit more of the air/fuel mixture to the engine. The further you open the throttle plates in the carburetor, the higher the engine speed becomes.
As previously stated, when the engine is not running, the throttle plates in the carburetor are closed. But, in order for the engine to idle, it is necessary to open the throttle plates slightly. To prevent having to keep your foot on the gas pedal when the engine is idling, an idle speed adjusting screw was added to the carburetor. This screw has the same effect as keeping your foot slightly depressed on the gas pedal. The idle speed adjusting screw contacts a lever (the throttle lever) on the outside of the carburetor. When the screw is turned in, it opens the throttle plate on the carburetor, raising the idle speed of the engine. This screw is called the curb idle adjusting screw, and the procedures in this section will tell you how to adjust it.
Fuel injected engines function in a similar manner. Vacuum is used to draw the air/fuel mixture into the engine, but fuel injectors are used to introduce this fuel just upstream from the appropriate intake valve. The throttle plate is still used to control engine speed by the amount of air allowed into the engine, then the computer control module will regulator fuel delivery based on input from various sensors (including the throttle position). Again, an idle speed screw is used to set the curb idle speed.
On carbureted engines, it is difficult for the engine to draw the air/fuel mixture from the carburetor with the small amount of throttle plate opening that is present when the engine is idling. Because of this, an idle mixture passage is provided in the bottom of the carburetor below the throttle plates. This idle mixture passage contains an adjusting screw which restricts the amount of air/fuel mixture that enters the engine at idle. the procedure given in this section will tell how to set the idle mixture adjusting screw.
See Figures 1, 2 and 3
Perform the following adjustments with the air cleaner in place. When adjusting the idle speed and mixture, the gear selector should be placed in Drive (D) on 1970-73 models equipped with an automatic transmission. Be sure to set the parking brake and block the drive wheels. On all cars equipped with manual transmissions and all 1974 automatics, adjust the idle speed with the gearshift in Neutral (N)
- Run the engine until it reaches normal operating temperature, then stop the engine.
Connect a tachometer to the engine as detailed in the manufacturer's instructions, but keeping the following in mind:
- On models equipped with a conventional ignition system, one lead (usually black) goes to a good chassis ground. The other lead (usually red) goes to the distributor primary side of the coil (the terminal with small wire running to the distributor body);
On models with transistorized ignition, connect one lead (usually black) of the tachometer to a good chassis ground. Connect the other lead (usually red) to the negative (-) coil terminal; NOT to the distributor or positive (+) side. Connecting the tach to the wrong side will damage the switching transistor.
- Remove the plug and install a vacuum gauge in the manifold vacuum port by using a suitable metric adaptor.
- Start the engine and allow it to stabilize at idle.
- Turn the idle speed screw until the engine runs smoothly at the lowest possible engine speed without stalling.
- Turn the idle speed screw until the vacuum gauge indicates the highest specified reading (please refer to the vacuum at idle chart) at the specified idle speed. (For idle speed information, please refer to the tune-up specifications chart located earlier in this section.)
- Tighten the idle speed screw to the point just before the engine rpm and vacuum readings drop off.
- Remove the tachometer and the vacuum gauge. Install the plug back in the manifold vacuum port. Road test the vehicle.
- In some states, emission inspection is required. In such cases, you should take your car to a diagnostic center which has an HC/CO meter, and have the idle emission level checked to be sure that it is in accordance with state regulations. Starting 1974, CO levels at idle are normally provided on the engine tune-up decal under the hood.
See Figures 4 through 10
The idle speed and mixture should be adjusted under the following conditions: the air cleaner must be installed, the choke fully opened, the transmission should be in Neutral (N), all accessories should be turned off, all vacuum lines should be connected, and the ignition timing should be set to specification. Be sure to firmly set the parking brake and block the drive wheels for additional safety.
- Start the engine and allow it to reach normal operating temperature (180°).
- Check the float setting; the fuel level should be just about even with the spot on the sight glass. If the fuel level is too high or low, adjust the float level. For details, please refer to Fuel System in this repair guide.
1983 and later A-series engines require a special type of tachometer which hooks up to the service connector wire coming out of the distributor. As many tachometers are not compatible with this hook-up, we recommend that you consult with the manufacturer before purchasing a certain type
- Connect a tachometer in accordance with the manufacturer's instructions. However, connect the tachometer positive (+) lead to the coil. Negative (-) terminal. Do NOT hook it up to the distributor or positive (+) side; damage to the transistorized ignition will result.
On 1980 and later models, all of which have tamper-proof idle mixture screws, merely turn the idle speed adjusting screw until the proper idle speed is obtained (for specifications refer to the tune-up chart earlier in this section). Disregard the steps following idle speed adjustment, as they do not apply to 1980 and later vehicles. Instead, shut the engine OFF and disconnect the tachometer
- Turn the idle speed adjusting screw to obtain the correct curb idle speed.
- Turn the idle mixture adjusting screw to increase the idle speed as much as is possible.
- Turn the idle speed screw to again obtain the correct curb idle speed, then if possible, turn the idle mixture screw to increase the idle speed again. Keep repeating this step until the idle mixture adjusting screw will no longer increase the idle speed.
- Shut the engine OFF , then disconnect the tachometer.
See Figures 11, 12 and 13
- Run the engine until it reaches normal operating temperature.
The air cleaner should be in place, all accessories should be turned off and the transmission should be in Neutral. Be sure to firmly set the parking brake and block the drive wheels
- Install a tachometer that is compatible with diesel engines.
- Disconnect the accelerator cable from the injection pump and then check that the idle speed is within specifications. If not, adjust it using the adjustment screw on the injection pump.
- With the tachometer still connected, check the engine maximum speed by fully depressing the adjusting lever on the injection pump. Maximum speed should be approximately 5,100 rpm.
- If maximum speed is not within specifications, turn the adjusting screw until it is.
- Connect the accelerator cable and adjust it so that there is no slack.
- Check to see that the adjusting lever is stopped by the maximum speed adjusting screw when the accelerator pedal is depressed all the way to the floor.
See Figure 14
Idle speed is the only service adjustment possible for fuel injected engines as the air/fuel mixture is controlled electronically by the computer control module
- Connect a tachometer (follow the tachometer manufacturer's directions) to the engine. Run the engine until the normal operating temperature is reached. Make sure that the air cleaner is installed, all pipes and hoses of the air intake system are installed and all vacuum hoses are connected.
- Check that all electrical connectors are fastened and tight. Switch off all accessories.
- Run the engine up to 2,500 rpm for about two minutes. Allow the engine to return to the normal idle speed. Adjust the speed as necessary by turning the curb idle speed screw (rear upper side of unit).