Idle adjustment sets the proper speed of engine rotation during vehicle standstill (closed throttle). Correct idle speed is important for proper engine operation and fuel economy. Idle speed also affects the function of engine-driven accessories such as the alternator, air conditioning compressor, engine cooling fan and water pump. Idle is adjustable on 1984-1986 carburettor-equipped models of the Jeep Cherokee. On 1987 and later fuel-injected Cherokee models, idle is controlled by the engine control computer and is not adjustable as part of routine tune-up and maintenance.
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Things you need
- Small (1/4-inch or adjustable) wrench or pliers
- Flathead screwdriver
Connect the tachometer to the vehicle following the tachometer manufacturer's instructions. Exact attachment depends on your tachometer's make and model. Be certain any wires are clear of the fan and other moving engine parts.
Set the parking brake. Shift the automatic transmission to "park" or manual transmission to "neutral."
Start the engine and allow it to run until it reaches normal operating temperature (upper coolant hose will be hot). Shut off the engine, as well as the air conditioner.
Disconnect the vacuum line from the carburettor sol-vac actuator (four-cylinder models) or the exhaust gas recirculation valve (V6 models). Plug the disconnected vacuum lines.
Start the engine. Check the idle speed with the tachometer. For manual transmissions, the shifter should be in "neutral." For automatic transmissions, the shifter should be in "drive." Have an assistant shift to "drive" while holding the brake pedal down.
Compare the idle speed to the specifications on the vehicle emission control information sticker, which is in the engine compartment. Normal idle speed is 700 to 750rpm. Use this value if the sticker is missing or damaged. If the idle speed is correct, you are finished. Shut off the engine, disconnect the tachometer and reconnect the vacuum lines.
Adjust the idle speed by turning the idle speed adjusting screw on the carburettor (V6 models) or by turning the 1/4-inch nut on the sol-vac vacuum actuator (four-cylinder models). Turn clockwise to increase idle speed, counterclockwise to decrease idle speed.
Shut off the engine, disconnect the tachometer and reconnect the vacuum lines.
Tips and warnings
- It is a good idea to make sure all other engine systems are properly functioning and adjusted before adjusting idle speed. For example, fouled spark plugs or a clogged fuel filter will cause erratic idle, making idle speed adjustment difficult and ineffective.
- Warmed-up engines are hot. Be careful not to touch the engine itself or any components of the cooling or exhaust systems. Severe burns can result.
- Do not wear loosefitting clothing, jewellery or long hair around a running engine. Loose items can catch in the fan or other moving parts and cause serious injury.
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