Checking idle speed

Part 1. Preparation and safety

Objective

• Check and compare engine idle speed with the manufacturer's specifications.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

• Work clothing - such as coveralls and steel-capped footwear
• Eye protection - such as safety glasses and face masks
• Ear protection - such as earmuffs and earplugs
• Hand protection - such as rubber gloves and barrier cream
• Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

• Always make sure that you wear the appropriate personal protection equipment before starting the job. It is very easy to hurt yourself even when the most exhaustive protection measures are taken.
• Always make sure that your work area/environment is as safe as you can make it. Do not use damaged, broken or worn out workshop equipment.
• Always follow any manufacturer's personal safety instructions to prevent damage to the vehicle you are servicing.
• Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

• Checking the idle speed can only be done with a tachometer, which measures the engine revolutions per minute (rpm). Most modern vehicles are fitted with a built-in dashboard tachometer, but if the vehicle you are working on does not have this feature, use a separate inductive type tachometer that attaches to the engine. The manufacturers of these tools provide specific instructions on how to do this. You will need to adjust this external tachometer for your particular engine cylinder configuration and type of ignition system. This will give you an accurate measurement of the number of engine revolutions per minute.
• "Kicking" the throttle once the engine has warmed up will help allow the engine to settle into its normal idle condition. Note the reading again. "Kicking" gives you an accurate reading of the normal idle speed. In some systems, the automatic choke system does not cut out until the throttle has been depressed. Also, "kicking" the throttle and letting the engine settle again will confirm that the vehicle does not have a sticky throttle cable that may give you a false idle reading.

Part 2: Step-by-step instruction

1. Start the engine and note idle speed
   Switch on the ignition and start the engine from cold. Without revving the engine, note the rpm reading from the tachometer once it has stabilized and is running smoothly.
2. Rev the engine briefly and note speed again
   Allow the engine to warm up to its normal operating temperature, and note the idle speed again. Kick the throttle briefly to rev the engine and let it settle back to idle speed, and note any change in the tachometer reading.
3. Compare readings with specifications
   Compare your idle speed readings with the specifications in the vehicle workshop manual. If your readings are outside the acceptable range, this could indicate a problem in the air or fuel system and should be reported to your supervisor.