Testing cylinder compression

Part 1. Preparation and safety

Objective

• Safely perform a cylinder compression test.

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

• Before performing a compression test, disable the ignition system to prevent electrical shock.
• Be aware of moving drive belts and accessories when the engine is being cranked.
• If you have a helper cranking the engine make sure they only crank it when you tell them to.
• Make sure the compression tester will safely handle the compression of the engine you are testing. Diesel (CI) engines operate at much higher compression pressures than Gasoline (SI) engines.
• 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

• Be sure to follow the vehicle and equipment manufacturer's compression testing procedure.
• Select the proper adapter for the engine you are testing. Make sure it matches the 'reach' of the spark plugs.
• When refitting the spark plugs to aluminum heads, use anti-seize compound on the spark plug threads.
• Reinstall the spark plugs by hand to prevent cross threading. Make sure the spark plugs thread into the hole 5 complete threads (once they 'start') before using a ratchet, or tension wrench to tighten them.
• Always torque the spark plugs to the manufacturer's specifications.
• Re-enable the ignition system, and start the vehicle to verify proper engine operation.

Part 2: Step-by-step instruction

1. Prepare the vehicle
   Locate the specified testing procedure for the vehicle you are working on, and check the engine is at normal operating temperature. Locate the spark plugs & remove the high-tension leads. Make sure you can identify the correct leads for each cylinder. Remove each spark plug and place them on a bench in the correct order.
2. Perform a ‘dry’ test
   Attach the compression tester to the first cylinder and have an assistant crank the engine at full throttle. Note your findings. Repeat this test for each cylinder in turn.
3. Perform a ‘wet’ test
   Insert a small amount of engine oil into the first cylinder and re-attach the compression tester. Have an assistant crank the engine again and note your findings. Remove the compression tester from the first cylinder and cover the cylinder opening with a rag to catch any residual lubricant from the cylinder already tested. Repeat this test for each cylinder in turn. Take your findings to your supervisor.
4. Inspect the spark plugs
   Inspect the spark plugs and, using an appropriate reference chart, note any problems with each plug.
5. Reassemble the engine
   Reassemble the engine with either new or cleaned spark plugs. Refit the high-tension leads in the correct order.
6. Start the engine
   Start the engine to make sure the leads have been refitted in the correct firing order and that the engine runs correctly.