How to Test Compression
Modified on: Wed, 8 Jan, 2020 at 4:37 PM
- Always wear safety glasses and proper PPE while working in the engine bay of your vehicle
- Be aware of pinch points such as belts and pulleys when cranking the engine
- If the engine is hot allow it to cool before removing spark plugs
- Exercise caution when screwing in spark plugs or the tester adapter - damaging the plug threads in your engine's head can be a costly mistake
- Before each test check the operation of the pressure release button as well as the condition of the o-rings
Tip: When installing plugs or the adapter, 'find the thread' by turning counter-clockwise while applying gentle downward pressure - when you feel a 'pop' you can stop and then carefully start threading it in clockwise. If you experience sudden premature resistance while installing the plug/adapter stop, back it out, and then try again.
Tip: Download the test sheet at the bottom of this page to record your results
- Ensure the battery is fully charged - you will be using the starter to turn over the engine during testing.
- Warm up the engine by letting it idle for a short period.
- Shut off the engine - if the engine/coolant reached full operating temperature it should be allowed to rest and cool slightly before continuing
- Disconnect the fuel system
Tip: The easiest way is to pull the relay/fuse for the fuel pump (see your user manual or your fuse/relay diagram for more information)
- Remove your spark plug wires
Tip: If it isn't immediately obvious which wire goes to which spark plug (e.g. if you have an ignition coil that serves all cylinders) mark/identify each wire to make reinstallation easier
- Disconnect your ignition system
Unplug the ignition coil wiring connector. See your vehicle's user/factory manual for more information.
Disconnect the coil wire from the distributor cap and ground it or disconnect the primary wire as appropriate. See your vehicle's user/factory manual for more information.
- Clear the area around the spark plugs of dirt & debris using compressed or canned air.
- Remove all spark plugs using a ratchet and the appropriate socket size for your vehicle
Tip: Record the cylinder each plug was removed from to aid in further troubleshooting
- If your vehicle is carburated open the choke
- If possible, secure the throttle wide open
Note: On newer vehicles with electronic throttle bodies the throttle should not be physically fixed as this may cause damage.
- Attach the appropriate adapter to the tester hose
Note: do not attempt to connect the tester hose directly without an adapter due to potential risk of piston contact during the test
- Measure Compression
- Screw the hose & adapter in to the spark plug hole until hand tight. The o-ring will seal the cylinder
- Connect the hose to the pressure gauge
- Ensure the gauge is displaying zero pressure - press the release valve button if the gauge is not displaying 0 psi
- Have an assistant hold the accelerator pedal to the floor and crank the engine 5-6 full compression strokes - on four stroke engines this will be 10-12 rotations of the crankshaft.
Try to keep the number of rotations consistent between each test
- Record the measured pressure as well as any extra notes (e.g. how many strokes it took to build full pressure)
- Press the pressure release valve
- Disconnect the pressure gauge from the tester hose
- Remove the hose & adapter from the plug hole and then repeat the above steps for all remaining cylinders
- Perform any re-tests are necessary
- Install your spark plugs
Note: Exercise caution to prevent cross threading, and tighten all plugs to the factory specified torque values for your vehicle
- Reconnect the ignition system & plug wires
- Replace the fuel pump relay
- Close the throttle and choke (as appropriate)
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