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How to Test Compression

Last Updated: Sep 27, 2018 03:53PM NDT

Important Information

 
  • 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.
 

Compression Test


Tip: Download the test sheet at the bottom of this page to record your results
 
  1. Ensure the battery is fully charged - you will be using the starter to turn over the engine during testing.
     
  2. Warm up the engine by letting it idle for a short period.
     
  3. Shut off the engine - if the engine/coolant reached full operating temperature it should be allowed to rest and cool slightly before continuing
     
  4. 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) 
     
  5. 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
     
  6. Disconnect your ignition system

    Standard/Ignition Coil
    Unplug the ignition coil wiring connector. See your vehicle's user/factory manual for more information.

    Distributors
    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.
     
  7. Clear the area around the spark plugs of dirt & debris using compressed or canned air.
     
  8. 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
     
  9. If your vehicle is carburated open the choke
     
  10. 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.
     
  11. 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
     
  12. Measure Compression
     
    1. Screw the hose & adapter in to the spark plug hole until hand tight. The o-ring will seal the cylinder
       
    2. Connect the hose to the pressure gauge
       
    3. Ensure the gauge is displaying zero pressure - press the release valve button if the gauge is not displaying 0 psi
       
    4. 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
       
    5. Record the measured pressure as well as any extra notes (e.g. how many strokes it took to build full pressure)
       
    6. Press the pressure release valve 
       
    7. Disconnect the pressure gauge from the tester hose
       
    8. Remove the hose & adapter from the plug hole and then repeat the above steps for all remaining cylinders
       
  13. ​Perform any re-tests are necessary
     
  14. Install your spark plugs

    Note: Exercise caution to prevent cause threading, and tighten all plugs to the factory specified torque values for your vehicle
     
  15. Reconnect the ignition system & plug wires
     
  16. Replace the fuel pump relay
     
  17. Close the throttle and choke (as appropriate)
     

 

Contact Us

90fdcc22fa2aef945851ac9ea6bff549@lemurvehiclemonitors.desk-mail.com
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