Compression Test Lab Notes

What is the purpose of a compression test?

The purpose is to tell you the mechanical condition of the internal parts of an engine.

• It is made to test the operation/condition of:

  1) Compression rings/piston rings

  2) Valves (intake & exhaust)

  3) Cylinder head gasket

  4) Block/Head/piston

  5) Timing Chain

What tools are needed for compression test:

• Compression test kit (red snap on tool box)

• 3/8’’ drive torque wrench

• 3/8’’ extension ( at least 6 inches)

• 3/8’’ Rachet/breaker bar

• 3/8’’ socket (student tool box, cabinet B)

Steps for compression test: Step 1

Connect the battery to the vehicle by connecting the negative battery terminal on the negative battery terminal.

Steps for compression test: Step 2

Disable the fuel system by disconnecting fuel injectors or remove he fuel pump fuse. (check fuse box or online) (use pliers, relay or fuse pullers).

Steps for compression test: Step 3

Start the vehicle to relieve residual pressure in the system. Vehicle might start up momentarily, then die. (skip if you where able to pull injectors)

Steps for compression test: Step 4

Disable ignition system by unplugging the electrical connector to all ignition coils. (make sure they are unplugged fully)

Steps for compression test: Step 5

Always mark the coils (ignition coils) to their cylinders they came out of. (tape and number them).

Steps for compression test: Step 6

Remove the ignition coils.

Steps for compression test: Step 7

Using compressed air, Blow out all the spark plug holes. (Before you pull the plug)

Steps for compression test: Step 8

Using the breaker bar extension & spark plug, remove the spark plugs. When removing, keep them in order of the cylinders they came out of.

Steps for compression test: Step 9

Thread compression tester into the spark plug hole by hand. DO NOT overtighten.

Steps for compression test: Step 10

Stop and get instructor to check your work!!!

Steps for compression test: Step 11

Open the throttle all the way by pressuring the gas pedal to the floor. Crank the engine until you see the needle stop moving. (How much did we squeeze the air in combustion chamber).

Steps for compression test: Step 12

Record your readings. Press the button on the side of the gauge and then remove the tester. (check service info for the amount it should be)

Steps for compression test: Step 13

Repeat the steps until you have readings for all the cylinders.

Steps for compression test: Step 14

Put the spark plugs back in and thread them in by hand (you can use the spark plug socket and extension, but not the ratchet.

Steps for compression test: Step 15

Use the 3/8’’ torque wrench and socket extension to finish tightening.

Steps for compression test: Step 16

Torque all spark plugs to factory spec (look up on service info)

Steps for compression test: Step 17

Put all the ignition coils back into the cylinder you took them out of.

Steps for compression test: Step 18

Re-connect your ignition fuel system.

Steps for compression test: Step 19

Disconnect the negative battery terminal. Key should be put back on the wall.

Steps for compression test: Step 20

When complete, have instructor check your work. Make sure everything is put back and be prepared to explain your readings to instructor.

Hoe do you diagnose your readings and what do the amounts mean?

• If all your cylinder readings are within 5-10% of each other, engine is in good condition.

• If 1 cylinder is bad you have a bad valve.

• If 2 adjacent cylinders are low you have a bad head gasket (leaking).

• If all readings are low, you have bad piston/compression rings.

• All crazy readings, you have timing chain is off (cam timing)