Activity Industry Sector

Activity Originally Created By: MaryRose Lovgren

The How and When To Compression Test

Part of Lesson Plan: Compression Testing (updated) by Michael Fleming

Activity Overview / Details

Refer to the student notes on Resource Materials. Students should have their notebooks ready for taking notes. On the white board, I always start with the four requirements for a cylinder to operate properly; Air, Fuel, Spark, and Compression. Write the triangle somewhere on the board. Since, they just watched the You Tube Video, list the steps for performing a compression test.

1) Locate what cylinder, if any, is misfiring (prior knowledge: cylinder balance test).

2) Warm up the engine to operating temperature.

3) Disable the ignition system "safety" (prior knowledge: disconnecting coil primary circuit).

4) Prop or hold the throttle at WOT (Wide Open Throttle).

5) Remove all spark plugs and check condition (lay them in the order of the cylinders).

6) Screw in compression gauge and hook up the remote starter

7) Crank the motor and record 1st and 5th cranking pressures. (I like to verbally make the noise of the motor cranking emphasizing listening for engine cycles.) The first pressure should be half of the last pressure.

8) Repeat for all the cylinders on the engine.

9) If there are any dead or low pressure holes, perform a wet compression test.

After writing the steps, Draw a depiction of a cylinder that includes: cylinder, head, head gasket, piston, rings, connecting rod, vales and a compression gauge attached. Next to it make a list of the parts that a compression test checks. Then draw two for cylinder motors; one with a blown head gasket readings and one with a dead or low compression cylinder. Discuss each example writing in the over (1st) under (5th) pressures.  I always verbally model the sound of the motor cycling 5 times.You can add a third motor with all the cylinders low. Make sure to discuss looking up the vehicle specifications and comparing them to the readings.

With the motor with a dead hole or third motor with low compression in all the cylinders, I draw an arrow pointing to an example of a wet compression test. I circle the dead hole and explain the need to do a wet compression test by squirting oil into the cylinder, thereby distinguishing if it's the rings or not (if pressure increases it is the rings). The fun part is I actually squirt oil right onto the white board in the drawn circle.

Then go back to the drawing of all the parts and draw circles on the rings to show how the oil is sealing the rings. Explain a wet compression test is necessary whenever a cylinder is low on compression.

Materials / Resource