Lesson Plan Industry Sector

Lesson Plan Originally Created By: Michael Fleming

Compression Testing (updated) by Michael Fleming

Part of Unit: Engine Performance Fundamentals

Lesson Plan Overview / Details

Students will use a compression tester to diagnose the internal components of an automotive engine.

Lesson Time

Lecture, Demo, Guided Activity
3 Hours

Objectives and Goals

  • Students will understand when to perform a compression test.
  • Students will understand and perform a compression test to properly diagnose the mechanical components of an engine.

Activities in this Lesson

  • Remote Starter Hooks / Set

    Tap into prior knowledge from a previous lesson(s) on remote starter, by starting the class with a starter.  After students arrive, place a starter in a vise, and hook up a remote starter and battery pack. If you have a confident student, have a student hook the starter up.  Activate the starter as if bench testing with the remote starter.  Ask the students to describe what was done. Next, ask them why you would use a remote starter. List all the reasons they come up with, and put a star by compression testing. If they don't mention it, then add it to their list and say that compression testing is one of the most common reasons to use a remote starter.

    For safety, stress that a remote starter can be potentially dangerous for the following reasons. First, it bypasses all the safety features on a vehicle; make sure the parking brake is applied, the vehicle is in park or neutral and the wheels are chucked. Also be careful of creating any sparks around the battery.

  • I always stress with my students the availability of information on the internet that was not around when I was a student.  I tell them the best technicians are the best at research, in other words, they excel at educating themselves and are capable of applying the new information to real situations. Show the students the following You Tube Video.

    Resources and Materials

  • 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.

    Resources and Materials

  • Have a four cylinder vehicle ready in a service bay with the necessary tools: 3/8 ratchet set with spark plug socket, spark plug wire pliers, compression gauge, pry bar (for propping accelerator pedal, remote starter, fender covers. Prep the car following the steps described in class. Perform "think alouds" so that the students understand how you are thinking. Pull the plugs and lay them in the order of the cylinders and do a thorough plug condition evaluation. Pay special attention to safety (blocked tires, vehicle in neutral or park, tools off or away from the battery).

    When performing the compression test on each cylinder, you do the first emphasizing 5 cycles. For each adjacent cylinder, let the students gradually perform the repair and evaluate the results or  mistakes.

  • Guided Practice Group Work

    Pass out the worksheet, and discuss what you expect them to do and write. Assign groups of 4 to 5 students per vehicle and explain they will get stamped when they show proficiency as a group in performing a compression test. It is best to also assign carts to each group with the required tools.

    Resources and Materials

  • I provide closure by creating dialog with the students in the group setting. After I stamped off a group on the activity, I ask the students to summarize what they have learned, and I ask for their thoughts on the usefulness of the tool (the how & when the skill would be used in an actual shop).


The instructor should rotate through the shop activity monitoring for safety glasses, proper compression testing procedures and diagnosis. Ask open ended questions to check each student's understanding. Each student should also be observed testing at least one cylinder correctly. After verification of all of these assessments, the instructor should use a slected stamp to stamp the worksheet and the completion of the skills.