How to Check Compression in Vehicles

by Contributing Writer; Updated June 12, 2017

Thee conditions needed Vehicles an engine to run correctly consist of fuel, electricity and compression. Compression has to exist in the Vehicles motor's cylinders in order to compress a combination of fuel and air into an explosive mixture. A hot spark drives the piston down, which in turn drives the main crankshaft Vehicles power. The piston must then rise to expel exhaust gases, compress and complete another power stroke. The piston, rings and valves must form a perfectly tight seal while doing this, or compression drops off and poor performance results.

Under The Hood:

 How to Check Compression in a Big Block

Park your vehicle on a level, paved surface and set the parking brake. An automatic transmission should be placed in "Park." Place a manual transmission in "Neutral."

Wrap a piece of masking tape around the spark plug wire and number the wires with a pen so you can put them back in the right place. Remove the spark plug wires and all of the spark plugs.

Fold up a small piece of cardboard. Open the throttle lever on the carburetor to the wide-open position and wedge the piece of cardboard between the lever and its stop.

Screw the end of the compression tester into the first spark plug hole on either side of the engine. Crank the engine for two or three seconds. Write down the gauge reading on a piece of paper. Take a reading from the rest of the cylinders in succession on that side of the engine. Move to the other side of the engine and take readings from front to rear.

Compare all of the readings to each other. The lowest number should be within 10 percent of the highest number. For instance, if your highest number is 150 psi, your lowest number should be no lower than 135 psi.

Check a low cylinder reading to determine if it is caused by bad rings or valves by using a trigger-type oil can to shoot a couple of squirts of motor oil into the spark plug hole. Take another reading. If the compression comes up, the rings are leaking. If the compression does not change, the valves are leaking.

Items you will need

  • Masking tape

  • Pen

  • Piece of cardboard

  • Ratchet and socket set

  • Compression tester

  • Piece of paper

  • Trigger-type oil can

  • Motor oil

 How to Check Compression in an Outboard

Secure the boat to the pier or dock if you're performing this test in the water. Place the manual motor tilt or electric trim in the full down, vertical position. If you have a key lanyard in your ignition, pull it. If not, disconnect your main ignition wire, then use a socket to remove the negative battery cable from the battery. Shut off the fuel supply valve.

Unclasp the snaps to the to top engine cowl case, or use a socket to remove the bolts, depending upon the fasteners. Set the cowl aside. If you have a multiple cylinder engine, unplug all of the spark plug wires from the spark plug tips. Use pliers to bend sections of coat hanger wire to fit into each wire connector and ground the other ends of the coat hanger sections to the engine block. Use a socket and wrench to remove all spark plugs from the engine.

Screw the adapter hose of a compression gauge into a spark plug hole. Tighten the adapter with an end wrench, but not overly tight. If your motor has an electric starter, temporarily connect the negative battery cable by hand, reconnect the ignition wire connector and insert the lanyard key. Make sure the choke is off.

Have an assistant turn the key to the "Start" position so the engine turns over, while holding the throttle in the wide open position. Let the engine turn over five to seven times then stop. Read the psi on the gauge and record it with pen and paper. Remove the compression gauge, screw it into the next cylinder and repeat same procedure. For a pull-rope recoil starter, you must pull the rope four or five times for each cylinder and record the psi.

Check your owner's manual under the section for compression specification requirements for your outboard motor. The psi listing in the manual will be the minimum allowable number for your engine. If any of the engine cylinders fall lower than 15 psi, you have compression deficiency in that cylinder, resulting from worn rings, a bad valve seat or worn valve faces.

Remove the coat hanger wires from the plug wire connectors. Screw the spark plugs back into their respective cylinders and initially tighten them with the socket and wrench. Refer to your owner's manual for the proper torque number on the plugs, and tighten them with a torque wrench.

Push the plug wires back onto the plug wire tips, seating the rubber boots firmly. Tighten the negative battery cable with a socket. Replace the engine cowl case and snap the clasps. Turn the fuel selector valve back on and replace the lanyard key.

Items you will need

  • Motor repair manual

  • Coat hanger wire

  • Pliers

  • Socket set

  • Ratchet wrench

  • Pen and paper

  • Compression gauge

  • End wrenches

  • Assistant (if applicable)

  • Torque wrench

 How to Check the Compression of a Yamaha 660 Raptor

Start your Raptor 660 and let it warm up to its operating temperature. This normally takes between three to five minutes if the ATV is idling in place. Stop the motor and turn off the ignition once your Raptor has warmed thoroughly.

Unplug the high-tension wire from the spark plug, located on the front of the motor's cylinder. Remove the spark plug with a socket wrench and a spark plug socket.

Screw the threaded end of your your compression gauge's hose into the cylinder's spark plug hole. Press the release button of the side of your compression gauge to reset its indicator needle to zero.

Turn your Raptor's ignition switch to the "On" position. Push the throttle lever forward to open the throttle completely. Press and hold the starter button to turn the motor over and build up up compression within the cylinder. Release the starter button after two to three seconds.

Take note of the cylinder's internal pressure indicated by your compression gauge. If the motor's compression is above 210 psi or below 163 psi, remove the compression gauge and squirt five to six drops of 10w40 motor oil into the cylinder. Reinstall the compression gauge and take another measurement.

Unscrew the compression gauge from the motor. Reinstall the spark plug with a spark plug socket and a socket wrench. Press the high-tension wire into place over the spark plug.

Items you will need

  • Socket wrench

  • Spark plug socket

  • Compression gauge

About the Author

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