What To Do When Your Check An Engine’s Cylinder Compression

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If your vehicle has been running roughly or losing power, there may be not enough pressure in one or more cylinders. To ascertain whether pressure is escaping from the engine, you need to check the compression inside the cylinders using a compression gauge, which measures the amount of pressure that the piston exerts around the fuel/air mixture before the spark plug fires the mixture. These gauges don’t cost much, and they’re easy to use. Some gauges screw into the spark plug opening, and others really need to be held in place.

If there isn’t enough pressure, it’s escaping through one of the valve openings (because the valve is improperly adjusted or worn), down past the rings on the piston or through a blown head gasket.

Here’s using a compression gauge:

Have someone sit inside the driver’s seat with the engine off, the gearshift in Park or Neutral, and the parking brake on.

The next phase depends on the type of distributor you have:

On vehicles with distributors: Pull the major wire leading to the coil from the middle of the distributor cap, and lean the metal connector against an unpainted metal surface as far from the spark plugs as possible.

On vehicles with distributorless ignitions: Disconnect the electrical connector at the ignition control module. Ask a mechanic if you’re not sure what to disconnect.

Disable the fuel injection system so that gasoline mist won’t spray out of the spark plug holes and possibly ignite.

Remove the fuse labeled “Fuel Pump”; then start the car and let it run until it stalls from insufficient gasoline.

Label and remove the boots that connect each spark plug wire and every spark plug.

If you get the plug wires mixed up, you can really screw the engine.

Remove all of the spark plugs and lay them down in a clean place.

As a way to insure which you return each one of these to its original cylinder as soon as the time comes, keep the labeled plugs.

Connect the starter switch to battery.

If you have a remote starter switch, connect one clip to the battery’s positive or “plus” terminal, and the other to small terminal of the starter solenoid.

Insert the compression gauge

It should match the hole in the engine where the first spark plug screwed in to the cylinder.

Checking compression.

Checking compression.

If you don’t use a remote starter switch, have a friend turn on the ignition until the engine cranks over about six times. Otherwise, press the button in the remote starter switch.

Be sure to keep the gauge plug firmly inserted while the engine is cranking. (The vehicle won’t run because the engine has been disabled.)

Look at the gauge and write down the reading, which will be in psi (pounds per square inch)., and then reset the gauge.

Repeat these steps for each of the other cylinders.

Don’t forget to reset the gauge and crank the engine each time.

After you’ve tested each cylinder, glance at the readings.

The lowest and highest shouldn’t vary by more than 15 percent. If one or more of your cylinders reads well below the rest, use a trigger-type oil can to transmit a good squirt of motor oil across the spark plug opening, and retest the compression of that cylinder with the gauge. In the event the reading is the same, the valves either are worn (and letting pressure escape) or are out from adjustment. In the event the reading rises dramatically when you insert the oil, you almost certainly need new rings in the piston in that cylinder. The cylinder definitely isn’t mechanically sound if the pressure recorded with the gauges is less than 100 psi.

Replace each spark plug in the cylinder it came from.

And ensure to put the appropriate spark plug wire boot back on each plug, make sure that the ignition is off prior to deciding to reconnect the spark plug wires. Screw the plugs in yourself to avoid damaging the threads in the aluminum valve cover.

If the “Check Engine” warning light comes on as soon as you perform a compression test and doesn’t disappear in a few days, have it reset at the dealership.