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Why We Remove All Spark Plugs for a Compression Test

Why Remove All Spark Plugs for Compression Test

Good compression for an internal combustion engine is 100 PSI. An engine is driven through cycles during a compression test. The engine is rotated to get a pressure reading.

After the test, the readings are compared to determine an engine’s health – There should be no more than a 10% variation for each cylinder between the lowest and highest readings.

You can remove just one spark plug if you’ve narrowed compression issues to one problematic cylinder.

But if that’s not the troubled cylinder. You’ll need to go through the remaining cylinders removing and replacing spark plugs until you find the cylinder with low compression.

In my opinion, it’s easier to remove all the spark plugs at once before performing compression testing.

Some of the reasons why all spark plugs are removed for compression testing are:

1. To Make Room for the Compression Tester

An internal combustion engine is a big air pump.

A compression test works by spinning an engine through its cycles.

To get the proper readings the compression tester goes in place of the spark plug in the cylinder.

As the engine builds pressure, the gauge on the compression tester will move and hold at the highest point on the gauge.

For accuracy, you can run the compression test two or more times per cylinder.

Then, record these measurements so you can compare them against one another.

There should be no more than a 10% variation between the highest and lowest readings from a single cylinder. All readings should be at or close to 100 PSI.

2. Prevents Unwanted Ignition

Running a compression test is a great way to get an idea of the condition of your valves, valve seats and piston rings.

An engine has the best compression when it’s at normal operating temperature to ensure you’re getting accurate readings.

Compression pressure is specified at cranking (rotating) speed.

But if the engine starts up during a compression test it throws off the readings.

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Removing all the spark plugs – as well as a few other fail safes we’ll talk about later – will prevent the engine from starting up as you crank the motor during compression testing.

3. Get the Correct Cranking Pressure

It’s important to keep altitude in mind as you perform a compression test.

For this reason, it’s better to contrast the readings you get from each cylinder against each other rather than trying to match an assumed compression number.

Cranking compression pressure is the pressure a cylinder builds when the valves are closed, and the piston moves up to compress the air and fuel mixture in a cylinder. It’s measured in PSI (pounds per square inch).

For a stable reading I would crank the engine 4 times. If the readings you’re getting are all over the gauge, then try cranking the engine for a total of 10 revolutions.

The key to accurate readings is to crank the engine the same number of times per cylinder.

4. For Accurate Compression Reading

Removing all the spark plugs for a compression test also results in accurate readings.

To get an accurate reading you need to allow the starter to cycle the engine through three full compression strokes.

Again, a compression stroke is when a piston travels up a cylinder to compress the air and fuel mixture.

Just before a piston reaches the top of its compression stroke under normal engine operation. The spark plug will emit a spark to ignite the air and fuel mixture – this is combustion.

But during a compression test, we don’t want this to happen. Which is why we remove all the spark plugs.

And since all the spark plugs will be removed it’s a good idea to wear protective goggles to protect your eyes from the air that will rush out of the now open spark plug ports as the engine rotates during compression testing.

5. Preserves Battery Life

Slow crank speed causes low compression.

It’s important to keep the battery voltage in mind as you do a compression test.

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If the battery is weak the starter will crank slower with each cylinder you test. And by the time you get to the last cylinder the starter will be cranking even more slowly throwing off your readings and making you think you have low compression in that cylinder when the cause is actually from a weak battery.

Battery voltage should stay above 10 volts during cranking.

A starter is a device that converts electrical energy into mechanical energy.

The battery in an automobile is responsible for providing the electrical energy.

Under the usual engine operation conditions, the starter is responsible for spinning the engine until it can start the combustion process to run on its own.

Removing the spark plugs ensures the engine doesn’t turn over.

6. Helps the Engine Spin Faster

Without the spark plugs in place during a compression test, air is pumped out of the now open spark plug ports during an up stroke as you’re cranking the motor.

Then air gets sucked into the engine during the down stroke (hence the need for the protective eye wear).

Since the lack of spark plugs means there’s no resistance, the engine is able to spin faster.

The open ports left by the spark plugs does present a problem, however: The opening makes it easy for dirt and debris to enter the engine. Specifically, the area where the pistons are moving up and down.

This can cause abrasions on the cylinder wall which can lead to future low compression.

Be sure to work in an environment where there isn’t risk of dirt and debris getting sucked into the engine.

Or at least cover the holes as you test each cylinder.

Don’t Forget to Remove the Ignition Coil Fuse Before a Compression Test

Ignition coils convert the low current from an automobile’s battery into enough power to ignite fuel and start the engine.

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But during a compression test, the goal isn’t to start the engine.

Plus, leaving the ignition coils in place during a compression test could short them out.

For their protection – and to ensure an accurate reading – you’ll want to either:

  • Pull the ignition coils out of the valve cover before you begin compression testing.
  • Disconnect the primary wires going to the ignition coils.
  • Or remove the ignition coil fuse.

You’ll Also Want to Remove the Fuel Pump Fuse

Without removing the fuel pump fuse, fuel will continue to spray into the cylinder during a compression test.

This can cause lubrication to be washed from the cylinder walls, which can not only create an improper reading.

But damage the engine. However, simply pulling the ignition and fuel pump fuse without also pulling spark plugs isn’t enough to get a good reading.

Leaving the spark plugs in will still result in low compression readings since the cranking speed will be low.

Other Things to Consider About Engine Compression: Spark Plug Size Affects Compression

The size of a spark plug can absolutely affect compression.

A loose spark plug can cause issues like compression leak, poor combustion and misfire.

Now, when it comes to the electrodes in a spark plug. The distance between them has less effect on compression and more to do with the strength of the spark necessary for igniting the fuel and air mixture necessary for proper combustion.

If the spark plug gap is too small there won’t be enough of a spark to complete the combustion process.

If it’s too wide the spark won’t be able to jump across the long distance between the electrodes.

Both issues can affect an engine’s ability to run properly.