Do all plugs have to be removed for a compression test?

Spark plugs are small parts in a car but play an incredibly significant role in-car functions. Without a properly working spark plug, the car might not be able to run as required.

Spark plugs supply the spark needed by the engine to ignite the fuel/air mixture producing an explosion that makes the engine run. However, there are queries on whether the spark plugs should be removed when the car undergoes a compression test.

The engine is driven in cycles during this test using a compression tester instead of a spark plug. The gauge in the car’s engine will continue to move up or down as per the pressures when the engine starts to move.

After a compression test, the readings obtained from each cylinder are compared to determine the engine’s health. But do you have to remove all spark plugs when doing this test? Check out

Technically, it is advisable to remove all the spark plugs while conducting a compression test. Doing by one cylinder itself is likely to fire up, thereby giving incorrect readings.

When all the spark plugs are removed, the correct cranking pressure can only be determined at the cranking speed. Leaving the plugs on during the test and pulling the fuel or ignition pump, the pump won’t run the pressure since the cranking speed will be super slow.

This means that if you need a consistent and proper reading from all engine cylinders, it is advisable to remove all the spark plugs before conducting the test.

However, this is not always necessary because you can only focus on the cylinder if you only have an issue with one cylinder.

But for the sake of your engine’s overall health, ensure you remove all spark plugs before the compression test. This will reduce the load on the starter engineer to make the engine spin faster and increase engine longevity.

Generally, you should remove the spark plugs to get accurate measurements from each cylinder.

Reasons why you should remove all spark plugs before an ignition test

The spark plugs should be removed from the car for a compression test because every spark plug hole will hold a compression tester for every cylinder to be tested.

The other reason is that removing the spark plugs allows the starter to easily spin the engine at a higher RPM than when done with the spark plugs in place.

Do spark plugs affect compression?

To some extent, a spark can affect compression. This is dependent on the spark plug size to the volume of the cylinder. A bigger spark plug will take the larger percentage of the cylinder if the engine is small, hence increasing compression. However, if the engine is bigger, a plug of a similar size will have less effect on the compression.

What should be done before removing the spark plugs for a compression test?

Spark plugs are useful in providing sparks for fuel ignition but they also prevent dirt from getting into cylinders. This means removing spark plugs from the engine will leave an open hole that may cause cylinders to get filled with dirt.

Cylinders are sensitive components and dirty should not contaminate their walls as they will get scratched or scored. If this happens, the cylinder’s compression ability will be poor.

Therefore, it can be a very critical process during a compression test trying to prevent dirt from getting into the cylinders.

So, what should be done before removing the spark plugs for a compression test? The key goal here is to prevent the happening of issues brought about by the absence of spark plugs in the cylinder.

Therefore, before removing the spark plug, use compressed air to blow around spark plugs to prevent contamination and dirt from entering the cylinder.

However, you must use pressure wash around the spark plugs if the engine is dirty before taking the spark plugs off. But take caution, such as avoiding pressure washing the whole engine; perhaps, some components may be damaged, such as wire connectors and another electrical wiring.

Remember to wear or use safety gear such as a face shield, gloves, and glasses when handling a pressure washer or compressed air machine.

What should I disconnect when doing a compression test?

One of the most key components you need to disconnect is the ignition coil fuse before doing a compression test. It is located in the fuse box.

The ignition coil fuse is placed so that it can receive or connect sparks. Therefore behind this ideal disconnecting ignition coil fuse helps prevent electrical sparks into the spark plug in the engine.

If you don’t know how to locate it, use the fuse box diagram or your owner’s manual.

However, your vehicle may lack an ignition fuse; so, you can check a big one located in the compartment of the engine. Its appearance is like a perched cylinder above the engine; unplug big wires by pulling them off the coil’s top part.

It is also important, and even a must you take out all spark plugs from the engine before starting the compression test. This will help to observe an accurate record in the cylinder you test.

Can you compression test just one cylinder?

You can do a compression test on a single-cylinder if it is the only one with problems.

How long does a compression test take?

Engines in good conditions should have a compression of over 100psi in every cylinder with less than ten percent variation between the lowest and highest readings.

Averagely, a compression test can take about twenty minutes with just some hand tools and a compression tester.


A compression test aims to determine how far a material can hold over a specific time when loading. This activity is done in car cylinders.

However, to achieve an accurate reading regarding each cylinder, ensure you take off all spark plugs. In addition, disconnect the ignition coil fuse to avoid electrical sparks being sent to the engine’s spark plug.

But you need to care more about dirt entering the cylinders through the spark plug holes when you remove them. Therefore, always clean around the spark plugs with a pressure washer of compressed air before removing them for the compression test.

Scroll to Top