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Compression Test: What It Is, and How to Do One

Alex Palmeri
May 3, 2018

compression test

A compression test measures the pressure in each cylinder of your car’s engine. It’s normally performed as a step in diagnosing an issue, but it’s also a great way of gauging the health of an engine and is often performed during a pre-purchase inspection of a used car or truck.

The test requires a compression gauge and various fittings to match the type of engine you’re working on. When connected to the engine, the gauge can read the pressure generated by the upward movement of the piston on the compression stroke. This indicates how well your engine's combustion chambers are sealed by the pistons, piston rings, valves and head gasket

If there’s a major drop in the compression on one cylinder or an overall drop across the entire engine, this could indicate major issues and may even warrant a complete engine rebuild or replacement.

A professional mechanic can perform a compression test for you, but you can also do it yourself if you have the right tools, instructions and experience.

Reasons to perform a compression test

Before we get into how a compression test is performed, let’s take a look at why you might need one. 

Your engine is misfiring. If your engine is running rough or misfiring and you have ruled out your ignition and fuel system as the cause, it may be time for a compression test. Low compression will cause an incomplete burn of the air-fuel mixture your car runs on, resulting in a misfire. This can act very similar to a bad spark plug, a faulty fuel injector or various other more common failures.

There's a loss of power over time. An engine with high mileage or poor maintenance can begin to lose compression over time. This will be present itself as an equal drop on each cylinder and is a good indication that the engine is worn out and in need of a rebuild.  

You see smoke from the exhaust. There are many reasons why you may see excessive smoke from the exhaust, including worn piston rings or a blown head gasket. A compression test will help pinpoint which cylinder is causing the issue.

You’re buying a used car. It’s always important to get a pre-purchase inspection before buying any used car. Some shops will offer a compression test as part of the inspection to ensure the engine is in good health.

Get it diagnosed by a professional
 

How to perform a compression test

The steps involved in performing a compression test will vary depending on your type of engine, so make sure to refer to a workshop manual for specific instructions on your car. 

While professional mechanics wouldn't consider a this procedure dangerous, consider this: When performing a compression test, failure to follow the correct steps can result in fire, electrical shock or severe damage to the vehicle. So read the instructions carefully and look up the definition of any word you’re unsure of. For example you will be “cranking” the engine, but not “starting” it. You will use the starter to do this, not turning the key. Steps like “grounding secondary ignition cables” and “disabling the fuel system” are critical and vary on different vehicles.

Also, keep in mind that with some modern engines, you can instead perform a relative compression test. This is more common with newer diesel engines and some gasoline engines, and it requires a high-quality scan tool. You can connect the scanner and crank the engine, and the car's computer will gather data from its various sensors to generate a pressure reading. It’s much faster than a manual compression test. 

The following steps refer to the traditional compression test method using a manual gauge. If you don’t have the right tools or don’t feel comfortable doing this yourself, you can always have a mechanic perform it instead.

Step 1: Get the right tools for your engine

Gasoline and diesel engines use different fittings and gauges, so make sure you get the proper tools for the job. Remember to wear safety glasses and gloves, and in most cases a second set of hands will be useful. Also, make sure you have the repair instructions for your specific vehicle handy. 

Step 2: Disconnect the fuel system

With every compression test, you’ll need to ensure that the engine won’t start during the test. This is usually done by disabling the fuel system by disconnecting a relay or fuse. This step is critical, so refer to the work instructions for your car or truck on how to prevent the engine from running. No liquid fuel or fuel vapor can be present while testing. Compression alone can ignite fuel. A bad plug wire can spark even if it’s properly grounded.

Step 3: Find the place where you’ll tap into the cylinder

On gasoline engines, this is done by removing the spark plug for the cylinder you want to test. If you are going to test the entire engine, it’s important to remove all of the spark plugs so you get even results. With diesel engines, you can remove the glow plug, and in some cases, you need to remove the fuel injector to screw in your gauge fitting.

Step 4: Choose the correct gauge fitting and screw it in

Most compression gauge sets come with multiple fittings, but most modern gasoline engines use the same fittings, as the spark plug threads are common across many makes and models. If your gauge fitting has a rubber seal, inspect it for tears before performing the test. A leaking gauge will lead to inaccurate results. Once you’ve found the right one and made sure it’s in good shape, thread it into the port.

Step 5: Crank the engine

With the gauge connected and your engine disabled from starting, you’re now ready to crank your engine. Use a remote starter button for this, not the key. Keep the key in your pocket throughout the procedure. Make sure the engine is clear of tools, rags and body parts before starting. Some engines will require the throttle blade to be opened while cranking. If this applies to your car, get a helper to sit in the driver's seat and hold the accelerator pedal down. 

The amount of time the engine needs to be cranked will vary depending on the manufacturer. Some say four seconds, others say seven seconds, and some instructions say to crank until a steady reading is showing on the gauge. The most important part here is that you crank the engine for the same amount of time or revolutions for each cylinder, and the battery is fully charged throughout the test.

Step 6: Record the results

After testing each cylinder, write down your readings and compare the results from all cylinders. You can find exact tolerances and specifications for the acceptable pressure differences in your work instructions, but know that any major variance can indicate an issue. A general rule of thumb is that any variance over 10% between cylinders requires further investigation. 

Let's use a six-cylinder engine as an example. If after testing each cylinder, five have a reading of 150 psi and one cylinder shows 135 psi or lower, you may have an issue. If the reading is very close to this threshold, perform the test again. In cases where there is a dead misfire or severe smoking, you will see a cylinder with very low or no compression at all. 

These are general instructions, and there are many factors that come into play when performing a compression test. Temperature, type of oil, time since the engine was last started and other variables can affect the results. The main idea here is consistency. If all your cylinders are reading close to each other, it’s a good sign of a healthy engine.

» MORE: Get an estimate for your engine repair

If you have low compression 

Modern engines with smaller displacements don’t have as much piston and cylinder wear as the big V8s of old. So, if you have a smaller engine with low compression, you can do another test and possibly avoid a full engine rebuild.  

A “wet” compression test can be done after the standard test. Squirt a teaspoon (or less) of 30W motor oil into each cylinder, then check the compression again. The oil will help seal any broken piston rings temporarily — so if compression goes up after this, even 5%, you’ll know your engine is in need of a complete overhaul.

However, if the numbers stay very close to the same and you know you’ve always serviced your car on time, you can get away with just a top-end rebuild — the head gasket and cylinder head of a engine — as opposed to the whole thing. 

That said, even a top-end rebuild can be expensive. If your car has a lot of miles or years on it, it might make more sense to invest in a new vehicle instead. Ask your mechanic for an estimate and his or her opinion.

Alex Palmeri

About the Author

Alex Palmeri worked nine years as a master technician at Mercedes-Benz of Chicago and is currently the foreman at a large fleet garage. He writes about automotive news, maintenance and racing, and runs a YouTube channel called Legit Street Cars.

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