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What is a Compression Test?

Your engine’s ability to produce power and run smoothly and seamlessly boils down to 4 major variables, those being fuel/air mixture, spark (in a gas engine), timing, and compression. Compression is the ability for the engine’s pistons and valves to take the air and fuel being ingested and do just that – compress it – either to the point in a diesel engine that the compressed fuel simply ignites on its own, or in a gas engine, where spark ignites the mixture using a spark plug.

If your engine loses compression on one or multiple cylinders, problems will follow. From a misfire to a massive reduction in power, there are some severe and complicated problems that can arise. A compression test is by far one of the most basic engine test procedures and starting point to understand exactly what your engine is doing, and where the problem may be.

The premise of a compression test is to test each cylinder to see in measured PSI exactly how much compression that particular cylinder is producing. From a single piston lawn mower engine, to a diesel or V8 engine, the process is entirely the same – measure what the cylinder is producing in PSI.

Understanding a compression test procedure is also understanding what you are trying to achieve, and that is consistency between the numbers of each cylinder, and looking for the outlier, or outliers. If you measure the tire pressure of 4 tires on your car, and the measurement of three of them is 32PSI, but one of them is 15PSI, you immediately know you have an issue with that one tire, and further testing or inspection is needed to determine why. A compression test is much the same.   

How Difficult is a Compression Test?

A compression test is simple to learn and only requires a few basic tools. From a mechanical perspective, you will need to know how to properly remove the spark plugs on the vehicle or equipment you are testing and properly disable the ignition system and fuel system, the latter if the vehicle is fuel injected. For carbureted engines, you can choose to disable the fuel supply and run the carburetor out of gas, but most simply perform the test without doing this.

How easy it is to perform the test rests solely on the ability to perform the above tasks. A compression test can be done on either a hot or cold engine, or an engine that has been running or not running that you are attempting to diagnose problems with.

How to Do a Compression Test

1

Get Your Equipment

Gather your tools, including insulated gloves (or nitrile) and safety goggles to protect yourself from hot parts and oil sprays. Keep in mind if working around a hot engine to be careful of hot engine parts like the exhaust manifolds. You will also need a good quality ratchet, an extension or two (a long one with a “wiggle joint” is a good idea on V6 or V8 engines), and a suitable spark plug socket – note that while a regular socket may fit, it’s best to use an actual spark plug socket designed to remove spark plugs. You will need a notepad to jot down your readings and label each cylinder appropriately on your notes.

A compression tester can be purchased or rented and includes a set of adapters that will screw into the spark plug hole. The adapter has a built in Schrader valve to trap pressure and a special gauge to show the compression readings. The Shrader valve in the adapter traps the compression in the gauge so you can do the compression test without an assistant. The gauge will also have a release button, which releases the trapped pressure after each reading.

Caution: If you do have an assistant, make sure he/she knows NOT to spin the engine until you’re absolutely ready, and always keep your hands clear of the belts and pulleys. Have paper and something to write with so you can record your readings as you go. Do some research and know how the cylinders are numbered so you can write the readings in order – you can find this information online if you don’t know.

2

Disable the Ignition and Fuel

When performing the test, you don’t want the engine to start during the test. With older vehicles using a distributor, you can disable the coil by disconnecting the positive terminal of the coil or unplugging the coil or coils completely. Avoid simply removing the coil-to-distributor wire as the coil is still charged and can still achieve a grounded spark and shock someone. If you have to remove the spark plug wires, first know how the cylinders are numbered and mark the wires with tape or a Sharpie or both to make sure you don’t cross them while putting them back later. On vehicles with Coil-On-Plug (most modern vehicles) you will simply need to disconnect the coil’s wiring harness as you remove each spark plug, or, you may be able to find a fuse that controls the ignition system.

For fuel injected cars, you will need to disable the fuel system. Begin by checking your fuse panel details, either in the owner’s manual or under the fuse box cover and disable the fuel pump by removing the fuel pump fuse or the fuel pump relay if the vehicle is fuel injected. Carbureted engines with mechanical fuel pumps don’t have a fuel pump fuse and will not need to have the fueling disabled unless you choose to do so.

Finding the fuel pump fuse or relay is easier on some vehicles than others. Once you believe you’ve disabled the fuel pump, verify that you’ve disabled the fuel system by starting the engine until pressure is no longer in the fuel rail. Unless removing the fuse or relay also disabled the injectors and the ignition system, the engine will start initially after the fuel pump has been deactivated but then it will spin without starting as the residual fuel pressure is depleted.

At this point, you can listen to how the engine spins to see if one cylinder picks up speed while the engine is spinning – if so, one of the cylinders could have low compression.

3

Remove the Spark Plugs

Using the ratchet, any necessary extensions, and the spark plug socket, screw all the spark plugs out. This is important because you don’t want other compression strokes slowing the engine spin during the compression testing process.

4

Get Your Compression Testing Kit in Order

Choose the appropriate gauge for either gas or diesel. A diesel compression test typically requires the removal of the glow plugs and a special adapter for that engine. Since diesel has a higher compression rating, it uses a special gauge. Screw the correct compression adapter into the glow plug or spark plug hole of the cylinder you plan to test first – it’s good to start with cylinder #1, but you can do it in any order – just write down your readings next to the correct cylinder number.

5

Begin the Compression Test

A compression test is performed by then cranking the engine. This will then allow the gauge to read multiple compression strokes from that cylinder. Allow the engine to spin until the gauge has received at least six “puffs”, then release the key. If the compression gauge is working properly, the needle will indicate the amount of max compression.

Caution: If you do have an assistant, make sure he/she knows NOT to spin the engine until you’re absolutely ready, and always keep your hands clear of the belts and pulleys. Remember to have paper and something to write with so you can record your readings as you go. Do some research and know how the cylinders are numbered (firing order) so you can write the readings in order – you can find this information online if you don’t know. Consider using an assistant or remote starter if you’re having trouble keeping track of the reading while turning the key.

Once you have a valid reading, record the cylinder number and PSI rating. Most gas engine’s compression should be between 125 and 175 PSI while a diesel will generally fall between 275 and 400 PSI. Remember to write down each of your findings. If one or more cylinders looks to be way off in PSI, you can re-run the test again just to check.

6

Finish the Test

Continue the process of inserting the adapter, cranking the engine, and recording the results for all your vehicle’s cylinders. If one of the readings is significantly lower, it can indicate a problem with that single cylinder. Multiple low numbers can be a sign of other engine issues. Remember that what you are looking for is consistency. Cylinder pressures should not fall outside of 15-20% of each other, meaning, if you run a test on a V6 engine and come up with 150, 155, 150, 145, 135, and 130, this would fall within an acceptable level of consistency. If you run the test and come up with 150, 155, 90, 90, 135, and 130, this would indicate an issue as the highest pressure cylinder is significantly higher than the lowest pressure cylinder.

Diagnosing Potential Problems

A lack of compression is generally caused by 4 things – worn or damaged piston rings, damaged valves, improper valve timing, or a blown head gasket. The following can help pin-point possible problems.

  • Individual Cylinders being low PSI: If you only have a low reading on one cylinder or the cylinders aren’t adjacent or next to each other, the problem is usually valve or piston ring related. By adding two or three squirts of engine oil from an oil can to a low compression cylinder through the spark plug hole, you can attempt to isolate the possibly of worn rings being the issue. Re-run the test, and if the compression increases once adding the oil, the chances are very likely piston rings are the culprit. Many multi-valve engines and turbo engines can run into issues with bent valves, or burnt exhaust valves, and even the slightest leak in a valve can cause low compression, and usually a misfire.
  • Adjacent Cylinders: Two side-by-side cylinders with low ratings could be a blown head gasket. If you notice other symptoms such as milky oil or cloudy exhaust fumes, the head gasket probably needs to be replaced.
  • All Cylinders: When all your ratings are below 100 PSI for gas engines or below 275 for diesel, you may have a valve timing issue. First, be sure the valves are all properly opening and closing and that you do not have a broken belt, chain, or gears. This can be done in most cases by removing the valve covers and inspecting when the engine is cranking, and/or remove the timing inspection cover, which is usually available on any vehicle with a timing belt. It’s not uncommon for a timing belt to skip timing as well, which will yield you improper valve timing. If you notice a lot of blow-by gas coming out of the oil filler cap with the engine running and compression is low, suspect worn out piston rings. In this case, the engine will need to be overhauled or replaced.

Keep in mind that low PSI numbers on any combination of cylinders almost always indicate a problem that should be fixed sooner rather than later. Should you need to have a mechanic perform the diagnosis, you can always choose from our list of Preferred Shops in your area that can help you diagnose any engine-related troubleshooting. Low PSI numbers almost always indicate a problem that should be fixed sooner rather than later.

The engine compression test is one that can be done by nearly anyone with a basic knowledge of cars or trucks. Find everything from safety equipment and socket wrenches to compression testing adapters and gauges at AutoZone.

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