DIY Compression Test – How To Perform The Right Way?

DIY Compression Test – How To Perform The Right Way?

July 22, 2021 0 By CarGuy

Are you feeling a loss of power in your car recently? Although everything is in check, something just isn’t right and your engine doesn’t work as it used to. Maybe it’s time for a DIY compression test.

A compression test will tell a lot about your car’s engine and its health. But before getting your hands dirty you need to learn how this procedure is done.

You need to learn what is a good compression test reading and what is a bad compression test reading. Whether you should compression test a cold engine or compression test a warm engine.

There is a lot to be learned before you jump into compression testing your engine. In this article, we are going to get through all of these things and will give you a detailed guide on how to do a DIY compression test on your vehicle. So, without further ado, let’s jump into solving this problem.

What Causes Loss Of Compression?

Loss of compression can be caused by not properly sealed combustion chamber. Loss of sealing ability can be attributed to a couple of factors.

Blown Head Gasket

Head gaskets are one of the things that fail most often in combustion engines. They are prone to failures because they are exposed to a lot of stress. Some small holes in the gasket can appear between the cylinders and cause compression loss. Other issues can be mixing oil with coolant and overheating.

Overheating is probably the biggest problem when you have a blown head gasket. The car overheats for some reason and these gaskets are one of the first things that fail inside an engine. Make sure that the gasket is ok and is not causing loss of compression.

Lack Of Lubrication

The first thing is probably the lack of lubrication. If you are running your car low on oil or you don’t change your oil very often may cause the lubrication process not to work properly. If there is a lack of lubrication and fresh oil, the piston rings will start to scratch the walls of the cylinders and cause loss of compression.

Worn Out Or Broken Piston Rings

Another issue that can cause loss of compression is a broken piston ring or O-ring. These piston rings under high load, or with age and miles that build upon them can cause the pistons not to seal properly and break. When O-Ring brakes, it can damage the cylinder in the process and this also can worsen the condition.

Holes In The Piston

Pistons usually are put against big stress. Especially in high compression engines. If the pistons are exposed to high loads, they can develop holes inside them and may cause the engine to lose compression. This happens rarely, and mostly occurs to some high-performance applications and racing engines. Even though, sometimes even passenger cars are not avoided by this issue.

Head Valves That Don’t Sit Properly

Valves can also be the culprit for the loss of compression. If there is too much carbon buildup on them, the engine will probably won’t be able to make compression. Valves usually sit tight in order to deliver proper compression. Another reason besides carbon buildup for ruining your valves can be high temperatures. Valves are exposed to extremely high temperatures because the combustion process is happening right below them.

Broken Rods, Worn Bearings, And Damaged Crankshaft

Broken rods can also cause loss of compression. When a rod brakes there is usually catastrophic engine damage, even though in some cases this may not be the issue. The rod can break and the cylinder will stop working. This can result in loss of power and you will also hear some strange sounds from the engine bay. This will potentially ruin your engine.

There is also a sound is also known as rod knock. When rods start knocking the bearings that hold the rod are done. There is a big play in the crankshaft and also the crankshaft and the cylinder walls will get scarred.

This problem will ask for big machine work to see if the block and the crankshaft are savable. If they are not, the engine is scrap metal.

If you decide to continue driving like this, your engine is probably going to grenade and all of the oil inside will leak on the road and your car will not be able to move.

So, if you notice something like this, the best idea is to do a DIY compression test and see if there is something that can be done to save this engine like light machining, new bearings, and pistons. These parts will make your engine run well again if problems are detected on time.

Symptoms Of Bad Compression

There can be a couple of symptoms of bad compression. Some of them are very clear and some of them need more troubleshooting to see what is the real culprit.

Loss Of Power

Loss of power is probably one of the biggest symptoms when it comes to loss of compression. If your engine does not have compression, you will notice how your car does not deliver power as it used to. If one cylinder fails you may notice much, but if others fail too. You probably won’t be able to move your car around.

Sounds Coming Out From The Engine

Another symptom of bad compression can be sounds that are coming from the engine compartment. You can identify if a car works on three cylinders by the sound it makes. Some people even use a stethoscope to see which cylinder is affected by the issue.

Engine Misfires

Engine misfires are also one of the things that happen when you have a loss of compression. There is too much air in the cylinder wall and explosions are not equal. Misfires can be caused by broken valves. If you have broken valves you have to replace them before the issue becomes bigger and you have to do more work on the vehicle than you have previously anticipated.

Failure To Start

If your engine is too far gone it can cause it not even be able to start. Compression is needed for the cylinders to work. And if there is not any compression, your engine is pretty much done. It will require a total rebuild or possibly a new engine.

What Is A Compression Tester?

A compression tester is a tool that will give you the compression readings for your particular engine. There are compression testing kits that come with everything you need in order to perform compression testing on your engine. This tester includes a special barometer to see how much compression your cylinder is making.

When you cranking the engine, the needle climbs up to match the factory spec. If there is no compression in the engine, or the compression is low. The needle will stay low and not go up.

What Are Good Or Bad Compression Readings?

This depends much on your engine’s compression ratio, but basically, everything that is above 120 bars is a good reading if you have an average compression engine. If your engine is a race engine, you will probably need more than 120 bars of pressure. Some engines even go to 160 and more. Just make sure that your compression readings are consistent and give similar results on every cylinder. So, you can have a clear estimate of which cylinder is affected by low compression.

Where To Get A Compression Tester?

You can get a compression tester basically in every car parts store. They are fairly simple to operate and don’t cost a lot of money. You can get them online also, there are a lot of offers on Amazon or other online retail store websites. But make sure that you get a quality compression tester. Don’t go for the cheapest options, some of them may not even work in the first place. It is also useful to read the reviews and learn more about the product. Just to be on the safe side.

How To Do A DIY Compression Test?

Doing a DIY compression test is a fairly simple process. You just need a compression tester and some free time to put in the work.

Step 1

Involves removing all the injectors if you have a fuel injection system, or if you have a carburetor, you can take off the fuel hose clamp and remove the fuel line.

Step 2

Involves removing all of the spark plugs and wires. Make sure that you remove all of them because if you leave the spark plugs, it not going to work.

Step 3

Involves placing the compression gauge into the spark plug hole. Start from one side and move from there.

Step 4

Involves cranking the engine. Make sure that you give enough crank to apply enough pressure, so the gauge will show you a reading. 4 revolutions minimum, you can do more but the battery will suffer and go empty.

Step 5

Repeat the step 4 process for all of the cylinders. Make sure that you write down the numbers and see which one is bad and which cylinder is ok. 120 bars is the perfect reading for most engines. The tolerances go to up to 100. Everything below that is considered low compression.

If your cylinder reading is zero, it means that there is no compression on that cylinder and your engine needs repair. If the reading is from 90-50 it means probably the piston rings or valves don’t seal anymore and the engine needs to be opened and checked in detail.

What Are The Next Steps?

The next steps involve some work. Depending on if you want to do the work by yourself or you want to take your car to a mechanic’s shop.

If you decide to do the job by yourself, the first thing to do is to remove the head that has the affected cylinder. After you remove the head, you will have a clear vision if it’s the head gasket, valves, or it’s something else like the piston rings. In most cases, the piston rings fail.

Replace Bearings, Piston Rings, Machine Work The Cylinder Walls

If you are thinking that the problem lies in the piston rings. The next thing to do is to flush the oil and remove the oil pan. Then you need to unbolt the affected piston and see the bearings and piston rings.

If the bearings are affected then you will need to replace all of them on every cylinder. If they are close to the copper, they will fail eventually. And you want to avoid this because it can lead to catastrophic engine damage and broken rods.

If the piston rings are broken, you need to replace them with new ones. Take a note that you also need to inspect the cylinder wall. The walls should be perfectly smooth. If they are not smooth, some honing will be necessary to remove the grooves. Honing can be done simply at home without an issue.

If the grooves are too big then you need to machine the cylinders and this can be costly because you need to remove the engine and take it to a shop and then find piston rings that will fit. This can be a pain to do.

If the grooves are not that big a simple honing will do the trick. After this, you need to find new piston rings and reassemble the engine. Just make sure that you torque everything to spec and also do a DIY compression test to make sure that everything is ok.


In this article, we have learned everything about low compression engines and also how to do a DIY compression test in your garage with common tools.

We also covered a ton of other symptoms and issues that can be connected with this problem like the valve seals, the bearings.

We covered every step you need to follow in order to check the compression of your cylinders. Remember to disconnect the fuel system and the spark plugs in order to be safe and you can start cranking each cylinder one by one. Measure each of them and compare the results, everything below 100 is a bad result and the engine needs to be opened.

And lastly, we covered what to consider as your next step. We listed all the possibilities that you may face during the search for a fix for your engine. Thanks for reading and start wrenching.