8 Common Causes of Low Compression in Car Engines (& How To Fix Them)

An engine must have high compression to ensure it functions properly. A higher compression rate equals better engine performance.

Because it is a costly issue, low compression can be a problem with your car's engine. It doesn't always happen like that.

We will be discussing what parts are most likely to cause low compression, and how to diagnose it. Let’s begin with the most common causes!

Low compression can be caused by damaged valve seals, or poor valves. Wearing piston rings and pistons are other common causes. Low compression may also be caused by an incorrect timing or hydraulic lifter.

While it may look like a lot of different reasons, it’s not all of them, unfortunately! This is an extensive list of common reasons for low compression in your vehicle's engine.

The causes of low compression

1. Flawed valves, or defective valve seats

Low compression can be caused by worn or damaged valve seats. It can impact both intake and exhaust valves.

Before the exhaust pressure is released, the valves seal the combustion pressure. Leakage of the valves, or on the seats will cause compression to leak out and into your exhaust.

In older engines, you had to resurface these valve seats several times during the car’s lifetime. Luckily, this has been improved on modern engines, and you should never have to do it during the car engine’s lifetime.

However, this can still occur on certain car engines models. In these cases, the valve seats need to be replaced. It is also possible that the valves are damaged or have holes. To fix these, it's necessary to replace them.

A leak-down tester is required to detect gas leakages at the exhaust or inlet pipe valves.

2. Piston rings that are worn or damaged

A worn out or damaged piston ring is another cause of low compression. There are usually 2-4 piston rings around each piston of your car's engine. These rings ensure there's very little oil buildup in your combustion chamber and close to zero compression.

Sometimes, the piston rings can become worn or damaged over time. It is rare for modern cars to have worn or damaged piston rings. The reason I said fortunately is that to replace piston rings you need to take apart the entire engine. It is expensive.

Older engines can trap piston rings in engine sludge, which will result in them not expanding and sealing properly to the walls of the cylinders.

You can fix it by removing the spark plugs and pouring some diesel in the cylinders. After letting the mixture sit for several hours, make another compression test. You will need to replace the spark plugs if they become damaged or worn.

Find out more about the 4 Signs that a Piston Ring is Bad

3. Pistons that are worn or damaged

Wearing or damaged pistons are another common cause of low compression in car engines.

Many pistons are made from aluminum alloy, and can withstand very high temperatures. They can develop hot spots when the engine runs too low or has engine knocking. The hot spots quickly burn through the pistons, causing gasses to leak into your combustion chamber.

High mileage engines can wear pistons, which will result in a decrease of compression for a longer time.

It is rare for the pistons to wear in modern engines. However, older engines are more susceptible. If it occurs in modern engines, the problem is usually caused by a defective fuel injector that causes a lean mixture to create enough heat to melt the pistons.

You can replace the piston rings and disassemble your engine if they are damaged or worn. Resurface also the cylinder walls.

4. A Faulty Hydraulic Lifter

Between the camshafts and valves, the hydraulic lifters can be found. In the old days – engines used solid lifters which had to be adjusted within a fixed timeframe.

Hydraulic lifters on the other side are self-adjusting and will ensure that there is no play between the lifter's shaft and camshaft. To control the play, hydraulic lifters have oil pressure.

If you have a high-mileage engine, it does unforcedly happens that the camshaft is creating holes in the hydraulic lifters – which will cause them to not being able to hold the oil pressure. They will then become compressed when they are opened by the camshaft without opening any valves.

If the valves don't open correctly, this can cause either no or very low compression.

To diagnose this, you often have to remove the valve cover to inspect the hydraulic lifters’ top. You may also need to take off the camshafts in order to view them.

5. Wrong camshaft timing

Low compression in all cylinders could be due to a defective camshaft timing chain or belt, or a wrongly installed belt.

The timing belt or chain uses the rotation of the crankshaft and transfers power to the camshaft to control the valves’ opening times. The combustion chambers' gases are controlled by the valves.

A faulty belt or chain, or incorrect timing, will cause it to fail to inject the correct amount of air-fuel mixture and to emit the appropriate exhaust gases. It will eventually cause low compression.

It is possible for the timing of camshafts to be incorrect. If this happens, the valves can hit the pistons. Most engines will bend the pistons.

Low compression in all of your cylinders is a sign that it's time to inspect the timing of your camshaft.

6. Head gasket damaged or worn

A large gasket seals the coolant and oil between the engine block and cylinder head. This gasket can fail, causing many symptoms such as oil in coolant and compression in coolant.

Each cylinder has a metal band around it in the head gasket. This can cause them to fail. Failure of this ring will result in compression flowing from one cylinder to another. Low compression can result, along with other unpleasant symptoms.

It is easy to measure this pressure by using a leak tester.

Learn more about the symptoms of a bad gasket

7. Broken or cracked cylinder walls

Both the pistons as well as the piston rings work in the cylinders. The piston rings must have a smooth surface within the cylinder walls in order to properly seal.

Unfortunately, parts can get sucked into the combustion chamber that shouldn’t be there. These can lead to deep scratches on the cylinder walls that can result in compression leaking down into the crankcase.

It is possible for a very older engine to have worn cylinder walls, which can cause problems with the sealability of piston rings. Although cracks can occur in the walls of the cylinders, it's very uncommon.

Aluminum blocks can be replaced with special tools. However, most engine blocks with steel blocks require that the block is either removed or replaced.

8. Walls of cylinder walls cleaned

Your car might not have been running for some time without any combustion in one of its cylinders. This could cause reduced compression.

To check, remove spark plugs from the cylinder. Then pour some oil in the cylinder to make sure it is lubricated. Next measure the compression and see if the result has changed.

How to Diagnose Low Compression

Low compression can be diagnosed using a variety of methods. A compression tester is sufficient to diagnose low compression issues. However, a leak-down tester or compression tester are more useful.

1. If you get low compression on all cylinders – Check the timing of the camshaft and the timing chain.
2. If your engine has low compression on one cylinder – Pour some oil in the affected container to check if it has improved the compression test. There are two main reasons why you might get higher pressure when adding oil to the cylinder. If your car has misfired for long periods, it might be because the fuel could wash away the cylinder walls. That can lead to low compression. A second possibility is poor sealing or stuck piston rings. While this may be possible in some instances, it is not recommended for all cases.
3. Take off the oil cap. The oil cap should be removed while the engine runs. You might feel a high level of overpressure and smoke. This could indicate that the pistons are leaking into your crankcase. If everything is functioning properly, there should be some underpressure at idle.
4. You can use a leak tester to find out where compression is leaking. After ensuring that the camshaft remains at its timing, close both valves. Then, put pressure in the cylinder to check for compression leakage.

It's possible to use this type of thing: