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8 Causes of Low Compression in a Car Engine (and How to Fix It)

For an engine to run properly, it must have high, healthy compression. The more air the engine can compress in each cycle, the greater the engine’s performance.

Low compression is something you don’t want to experience with your car’s engine because the root cause is an expensive problem. But is it always like this?

In this article, we will discuss the common parts that can cause low compression and how you can properly diagnose your low compression. Let’s start with the most common causes!

The most common cause of low compression is due to damaged valve seals or faulty valves. Other common causes are worn piston rings or pistons. A faulty hydraulic lifter or incorrect camshaft timing can also cause low compression in rare cases.

While this may sound like many different reasons, unfortunately, these are not all! Here is a more detailed list of the most common causes of low compression in your car engine:

Causes of low compression

1. Defective valves or valve seats

Soot valves

The most common cause of low compression is worn valves or valve seats. This can affect both the exhaust and intake valves.

The valves seal off combustion pressure before it is released into the exhaust. If there is a leak at the valves or valve seats, compression will leak into the exhaust or intake during the combustion process.

On older engines, you had to repair these valve seats many times over the life of the car. Fortunately, this has been improved in modern engines and you should never have to do this over the life of the car’s engine.

Unfortunately, it can still happen on some car engine models that the valve seats become worn and need to be rebuilt. Another common cause is holes or other damage to the valves, and this requires replacement.

You will need a leak tester to identify any gas leaks from the air intake manifold valves or exhaust pipe.

2. Worn or damaged piston rings

Piston ring function

Another common cause of low compression is worn or damaged piston rings. Around each piston in your car engine, you have 2 to 4 piston rings. These rings ensure that there are no compression leaks in the crankcase and no oil leaks into the combustion chamber.

Sometimes piston rings become damaged or worn over time. Fortunately, it is not very common for modern car engines to become damaged or worn out. Why I say fortunately is because to replace the piston rings; You have to dismantle the entire engine, which is an expensive job.

The piston rings can become clogged with engine sludge if the engine is old, which will prevent them from expanding and sealing properly against the cylinder walls.

To solve this problem, you can remove the spark plugs, pour some diesel into the cylinders and let it sit for a few hours, then do a new compression test. If they are damaged or too worn, you will unfortunately have to replace them.

Read more: 4 symptoms of a bad piston ring

3. Worn or damaged pistons

car pistons

Another common reason for low compression in a car engine is worn or damaged pistons.

Pistons are often made of aluminum alloy and are strong enough to withstand high engine temperatures. However, they can develop hot spots if the engine runs too lean or the engine knocks. These hot spots quickly burn holes in the pistons, causing gas to leak into the combustion chamber.

Pistons can also wear out on high-mileage engines, which will result in lower compression over a long period of time.

Fortunately, piston wear is quite rare in modern engines and tends to occur more in older ones. This is primarily due to a faulty fuel injector creating a lean mixture that generates enough heat to melt the pistons if it occurs in a modern engine.

If your pistons are worn or damaged, there is nothing you can do other than take the entire engine apart and replace them along with the piston rings. You also need to coat the cylinder walls.

4. Defective hydraulic lifter

hydraulic pusher

Hydraulic lifters are installed between the camshaft and valves. In the past, engines used solid tappets that had to be adjusted within a fixed time frame.

The hydraulic tappets, for their part, are self-adjusting and always ensure that the clearance between the tappet and the camshaft is perfect. The hydraulic tappets are filled with oil pressure to control this play.

If you have a high mileage engine, it will happen without force that the camshaft creates holes in the hydraulic lifters, which will prevent them from maintaining oil pressure. This will only cause them to compress when the camshaft opens them without opening the valves.

Therefore, when the valves do not open properly, it can lead to weak or no compression.

To diagnose this, it is often necessary to remove the valve cover to inspect the top of the hydraulic lifters. Sometimes you also have to remove the camshafts to see them.

5. Poor camshaft timing

Broken timing belt

If you are experiencing low compression in all cylinders, this may be due to a faulty camshaft timing belt or chain, but it may also be due to a poorly installed belt or chain.

The timing belt or chain uses the rotation of the crankshaft and transfers power to the camshaft to control the opening times of the valves. The valves control the exit and entry of gases from the combustion chambers.

When the belt or chain is faulty, or the timing is incorrect, it will not be able to inject the air-fuel mixture at the correct time and it will not be able to release the exhaust gases at the correct time. This will eventually lead to low compression problems.

If the camshaft timing is very bad, the valves can also hit the pistons and bend them in most engines, which will not cause compression in your engine.

If you have low compression in all cylinders, it’s definitely time to check your camshaft timing.

6. Worn or damaged head gasket

head gasket

Between the engine block and the cylinder head, a large gasket is installed to separate oil, coolant, and compression. If this gasket fails, you may experience many symptoms such as oil in the coolant, compression in the coolant, etc.

There is a metal ring around each cylinder at the head gasket, which unfortunately can fail. If this ring fails, compression will flow from one cylinder to the other. This can lead to low compression and many other symptoms.

This can be easily measured with a leak tester to see if pressure passes from one cylinder to another.

See more: Symptoms of a bad head gasket

7. Broken or damaged cylinder walls

Cylinder replacementThe pistons and piston rings rotate in the cylinders. For piston rings to seal properly, they need a good surface area inside the cylinder walls.

Unfortunately, the combustion chamber can suck in parts that shouldn’t be there. This can cause deep scratches on the cylinder walls, which can cause compression to leak into the crankcase.

If you have a very old engine, it may also happen that the walls of these cylinders are worn and therefore the piston rings cannot seal properly. Cracks may also appear on the cylinder walls, but this is very rare.

On aluminum blocks, you can often replace the cylinder walls with special tools alone, but unfortunately on most engines with steel blocks, you must either replace the engine block or bore out the cylinders to remove the damage.

8. Cleaning the cylinder walls

engine cylinders

If your car has been running with no combustion in one cylinder for a period of time, the gasoline may have washed the oil off the cylinder walls, which can cause lower compression in that specific cylinder.

After checking, you can remove the spark plugs and pour a small amount of oil into the affected cylinder, then measure the compression again to check whether it has improved or not.

How to diagnose low compression

Comprehension test

There are a few methods you can use to determine where the low compression problem lies. To diagnose low compression problems, you need at least a compression tester, but preferably a leak tester.

  1. If you get low compression in all cylinders – check camshaft timing and inspect the timing chain or timing belt.
  2. If your engine has low compression in one cylinder – Try pouring oil into the affected cylinder to see if the compression test results improve. If you get a higher pressure after pouring oil, there are basically two reasons for this. The first is that if your car misfires over a period of time, the fuel can wash away the cylinder walls and cause low compression. The second reason is that the piston rings are sealing poorly or are stuck. In some cases, this can fix the problem, but in most cases, it is necessary to replace the piston rings.
  3. Remove the oil cap. Remove the oil cap while the engine is running. If you can smell high overpressure in there with smoke, compression is leaking through the pistons into the crankcase and you may need to replace the piston or piston rings or repair a crack somewhere. There should be a slight vacuum in the crankcase at idle if everything is working correctly.
  4. Use a leak tester to check where the compression is leaking. Make sure the camshaft is on when both valves are closed, then apply pressure to the cylinder and listen for compression leaks in the intake, exhaust, or crankcase ventilation.

You can use something like this:


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