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

Low compression is often a sign of an old and worn car engine, but it could happen for other reasons also. Here're the common ones

Causes Of Low Compression

For an engine to function properly, it needs to have a high and healthy compression. The more air the engine can compress each cycle is equal to higher engine performance.

Low compression is something you do not want to experience with your car engine because an expensive issue mostly causes it. But is it always like this?

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

Causes of Low Compression

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

While it may look like a lot of different reasons, it’s not all of them, unfortunately! Here is a more detailed list of the most common causes of low compression in your car engine:

1. Faulty valves or valve Seats

Sooty Valves

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

The valves are sealing the combustion pressure before its released into the exhaust. If there is a leak on the valves or the valve seats, the compression will leak out into your exhaust or the intake during the combustion process.

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.

Unfortunately, it can still happen on some car engine models that the valve seats are getting worn, and they need to be refaced. Another common cause is that it occurs holes or other damages on the valves, and to fix this, you need to replace them.

You will need a leak-down-tester to identify any gas leakages from the valves in the air inlet manifold 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 are making sure that close to no compression will leak into the crankcase, and they also make sure that no oil will come up in the combustion chamber.

It happens that the piston rings get damaged or are getting worn by age. Luckily, it is not very common in modern car engines that they get damaged or worn. Why I say luckily, is because to replace the piston rings; you have to disassemble the whole engine, which is a costly job.

The piston rings can get stuck by engine sludge if the engine is old, and this will cause them to not expand and seal properly against the cylinder walls.

To fix this, you can remove the spark plugs, pour a little bit of diesel into the cylinders, and let it stand for a couple of hours, then make a new compression test. If they are damaged or too worn, you need to replace them, unfortunately.

Learn more: 4 Symptoms of a Bad Piston Ring

3. Worn or Damaged pistons

Car Pistons

One more common reason when it comes to low compression in a car engine is worn or damaged pistons.

Pistons are often made from aluminum alloy and are sturdy enough to withstand high engine temperatures. However, they may develop hot spots if the engine is running too lean or have engine knocking. These hot spots soon burn holes in the pistons causing gases to leak into the combustion chamber.

Pistons can also get worn in high mileage engines and this will cause a lower compression over a long timeframe.

Luckily, it is quite rare that the pistons are getting worn in modern engines, and it usually happens more in older ones. It is mostly because of a faulty fuel injector causing a lean mixture that creates enough heat to melt down the pistons if it happens in a modern engine.

If your pistons are worn or damaged, there is nothing else to do than disassemble the whole engine and replace them and the piston rings. You also have to resurface the cylinder walls.

4. Faulty Hydraulic Lifter

Hydraulic Lifter

The hydraulic lifters are installed between the camshaft and the valves. In the old days – engines used solid lifters which had to be adjusted within a fixed timeframe.

Hydraulic lifters, on the other hand, are self-adjusted, and they will always make sure that the play between the lifter and the camshaft is perfect. The hydraulic lifters are filled with oil pressure to control this play.

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. This will cause them to just getting compressed when the camshaft opens them without opening the valves.

Therefore when the valves are not opening properly, it may cause no or low compression.

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

5. Wrong camshaft timing

Broken Timing Belt

If you experience low compression on all cylinders, it may be caused by a faulty camshaft timing belt or chain, but it could also be caused by a previously wrong installed belt or chain.

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

When the belt or chain is faulty, or the timing is wrong, it will fail to inject air-fuel mixture at the right time, and it will fail to release the exhaust gases at the right moment. This will eventually lead to low compression problems.

If the camshaft timing is very wrong, it can also cause the valves to hit the pistons and bend them in most engines, which will cause no compression in your engine.

If you have low compression on all cylinders, it is definitely time to check the 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 the oil, coolant, and compression. If this gasket fails, you can experience a lot of symptoms like oil in coolant, compression in coolant, etc.

There is a metal ring around each cylinder in the head gasket, which unfortunately can fail. If this ring fails, it will cause the compression to flow from one cylinder into another. This can cause low compression and a lot of other symptoms.

This can easily be measured using a leak-down-tester to see if the pressure is blowing from one cylinder into another.

See more: Symptoms of a bad head gasket

7. Cracked or damaged cylinder walls

Cylinder Replacement

The pistons and the piston rings are running in the cylinders. For the piston rings to seal properly, they need a good surface inside the cylinder walls.

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

If you have a very old engine, it can also happen that these cylinder walls are just worn, and therefore the piston rings will not be able to seal properly. Cracks in the cylinder walls can also appear, but it is very rare.

In aluminum blocks, you can often replace the cylinder walls alone with some special tools, but unfortunately, in most engines with steel blocks, you have to either replace the engine block or bore the cylinders to get rid of the damages.

8. Cleaned cylinder walls

Engine Cylinders

If your car was running without combustion on one cylinder for a while, the petrol might have washed away the oil from the cylinder walls and this can cause lower compression on that specific cylinder.

The check this, you can remove the spark plugs and pour a small amount of oil into the affected cylinder and then make a new measurement of the compression again to check if it improved or not.

How to diagnose low compression

Compression Test

There are a few methods that you can use to find out where the problem is when it comes to low compression. To diagnose problems with low compression, you do at least need a compression tester, but more preferably a leak-down-tester.

  1. If you get low compression on all cylinders – check the camshaft timing and inspect the timing chain or timing belt.
  2. If your engine has low compression on one cylinder – try pouring some oil into the affected cylinder to see if the compression test results get better. If you get a higher pressure after pouring in the oil, there are basically two reasons for this. The first one is that if your car is running for a while with misfires, the fuel might wash the cylinder walls, which causes low compression. The second reason is that the piston rings are sealing poorly or were stuck. In some cases, this can fix the problem, but you have to replace the piston rings in most cases.
  3. Remove the oil cap. Remove the oil cap while the engine is running. If you can feel high overpressure there together with smoke, the compression is leaking through the pistons down into the crankcase, and you might have to replace the piston or piston rings or repair a crack somewhere. There should be a slight underpressure in the crankcase at idle if everything is working properly.
  4. Use a leak-down-tester to check where the compression is leaking. Make sure that the camshaft is at the timing when both valves are closed, then put pressure into the cylinder and listen for the compression leaking out in the intake, exhaust, or into the crankcase ventilation.
Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.

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