Low Engine Compression: Signs & Causes


Aug 17th, 2020

Low Engine Compression: Signs & Causes

You may not be familiar or interested in understanding engine compression until your vehicle develops a problem of low engine compression.

In an unlikely event that this happens to you, you'll be forced to understand it better as you try to fix the problem. It's a good idea, as a car owner, to be aware of car problems, so you're not caught unawares. In this article, we take a look at a major issue that affects car engines – low engine compression

What is engine compression?

Before we get down to the nitty-gritty of low engine compression, let's have a common understanding of what engine compression is. Internal combustion engines work based on the principle of compression. 

Fuel is mixed with air before the mixture is ignited in the combustion chamber. It is this combustion that compresses the engine cylinders and powers the vehicle. Up to this point, you can tell that low compression would create a problem. Your car may even fail to start, and in case it starts, you won't drive it for long.

What are the common signs of low engine compression?

Generally, it is easy to tell when your car has low compression problems. You'll experience issues on the road or when you try to start the engine. Symptoms of low compression vary depending on the number of cylinders affected, whether one or all.

Here are some of the signs that point to low engine compression:

1. Misfires

Engine misfires are a common sign of low engine compression in one cylinder. These occur when the engine skips one of the processes of the combustion cycle. In the case of low compression, the compression process is skipped or sometimes not executed well. 

2. Loss of power

Besides making the engine run rough and the vehicle jerk as it moves, misfiring also leads to power loss. Power loss in an engine is directly proportional to the number of cylinders affected.

For instance, if 2 out of 4 cylinders are misfiring, there'll be a 50% reduction in power. Sure, you may be able to take the car for a drive with a 25% power reduction, but you likely won't be able to drive in hilly/elevated terrains at the normal speed. 

3. Poor fuel economy

The higher the compression ratio in a vehicle, the better its fuel economy will be, generally. It follows that when your car has low compression, fuel economy will be significantly affected. Your engine will burn up more fuel to cover a specific distance than it would in typical scenarios. If you notice reduced fuel economy, it's a good idea to check for signs of low engine compression

4. Failure to start

In case all engine cylinders have low compression, then your vehicle won't start no matter how hard you try to power it up. The reason for this is that your engine won't have sufficient pressure to ignite the fuel-air mixture.

What causes low engine compression?

1. Holes in the piston

Pistons are usually made from an aluminium alloy. Although they are strong enough to withstand the power of combustion and high engine temperatures, they become weak over time. They start developing hot spots that burn holes through them, causing gas leakage into the combustion chamber leading to low engine compression

2. Leaky valves

Each engine's cylinder has an intake and exhaust valve. The former allows fuel and air in whereas the latter lets out gases resulting from the combustion process. When these valves become worn out or exposed to too much heat, they may start leaking gases out prematurely. Just as with holes in the piston, this causes low engine compression.

3. Worn timing belt

A worn-out or broken timing belt is incapable of turning the camshaft properly. This, in turn, means that the intake and exhaust valves wouldn't be able to open and close as expected. The combustion process is, therefore, impeded since no gases are being expelled.

4. Head gasket failure

A faulty or broken head gasket leaves a small hole between the cylinder and the cylinder head. Gases in the cylinder can leak out through this hole. When this happens, your vehicle will have low engine compression. Note that it is possible to have compression leak in two cylinders if the head gasket fails between those cylinders.

5. Bad piston rings

Pistons have rings that seal the piston to the cylinder wall. The top ring does most of the sealing while the secondary ring catches what the top ring may have missed. There's a third ring, oil control ring that removes any oil splashes on the cylinder wall when the engine runs.

These rings slowly wear out over time and typically fail at the same time. Also, overheating can cause damage to the rings or even break them. If this happens, gases may start leaking out because the rings can no longer seal the pistons effectively. This leak causes low engine compression

6. Broken camshaft

A broken camshaft will cause failures in the crankshaft, making it incapable of pushing the piston properly. The absence of pressure will lead to low engine compression. The chance of this occurring is rare compared to the other causes, but it still does lead to low compression.

7. Faulty camshaft lobe

Each valve has its lobe that helps in opening and closing of the valves in the cylinder. Wearing out of the lobe could make it get stuck in one position. The lobe may get stuck in a closed position, which means it won't open. Low compression, however, occurs when the lobe gets worn and stuck in an open position. 

Will an engine run even with low compression?

This depends on the cylinders that have low compression. If only a single cylinder has low compression issues, the engine will likely run. However, you'll notice a decline in engine power as you drive, and you may not be able to operate the car in some terrains.

Is it safe to drive with low compression in one cylinder?

You may able to drive your vehicle even with low compression in one cylinder, but it is neither safe nor advisable. When you notice any signs of low engine compression, carry out a compression test and find out exactly where the problem is. Once you know what the issue is, fix it as soon as possible to avoid further damage.

In case it involves replacing some car parts, don't worry so much. Here at Carpart.com.au, we will help you find the parts you're looking for at the best deal.


Low engine compression is detrimental to your engine and car. You'll experience problems like misfires and reduced fuel economy that will only get worse with time. This is why we recommend fixing low engine compression as soon as possible to avoid costly repairs.

By Sam O.