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


Compression in a car engine refers to when air and gas mix in the engine cylinders. This process is necessary for the car to move and run. If there is any problem with the compression process, you can expect to experience all kinds of car problems.

It will be easy to tell when you have a low compression problem because you may have difficulty trying to start the engine. Either that or the engine will not run well while driving the vehicle on the road.

The worst case scenario is that the car will not start if there is no compression in all cylinders.

Generally, if you have low compression in one cylinder, the engine will start, but you will probably misfire and your vehicle will run rough. If you don’t feel any compression in ALL cylinders, your engine won’t start.

Top 5 Reasons for Low Compression in a Car Engine

There are many reasons why low compression may exist in a car engine. Sometimes one engine cylinder will have low compression and other times ALL cylinders may have low compression.

All you have to do is understand the possible main causes of low compression in a car engine, then repair or replace whatever has been damaged. Below are the top 5 causes of low compression in car engines.

#1 – Holes in the piston

a hole in a piston

You probably know that engine cylinders contain pistons. These pistons are usually made of aluminum alloy and are supposed to be able to handle the power of combustion.

However, if the engine overheats, hot spots will reach the piston. After a while, these spots will burn holes through the piston. As soon as this happens, gas leaks through these holes and causes low compression.

#2 – Exhaust valves

There are exhaust valves and intake valves at the top of each cylinder. Air and fuel enter the intake valve for the combustion process. The resulting gases leave the exhaust valve.

If the valves overheat, they can begin to leak gas prematurely. When that happens, you have low compression.

More commonly, valve seals can wear over time, allowing gases to escape, causing cylinder compression to decrease.

Related: 6 Causes of Oil in Spark Plug Wells

#3 – Time zone

bad timing belt

Every engine has a timing belt or chain that connects the camshaft and crankshaft. If the timing belt was broken or damaged, the camshaft could no longer rotate.

This means that the exhaust valve or intake valve cannot open or close properly. As a result, combustion in the cylinders will be destroyed and gases will not be able to be released. Then you have low compression because of that.

#4 – Head gasket failure

blown head gasket

There is a gasket between the area at the top of the engine where the cylinder head attaches. If for some reason the head gasket becomes defective and begins to break, it will leave a small hole between the cylinder and the head.

This is called a blown head gasket and causes gases to escape from the cylinder through the gasket hole. Then you will have low compression and low performance. If the head gasket fails between two cylinders, it can cause a compression leak in both cylinder heads.

#5 – Defective piston rings

signs of bad piston rings and valve seals

Overheating can cause the piston rings to seize or become damaged. This will cause carbon dioxide to leak through the rings as they can no longer seal them inside the cylinder. As you probably already know, when this type of leak exists, it results in low compression.

How to set low compression

The first thing you should do is use a compression gauge and test to see if your engine has low compression. This process usually takes 45 minutes, so make sure you have some free time.

If you don’t have a compression gauge, you can purchase one or take the car to an auto repair shop and have them test the compression for you. If they find that the compression is low, the next step is to inspect the cylinder, piston, valves, and gasket to see if any of them are damaged or broken.

From there, you can replace anything that has been damaged. However, this will be a long and expensive job, as it involves disassembling the engine. Get ready for it.

Here is a good video explaining how to properly perform a compression test:



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