The only way to guarantee an engine is healthy – without taking it apart – is with a compression test. While it’s normal for an engine to lose some compression over time, anything more than normal indicates that an engine will need a full rebuild or a replacement. This is why many used engines are sold with a report indicating their compression. However, if one is in the market for a used vehicle, it seldom comes with a compression reading. 

Engines won’t always show signs of problems aka low compression until the issue gets quite severe. So how do you tell if the engine has sufficient compression in such a situation? Luckily, there are a few ways to tell if you have compression issues, and we’ve listed them below. However, we recommend reading a bit more about the fascinating thing that is compression before getting to it.

What Is Engine Compression?

Engines would be of no use if they didn’t have compression. It doesn’t matter what type of fuel an engine uses, it depends on compression to function. Without it, the piston won’t be able to prepare the fuel/air mixture for combustion while pressures created through said combustion would simply escape, failing to drive the piston down with enough force. 

Here’s how an engine works. A fine mixture of fuel and air is injected into each of the engine’s cylinders and this mixture is then compressed into a small volume with the help of compression. This might not sound like a big deal, but adequate compression helps it achieve the high pressure required inside the cylinder.

It is at this stage that the spark plug comes in as it handles the remainder of the job. The spark plug ignites the pressurized mixture of fuel and air to create an explosion which helps drive the engine. Where the fuel type plays a role when it comes to the specific amount of compression required. With gas or petrol engines, the combustion cycle can be completed with just around 140-180 pounds per square inch (PSI).

That said, compression varies from vehicle to vehicle. Multiple factors like the vehicle type and application dictate how much an engine would require. Manufacturers develop the engines while keeping the required compression in mind. Many gasoline or petrol-powered engines even use higher compression figures like 200-220 PSI. One can easily find the compression of their vehicle in the service manual.

While it would seem advantageous to have as high compression as possible, that’s quite the contrary with gasoline engines. Extremely high compression can lead to problems like detonation and pre-ignition. Both of which can be quite harmful to an engine. Speaking of higher compression figures, diesel engines are usually designed to achieve over 350 PSI. This is because diesel engines don’t rely on spark plugs for ignitions. Instead, the compression itself helps ignite the mixture of diesel and air.

How Does Engine Compression Affect Performance?

When an engine has higher compression it will increase the combustion efficiency. It’s pretty simple — the higher the pressure of the air and fuel mixture, the better thermal efficiency.

That being said, as mentioned earlier, gasoline engines can encounter detonation and pre-ignition with higher compression. However, this can be combated with higher octane fuel. With low-octane fuel and a high-compression engine, the fuel will ignite before the piston completes its stroke. Whereas high-octane fuel would ignite later and allow the piston to complete its movement. 

This is the reason why high-performance cars need high-octane fuel. They have engines designed to run much higher compression than regular cars. Similarly, when it comes to performance tuning, engines are usually completely rebuilt to be able to have higher compression and make more power. On the flip side, when an engine loses compression for whatever reason it underperforms and damages itself while it’s at it.

What Causes Compression Issues?

Compression issues can be caused due to several inconsistencies with the engine. The majority of internal engine problems can eventually lead to problems with the workings of an engine which can develop into compression issues.

Excessive Carbon Buildup

Yes, something as simple as carbon buildup can lead to serious compression issues. The most common cause of carbon buildup is improper maintenance — particularly skipped or late oil changes. In addition to inconsistent oil changes, bad fuel quality will also lead to excessive carbon buildup.

Dropped Valve Seat

Another cause of compression issues is a dropped valve seat. A dropped valve is also referred to as a cracked valve seat. When this occurs it will let the hot gas inside the chamber leak. A consequence of this is a burnt valve seat and valve. It will also lead to abnormal expansion of certain components which will cause irreversible damage and compression issues.

Broken Valve Spring

It’s not just the valve seat that causes compression issues. A broken valve spring can also cause a similar issue. The valve spring handles the operation of the valves. There are times when these springs become brittle and break. This affects valve operation and causes big compression issues.

Broken Valve

That’s not all, the valves themselves have also been known to go bad. When this happens, the head of the valve can come undone from the stem. As a result, the head of the valve will fall into the cylinder leading to compression issues and heavy damage.

Damaged Piston

Internal combustion engines use multiple pistons and when even one of them has damage, it will lead to compression issues. Overheating is one of the causes of piston damage. The most common form of damage is a hole at the top of the piston. As you can imagine, this causes compression issues.

Broken Camshaft

A broken camshaft might be one of the most serious problems on this list, but it has to be mentioned as it will lead to compression issues along with the failure of multiple other components. Keep in mind that a smaller problem like a weak or blown head gasket is what causes this.

Main Symptoms of Compression Issues

Given the drastic causes of compression issues, it shouldn’t be surprising that the symptoms are of a similar level as well. While many of the causes will ultimately cause complete engine failure, there are symptoms you need to watch out for:

• Starting Issues
• Reduction In Power
• Increased Fuel Consumption
• Misfiring Engine

How to Fix Compression Issues?

If you are at the point where you think you’re dealing with compression issues, we recommend checking the engine by doing a cylinder leak-down test. One can either rent or buy the kit required to carry out a cylinder leak-down test as it’s quite inexpensive. 

This test will reveal the exact compression of each cylinder. With this information, you will be able to detect the location and scale of the compression issues. Your next step would be to fix whatever it is that caused the compression issues.

Tell Us What You Need!

Once you determine what you need to fix the compression issues, visit Newparts.com so that we can help you with the next step. Sourcing high-quality parts is our specialty! Simply select your vehicle’s year, make, and model using our vehicle selection tool, and we’ll show you parts that are a guaranteed fit!