Low compression is bad news because it means your car is suffering from an internal engine issue—and that almost always leads to a hefty repair bill. The root cause of the problem could be one or more costly repairs, ranging from worn-out piston rings to a blown head gasket.
So, hopefully, your engine isn’t suffering from low compression. But if it is, you’ll likely notice one or more troublesome symptoms that you’ll want to address right away.
What is Engine Compression?
A gas-powered internal combustion engine needs three primary ingredients to run: an air-fuel mixture, spark, and compression. Simply put, compression is the pressure created inside each of the engine’s cylinders. The pressure raises the temperature of the air-fuel mixture and helps to atomize the fuel so that the mixture is easier for the spark plug to ignite.
Compression is created as the piston moves upward in the cylinder—during what’s referred to as “the compression stroke”—to pressurize the air-fuel mixture. During this phase in the engine’s four-stroke process, the valves must be closed so that the cylinder is completely sealed. The piston rings must also provide a tight seal between the piston and cylinder wall, and the head gasket must seal off the combustion chamber.
Anytime the cylinder is not sealed during the compression stroke, a significant amount of the air-fuel mixture escapes from the cylinder before being pressurized. The result is a loss of compression and incomplete combustion in that cylinder. That incomplete combustion is referred to as a misfire.
Volumetric efficiency—the ability of the engine to draw air into the cylinders—can also affect compression. When volumetric efficiency is low due to an engine mechanical problem, there isn’t as much air in the cylinder to be pressurized, so compression is lower.
Signs of Low Engine Compression
If your car’s engine compression is low, you’ll likely notice one or more of the following symptoms.
Note: Because other problems can present the same symptoms as low engine compression, you (or your mechanic) should perform a thorough diagnosis of the vehicle before performing any repairs.
Illuminated Check Engine Light
The engine management computer, which is often referred to as the powertrain control module (PCM), continuously monitors the engine for misfires. If the module detects a misfire caused by low compression or anything else, the device turns on the check engine light and stores a corresponding diagnostic trouble code (DTC) in its memory.
Vehicle Runs Rough and Misfires
Low compression causes incomplete combustion—also known as a misfire. When one or more of the cylinders are misfiring, you’ll notice that the engine runs rough. You may also notice a hesitation upon acceleration and an overall lack of power.
Engine Turns Over Quickly But Doesn’t Start
When there’s low (or zero) compression on most or all of the cylinders, you’ll often find that the engine turns over (cranks) faster than normal but doesn’t start.
What Causes Low Compression?
Low compression can stem from any type of internal engine problem that allows the air-fuel mixture to escape from a cylinder or reduces volumetric efficiency. The most common causes of low compression include:
Worn/Damaged Piston Rings, Pistons, and Cylinder walls
A piston has three rings, two of which are primarily designed to act as a seal that prevents compression loss between the piston and cylinder wall. If the rings are worn or the cylinder wall is damaged, there won’t be a good seal, resulting in a loss of compression. A piston that is broken or has a hole in it can also cause low compression.
Valve and Valve Train Problems
Anytime one of the engine’s valves doesn’t seal properly, the corresponding cylinder will suffer from a loss of compression. The root cause of the problem could be anything from carbon buildup behind the valves to a dropped valve seat.
The valvetrain—the components that work together to open and close the engine’s valves—can also cause a loss of compression due to low volumetric efficiency. For example, a worn camshaft lobe or broken rocker arm could limit the amount of air entering the engine, resulting in low compression.
Faulty Head Gasket
A head gasket that is breached between two cylinders (or a cylinder and a coolant passage) can result in a loss of compression.
Issues with the Timing Belt or Timing Chain
How Do You Check Compression?
Although there are more advanced test methods, such as a relative compression test, most professionals and DIYers check cranking compression with a mechanical gauge. The readings on the gauge can then be compared to the vehicle manufacturer’s specifications, which you can find in a repair manual or repair database.
The video below demonstrates performing a cranking compression test:
You can also perform what’s known as a running (or dynamic) compression test to measure the engine’s volumetric efficiency. A running compression test can sometimes pick up issues that wouldn’t show up on a cranking compression test.
As a general rule of thumb, the readings obtained during the “snap throttle” portion of the dynamic compression test should be approximately 80% of the cranking compression readings.
The video below demonstrates performing both a cranking and a dynamic compression test:
How Do You Fix Low Engine Compression?
There’s no quick-fix for an engine that has low compression. You (or your mechanic) will need to determine the source of the compression loss, then perform the necessary repairs to the engine to fix the problem.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.