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Diagnosing Low Cylinder Compression – Symptoms and Causes

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By Jake Mayock

Cylinders are the heart of any internal combustion engine. The cylinders are where air and fuel are compressed and ignited, creating combustion which is what creates power. One of the most important roles of the pistons inside the cylinders is compressing the air and fuel mixture. However, various engine problems and maintenance items can cause the cylinders to lose compression.

Losing compression in one or more cylinders can cause significant performance and drivability issues. The severity of low cylinder compression can range from needing a full engine rebuild to simply needing a new head gasket. Unfortunately, the majority of compression issues are related to the internal components of the engine which can make it very costly to repair. This guide is going to discuss how cylinder compression works, symptoms of low compression, and what the primary causes of low compression are.

How Cylinder Compression Works

Internal combustion engines used in cars go through four different strokes: intake, compression, combustion, and exhaust. The first stroke is the intake stroke, which uses the air intake system to draw air into the engine's cylinders, also known as the combustion chamber. The second stroke is compression. The intake and exhaust valves are completely closed, and the piston compresses the air and fuel mixture which is then ignited, creating combustion.

Compressed air is significantly more combustible. Therefore, low cylinder compression can result in the cylinder not being able to combust the air and fuel inside of it. Low compression is typically caused by a leak that allows air to seep out of the cylinder which releases the pressurization of the cylinder, making the piston incapable of compressing the air within it.

Symptoms of Low Compression

It's possible to lose compression in just one cylinder, or all of them depending on what is causing the issue. The symptoms will usually be greater and more noticeable when more cylinders have lost compression. If one cylinder has lost compression you will likely still be able to drive around, just with less power and a rough idle. However, if multiple cylinders have lost compression the engine probably won't run well enough to be drivable. With that being said, driving on low compression is not a good idea.

Here are the most common symptoms associated with bad cylinder compression:

    • Loss of performance
    • Rough idling and cylinder misfires
    • Slow starts or cranks but doesn't start
    • Decreased gas mileage

Low cylinder compression can be difficult to diagnose since the symptoms above are also frequently caused by other engine problems. Bad spark plugs or ignition coils can cause performance issues, misfires, and starting issues. A bad battery, starter, or battery connection could cause cranking issues. And loss of performance and decreased gas mileage can be attributed to a long list of other problems.

Therefore, the only accurate way to diagnose low compression is to compression test your cylinders. Since this isn't the easiest thing to do, we recommend running down the list of other potential causes before testing compression. It's also worth mentioning that low compression is more common on older high mileage vehicles, compared to newer vehicles.

Causes of Low Cylinder Compression

Cylinders can leak air from a number of different engine problems. Unfortunately, the majority of the causes are related to issues with the pistons or valves which can require an engine rebuild to fix. However, there are a few causes that can be fixed without needing significant engine work.

    • Cracked Pistons
    • Leaking Intake or Exhaust Valves
    • Scored or Cracked Cylinder Walls
    • Timing Belt/Chain Failure
    • Blown Head Gasket
    • Bad Camshaft

Once you have determined you have low compression, one of the biggest challenges can be determining what is causing it. The majority of these components are all internal and therefore it can require a lot of disassembly of the engine to get to the root cause of the problem. Because of this, on high-mileage and low-value cars, it is common to just replace the engine once compression is lost.

1) Cracked Pistons

Pistons sit within the cylinder and are the driving force for compression. Because pistons sit within the combustion chamber, they are prone to very high temperatures. These high temperatures can cause hairline cracks to form on the piston. When these cracks form, it creates a gap between the piston and the cylinder wall. This lets air escape, or leak, causing a loss of compression. Pistons usually crack one at a time unless you have a major overheating event.

Another piston-related issue is bad piston rings. Pistons generally have three rings on them. These rings seal the piston to the cylinder wall. The piston rings can wear down and cause the piston to not seal properly with the cylinder wall, resulting in a loss of compression.

2) Leaking Valves

Each cylinder has its own intake and exhaust valves. The intake valves let air into the cylinder while the exhaust valves carry air out to the exhaust system. High temperatures can warp or crack the valves causing them to not seal properly. Additionally, the valve springs, retainers, rocker arms, or valve seats can go bad and also cause an imperfect seal between the valve and cylinder.

Depending on which part of the valves fails, it can be as simple as replacing the rocker arms or require the valve seats to be re-machined.

3) Scored and Cracked Cylinder Walls

Just as the pistons can crack, so can the cylinders themselves. As we mentioned above, the pistons are sealed to the piston walls via piston rings. This ensures an air-tight seal. Heat combined with contact from the piston can cause the cylinder walls to crack. When this happens, you either need to bore out and re-sleeve the cylinders or replace the whole block. Both options are quite expensive.

Alternatively, a lack of oil and proper lubrication can cause the pistons to scrape against the walls and "score" them, or shave metal off of them. This is usually most detectable from inspecting your used engine oil for metal shavings.

4) Timing Belt & Chain Failure

Timing chain or belt failure can throw the timing of the pistons and valves off. Timing belts are prone to snapping or stretching, while belts are prone to jumping teeth and stretching. This causes the intake and exhaust valves to not be able to fully close, creating air leaks. This will cause rough idling, knocking noises from the engines, bad misfires, and a really poorly running engine.

If you have a more catastrophic timing belt or chain failure, it can result in the pistons colliding with the valves. This will result in severe engine damage and likely require a full rebuild. Timing chains and belts fortunately don't tend to fail immediately. They usually stretch or jump chains first before completely failing which gives you a chance to fix the issue before causing serious damage.

5) Blown Head Gaskets

An engine's cylinder head is sealed to the block with a head gasket. Gaskets are known to deteriorate over time which can cause a gap between the block and heat, allowing air to escape from the cylinder. A bad head gasket usually results in compression losses from all the cylinders the gasket covers. V6 engines for example usually have a gasket covering cylinder 1-3 and another covering 4-6.

Fortunately, a blown head gasket is probably one of the more common causes of low compression. Additionally, it is also one of the easiest to fix, at least compared to the other potential causes. It usually required simply replacing the gasket, and possibly having the head machined.

6) Camshaft Issues

The last potential cause of low compression to discuss is the camshaft. There are two primary issues here: a broken camshaft and worn camshaft lobes. Breaking a camshaft isn't very common, unless you are pushing serious power. However, a broken camshaft can't push the pistons upwards which makes it impossible for compression to take place.

Each valve has its own camshaft lobe which opens and closes the valve. Over time this lobe can wear down causing it to get stuck "closed" which means it can't open the valve. It can also get stuck open, which is what causes low compression as it leaves a valve permanently open, letting air escape easily.

Low Cylinder Compression Summary

Engine cylinders need to compress air and fuel to create combustion. Small air leaks in the cylinder can cause it to lose pressure and the ability to compress air. This results in low compression which causes a number of performance and drivability issues. Loss of performance, rough idling, cylinder misfires, and starting issues are all common symptoms of low compression.

Unfortunately, low compression can be caused by a number of engine problems. The majority of the problems relate to the internal engine components like the valves, pistons, and cylinder walls. This makes repairing low compression problems costly from both a parts and labor perspective.

The easiest way to diagnose low compression is to have your engine compression tested. Once you have confirmed low compression issues, the next step is determining which specific component is causing the issue. Due to the challenge of finding and repairing low compression issues, it is not common for owners to either swap in a replacement engine or opt for a full engine rebuild.