Low Engine Compression: Causes, Diagnosis and Fixes


 In order to fully understand low engine compression, it is necessary to understand the concept of compression and its importance in the overall running of your car’s engine.

Key parts in a car’s engine

The engine has a set of what could be compared to cooking pots called cylinders.  Most cars have between 4 and 6 cylinders though some have as few as 2 and others have as many as 12. This is where the power which makes the car move is created.

The cylinders are made using extremely strong metal and are each sealed shut. At the bottom of each cylinder is a tight-fitting piston which is a plunger-like component attached to the crankshaft wheel.

It slides up and down to close and open the cylinder. At the top of the cylinder are 2 valves (or gates), an inlet and an outlet. The former opens to let in a mixture of fuel and air into the enclosure and the outlet allows exhaust fumes out.

What Is Low Engine Compression?

How the engine makes power

The ‘cooking’ action which happens in the cylinder is a four-stroke process which is repeated over and over to drive the car.

Stroke 1. Intake: The piston pulls down in the cylinder by movement of the crankshaft wheel. As long as the car is moving the crankshaft wheel is turning. This creates a void inside the cylinder and opens the inlet valve which lets in the air and fuel mixture.

Stroke 2. Compression. Once the fuel and air mixture is let in, the inlet valve closes. At the same time the piston moves up into the cylinder therefore compressing the fuel and air mixture considerably. The higher the compression the more flammable the gas is. When the piston gets to the top, the spark plug fires the mixture.

Stroke 3. Power.  The result is a small explosion which gives off hot gas. This forces the piston down and out thus causing the crankshaft wheel to turn. This turning is what essentially drives your car.

Stroke 4. Exhaust. The final step is the opening of the outlet valve at the top of the cylinder. At the same time, the continually turning crankshaft wheel again pushes the piston up and into the cylinder. This time it pushes exhaust fumes produced by the explosion out through the outlet valve.

Problems arise when stroke 2 doesn’t happen as it should. The cylinder’s ability to confine the explosion is measurable and is defined as compression. When there is a leak out of the cylinder, compression is decreased. This is defined as low compression or compression loss and can drastically lower power output of the engine.

Causes of Low Compression in an Engine

Low compression in an engine could be caused by a number of faults. Sometimes, low compression is in only one cylinder and in other instances it is in a few or all cylinders. These are the common reasons behind low compression.

Hole in piston

The piston in a cylinder is supposed to seal it completely when it slides in to compress the air and fuel mixture. Pitons are made of aluminum alloy, a strong metal which should be able to withstand standard heat levels in the system.

If however, there is some overheating in the engine caused by other faults, the piston metal may not be able to withstand the heat. Hot spots develop on this metal and if overheating continues, holes are burned through the metal at these spots.

Once holes are formed, the usual required pressure created when the piston slides into the cylinder cannot be achieved. Gas leaks through the holes and this causes a drop in compression levels.

Leaking inlet and outlet valves

At the top of each cylinder are the intake valve and the exhaust valve. These valves must be able to seal the system tightly for sufficient compression to be achieved. If there is overheating in the system, these valves sometimes start to leak. Even small amounts of gas let out causes a reduction in compression.

Broken or faulty timing belt

The timing belt or chain connects the camshaft to the crankshaft. The belt keeps the two turning as they should. If the belt is faulty or broken, the crankshaft stops turning. This in turn makes the piston stop sliding into and out of the cylinder. The result is either a stalled system or an inefficient one where the crankshaft doesn’t push the piston far enough so only a little compression is created in the cylinder.

A timing belt is designed to last between 60,000 and 100,000 miles. If you have had yours for about this long, go ahead and replace it. Another problem could be tension on the belt. Some engines have belts which are supposed to be manually tightened. If your timing belt is causing low compression because of this, you can tighten it yourself or take it to a technician who can do it fairly quickly.

Blown head gasket

A head gasket works by sealing the cylinder head to the engine block. When it fails, sometimes between two cylinders, it allows gas to leak out of the cylinder when it is compressed by the piston. The result is low compression in both cylinders.

Faulty piston rings

If the ring on any one of the piston breaks, it compromises the piston’s ability to lock gas in the cylinder when it compresses it in the cylinder. Piston rings often break as a result of overheating. Excess heat weakens the metal and they eventually break.

How can you identify a broken piston ring? With the engine running, remove the oil fill cap. If a piston ring is broken, after a few seconds, there will be a little smoke coming out of the engine.

Broken Camshaft

If the camshaft is broken, it stops the crankshaft from moving as well. Just like happens in the case of a broken timing belt, the crankshaft cannot push the piston into the cylinder so no pressure can be created.

A broken camshaft is not as common as other causes of low compression but it does happen.

Faulty camshaft lobe

This part is responsible for opening and closing the intake and exhaust valves. Each valve has a lobe.  When one of the lobes wears out it could get stuck in a single position. If it gets stuck in a closed position, then it doesn’t open at all. In the case of low compression, it could be caused when this lobe wears out and gets stuck in an open positon.

This causes gas or the air and fuel mixture to leak out of the cylinder when it is compressed, therefore causing reduced compression.

Symptoms of Low Engine Compression

Low engine compression in one cylinder will cause different symptoms from when low compression is in all cylinders.


Low compression in one cylinder will cause misfires.  What does it mean when a car misfires? It happens when the engine skips one of the processes in the combustion cycle. In this case the compression step is either skipped or not executed properly. The result is rough running and jerking as the car moves.

Loss of power

Misfiring causes a significant loss of power in the car. If for instance one cylinder in a four cylinder engine is misfiring, it causes a 25% reduction in engine power. You will be able to drive the car from one place to another but you may not be able to take up hill climbs at your usual high speed.

Poor fuel economy

The general science here is that the higher the compression ratio, the better the fuel economy in an engine. Compression ratio refers to a comparison or ratio of the volume of air in the chamber with the piston at the top of the cylinder and volume of the same air with the piston is at the bottom. The higher the compression the higher the ratio.

From this it is clear that when compression is low, fuel burning efficiency is reduced therefore requiring more fuel to cover the same distance.

Failure to start

If there is low compression in all cylinders, then the car just won’t start at all. There simply isn’t enough pressure on the fuel and air mixture to ignite it.

How to Diagnose Low Engine Compression

There are 2 main ways of checking compression in a gasoline engine (the process is different and more complicated for diesel engines).

Using a manual compression gauge

A compression gauge, also called a compression tester is a tool used by inserting it into each spark plug hole. The pressure reading obtained can be used to determine if the cylinder, piston and other parts are in good condition.

Make sure the engine is warm. Preforming this test on a cold engine will give wrong results.

  • Disable the ignition module
  • Remove the first spark plug and insert the tester into the cylinder hole
  • Hold the throttle to a completely open position to let the engine get adequate air intake
  • Allow the engine to crank for at least ten revolutions to give an accurate reading.
  • Repeat the same for each cylinder
  • Record readings from each cylinder

It is unlikely that you will get similar readings for all cylinders. Readings should not vary by more than 10%. If there is a variance of more than 10% on any of them, there is need for more specialized testing.

Using an electronic engine analyzer

The tool causes a short in one cylinder at a time with the engine running. It then calculates the drop in revolutions per minute. When analysis of all cylinders has been done, you can tell which cylinders are working harder than others. Those with higher compression readings are working harder and those with low compression are not working as hard.

Recommended compression reading. What figures are you looking for when you do a compression test? What figures are good and which are bad? As a general rule, you want to have compression readings of 135 psi or higher. Compression readings of 85 or below are considered bad, requiring professional attention.

If you don’t have a pressure gauge or electronic analyzer, take your car to an auto repair shop and have them carry out an engine compression test. It costs between $128 and $162.

How to Fix Low Engine Compression

Fixing low engine compression in your car boils down to identifying the cause and resolving it. Ifit is caused by a broken timing belt, replace it. Replace or repair any part found to be causing low compression.

In some instances such as when low compression is caused by a hole in a piston, there is an underlying problem causing the offending part to fail. A hole in a piston is probably caused by overheating in the engine. Even before you get to replace the piston, find out what is causing overheating.

What Causes Low Compression in Diesel Engines?

Low compression issues can be experienced in gasoline and diesel engines as well.

Most common causes are the same as those discussed in gas engines though some causative issues are specific to diesel engines. These include:

Leaks past or through glow plugs

Glow plugs are a component of diesel engines whose function is to preheat engine cylinders to enable combustion take place more easily. A diesel engine can have as many as 10 glow plugs. There is one in each cylinder.

When fuel leaks past glow plugs, compression is reduced.

Leaks past or through fuel injectors

The fuel injection system is more or less the heart of a diesel engine. It pressurizes and then injects diesel onto highly compressed air in the combustion chamber. It adjusts fuel quantity released depending on engine speed and optimizes timing so only what is required is released into the chamber.

When fuel leaks past injectors, the system cannot achieve the required high compression.

Symptoms of Low Compression in a Diesel Engine

Diesel engines are considered hardy and more reliable than gasoline engines. Compression in cylinders is necessary whichever kind of engine you have. When it doesn’t happen as it should, any engine will start showing signs that something is amiss. These are some telltale signs of low compression in diesel engines.

Problems starting up

Starting problems could be caused by several issues. One of them is low compression. If for some reason the air in the cylinders is not sufficiently compressed, it could take more than a few tries to get the engine running. If compression is very low, the engine may not start at all.

White smoke

In a diesel engine, ignition of the fuel is caused by elevated temperature of air in the cylinder. These high heat levels are achieved through intense mechanical compression of the air. This is unlike a gasoline engine in which ignition is sparked by a spark plug.

White smoke coming out a diesel engine usually means that fuel is not burning properly due to a lack of sufficient heat in the combustion chamber.

It then follows that white smoke can be blamed on low compression which causes low heat and consequent poor combustion.

Loss of power

Does your vehicle seem to be struggling to take a hill you usually cruise up at high speed? This is a classic sign of low compression in one or some cylinders. If air in the combustion chamber is not being compressed enough, only a fraction of the engine power needed can be produced.


Compression is an essential element in the combustion process used to power your car. In gasoline engines, the air and fuel mixture has to be compressed before a spark is introduced to get the engine running. In diesel engines, compression is even more important because these engines need a higher level of compression to achieve the engine start.

The higher the compression level, the more power you can get out of the engine and the more fuel efficient it becomes. When compression is low, fuel efficiency decreases, the car loses power and in some cases doesn’t start at all.

When your car shows signs of low compression, it is best to take it to a professional immediately to prevent any further damage.


Will an engine run with low compression?

It depends on the number of cylinders with low compression. If one of a few cylinders have low compression, the engine will run but you will notice a drop in engine power and rough engine running. If all the cylinders have low compression, the car won’t start.

How much does it cost to fix low engine compression?

Fixing low engine compression means fixing whatever problem is causing it. For starters, it is advisable to have a professional carry out an engine compression test which costs between $128 and $162. After that the cost depends on what faulty part needs to be repaired or replaced and the make of your car.

Some can be quite expensive. Replacing piston rings for instance costs between $1,500 and $2,500. The bulk of the cost is for the technician because this replacement can be quite complicated. The engine must be disassembled and cylinders have to be reconditioned.

How do you start a car with low engine compression?

If low compression is caused by fuel leaking into cylinders, it will help to put a little oil in the cylinders. Before you do this, remove the spark plugs and set them out to dry. Once they are dry, put in some oil then put them back in place and try to start the engine.

It should start easily though you may notice some blue smoke coming out of the exhaust pipe for the next few minutes. This is because of the oil burning in the combustion chamber.

Can a bad valve cover gasket cause low compression?

If your valve cover gasket is faulty or has been improperly applied, oil leaks. It could leak all the way into spark plug wells and then seep into spark plug tubes. This could interfere with compression in the combustion chamber. 


Please enter your comment!
Please enter your name here