Low Compression Engine: 7 Main Causes & How to Fix

by Joshua Thomas

If you find that your car seems to be a little under the weather, the engine seems to misfire every time you start the vehicle or it just does not perform as well as it typically does, the reason for all this might be low compression.

Cars require a precise mixture of air and fuel to operate smoothly. The mixture is then compressed before the spark plugs can ignite it. The compression also has to be right, and so if it is low, you can expect to have issues.

But, the good news is that low compression is not a catastrophic vehicle problem as it is fixable. However, before you get to fix the issue, you first need to understand the different causes of low compression.

Low Compression Engine: 7 Main Causes & How to Fix
Low Compression Engine: 7 Main Causes & How to Fix

Causes of Low Compression Engine

Different things can go wrong in the engine, and hence leading to low compression which then affects the car's power output and overall performance.

And so if you think you have an issue with low compression, you need to first understand the causes as this will ensure you know what to fix. Here are some of the main causes of low compression in the engine.

1. Worn Piston Rings

Worn piston rings are one of the most common causes of low compression given the role that they play. These rings help prevent gases from leaking through, and so when they are damaged, the gases can seep through and hence leading to low compression.

Overheating is often the main cause of this problem as being exposed to the high temperatures for long causes the rings to break.

Besides the gas leakage, worn piston rings will also prevent the pistons from moving correctly, which can be a contributing cause for low compression.

While the pistons themselves are often made from a durable aluminum alloy material, they can also get damaged by overheating. If they develop holes, gas can leak through these holes and cause low compression.

2. Leaky Valves

When a valve does not shut all the way like it is supposed to, it will result in a leakage, which then translates to low compression.

With leaky valves, some of the gas and air mixture will escape through the hole as the piston compresses it, which means you will often have a lower than normal compression.

And because all the cylinders in your engine will have their valves, you need to inspect each one thoroughly when you suspect that a leaky valve might be the source of the low compression.

3. Worn Out Timing Belt/Chain

The timing belt connects the camshaft to the crankshaft and is also tasked with controlling or determining which pistons go up and down.

When the timing belt wears out, it means that neither of these functions will be done correctly, which can then mean you end up with low compression.

A bad timing belt means that the connection between gears that are tasked with moving the pistons up and down will be broken, and the gases cannot be released which then causes low compression.

Worn timing belts also tend to be loose, which means it will not be able to spin the shafts enough between the firing which can also cause low compression.

4. Head Gasket Issues

A problematic gasket is one that is not aligned properly or is a little dilapidated, and it will come with several issues for your vehicle with the key among them being low compression.

When you have head gasket issues, it is easy for the gas to leak through any small holes or gaps between the cylinder and head, which lowers the compression and prevents the engine from performing optimally.

The head gasket will often develop issues as a result of overuse which causes the development of small cracks that then allow pressure to leak, and hence causing low compression. You can easily test for pressure leaks in the head gasket using a pressure tester. 

5. Cracked Cylinder Wall

The combustion cylinders need to be airtight for efficient combustion of the air and fuel mixture. Hence, with a cracked cylinder wall due to things like exposures to high heat, you can end up with low compression as pressure is lost.

Bubbles in the radiator when the engine is running is one of the most reliable signs that you have a cracked cylinder wall.

The bubbles are gases from the combustion chamber that will be leaking through the cracked wall into the cooling system.

6. Dropped Valve Seat

The valve seat is that small and super hard metal head that will be pressed into the cylinder head. Cylinder heads are made from aluminum, but the valve seat uses a different metal material.

This material difference means that the two will expand at different rates when subjected to the high heat generated when the engine is in operation. This expansion difference can cause the valve seat to fall out of the cylinder head.

A dropped valve seat leads to low compression as the air will escape into the valve port. When you detect this issue is what is causing then low compression, you need to remove and repair the cylinder head.

7. Bad Camshaft

The camshaft is what opens and closes the intake and exhaust valves using the camshaft lobe. And there is one camshaft lobe for every valve.

When the camshaft lobe is damaged or wears out, it does not open the valve well, which means the cylinder will not be able to take in air or allow the exhaust gasses to escape and hence resulting in low compression.

If the camshaft is also completely broken on its forward most journal, it will not turn much just like when you have a broken timing chain. And while this is quite a rare occurrence, when it happens, you have to replace your camshaft.

How to Diagnose Low Compression

How To Diagnose Low Compression
How To Diagnose Low Compression

With the low compression causes above in mind, the next important thing is to know how to diagnose this common engine problem. And this is because the symptoms of the issue such as a reduction in overall performance and misfiring are also signs of other engine issues. Here is a step by step guide for diagnosing the problem.

Step 1: Check the Timing Belt

The first step when you suspect that you have low compression should always be to check the timing belt as this is one of the most common causes of the problem.

Checking the timing belt and chain is even more important if you are having low compression in all the cylinders.

Step 2: Pour Oil Into Cylinder with Low Compression

In cases where you are experiencing low compression in only one cylinder, you should start by pouring some oil into it to see whether there will be any changes.

If the compression increases, it might be a sign that the piston rings were stuck and the oil might be enough to get them moving again, and hence solving the problem.

Step 3: Remove Oil Cap

The other step when diagnosing low compression will be to remove the oil cap and check the pressure as the engine is running.

If you can feel a lot of overpressure, as the engine is running and see smoke, this is a sign that there is a compression leak through the piston and into the crankcase. 

But if everything is working okay, you should feel a slight underpressure in your crankcase when the vehicle is idling.

Step 4: Do a Leak-Down-Test

If you are still unable to diagnose the problem even after following the steps above, the only other alternative is to use the leak-down-tester.

For this test, you should make sure that your camshaft is at timing when both valves are closed and then apply some pressure to the cylinder and listen keenly.

If you can hear the compression leaking in the exhaust, intake or crankcase ventilation, then you have a low compression problem.

How to Fix Low Compression Engine

The last stage when dealing with the low compression issue is doing the actual fix. And like the diagnosis stage, there are a few steps that you need to follow whether you are fixing it on your own or using the services of a professional mechanic.

Step 1: Ensure You Have Low Compression

When fixing the low compression issue, the first step should always be to make sure that it is what you are dealing with and not any other engine problem given the similarities in symptoms.

The four steps above should be very useful in diagnosing the low compression issue. However, a compression tester will provide a more accurate diagnosis. The tester typically takes around 45 minutes to diagnose the problem and it is highly reliable.

Step 2: Determine the Underlying Issue

Once you ascertain that you have low compression, the next step will be to determine the underlying issue causing it as it can be anything from worn piston rings to a bad camshaft.

Here you will need to do a physical inspection of the components to check for any type of damage that may be causing or contributing to the problem. And this should also be a fairly easy step if you know what each of these components looks like when damaged.

Step 3: Repair/Replace the Problematic Part

Next, you should repair the problematic part if it is fixable, and if not, you will have to replace it with a new one to eliminate the problem.

And while some parts will be simple to replace such as timing belts, others might require you to remove the entire engine, which means that this will be a job best left for a professional mechanic.

Step 4: Test Drive the Vehicle

To be certain that your car no longer has the low compression engine problem, you need to take it for a test drive. During the drive, you should check whether the overall performance has improved and if there are still other common low compression symptoms such as misfiring.

If the vehicle is driving fine, it means that everything is working okay. But, if you want to be even more certain there is no low compression, you should do another round of diagnosis by following the steps highlighted above or do the more comprehensive compression test.

Note: If the low compression problem is not fixed even after replacing the problematic parts in the steps above, this might be an indication you have a more serious engine problem. Hence, the wise idea will be to take the vehicle to the auto repair shop for inspection by a licensed mechanic.


Low compression in your engine will affect the vehicle's performance significantly, and it will also often come with other problems like misfires, but the good news is that it is a fixable issue.

To deal with this common issue effectively, you will only need to first determine the cause which can range from a worn-out timing belt to leaky valves and gasket. Also, you need to know how to diagnose the problem to ensure that it is what you are facing and not any other similar issues.

When it comes to fixing the problem, the most effective solution is replacing the damaged or worn part. If you can do it on your own the better as you save time and money, but the wise idea is always to use the services of a professional mechanic.


  1. Low Compression in a Car Engine - Mechanic Base
  2. Reasons for low compression - 2CarPros

About Joshua Thomas

Joshua Thomas just simply loves cars and willing to work on them whenever there's chance... sometimes for free.

He started CarCareTotal back in 2017 from the advices of total strangers who witnessed his amazing skills in car repairs here and there.

His goal with this creation is to help car owners better learn how to maintain and repair their cars; as such, the site would cover alot of areas: troubleshooting, product recommendations, tips & tricks.

Joshua received Bachelor of Science in Mechanical Engineering at San Diego State University.

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