Engine compression is essential to have effective combustion, where the air-fuel mixture is compressed and ignited to make power. If compression is low, your engine will operate less efficiently and produce less power.
Read on to discover the most common causes of low compression in your engine, the symptoms you will likely experience, and how to troubleshoot where exactly the problem is coming from.
Symptoms of a Low Compression
Before we start learning about causes, it may be helpful to identify the symptoms of low compression. The following are various sing of low compression:
- Difficulty starting the engine, or the engine is unable to start
- Rough idle, especially when the engine is cold
- Misfiring cylinders (the engine stutters or hesitates)
- Loss of power and acceleration
- Reduced fuel economy
- Check engine light illumination
- Excessive smoke from exhaust
The symptoms we look for all vary when a compression problem affects one cylinder, multiple cylinders, or how much compression is lost.
Causes of Low Compression in a Car Engine
Understanding what is causing low compression is the first step toward fixing it. Problems vary from regular wear-and-tear issues to serious mechanical failure.
Below are the most common causes of low engine compression, with the most common listed first.
#1. Worn Piston Rings.
Piston rings seal the piston to the cylinder wall to contain combustion pressure while preventing oil from entering the combustion chamber. Piston rings wear from friction and eventually wear out.
Overheating can cause damage to rings faster than normal operation will, but wear and tear will eventually cause all piston rings to lose tension and sealing performance.
When piston rings do not seal and combustion gases can escape the piston and into the crankcase, compression in the cylinder will be reduced.
#2. Leaky Valves.
Valves control the intake of the air-fuel mixture and exhaust gas in and out of the cylinder. Valve seats and valve seals eventually wear with normal use and can be damaged by overheating.
Valves can also accumulate carbon deposits that prevent proper sealing abilities. When valves never close 100%, there will be a leak of compressed gases into the gaps during the compression stroke.
Leaky valves are especially common on engines that did not receive regular valve adjustment or on engines that have suffered overheating.
Valve leakage can be detected from a “hissing” sound during a compression test and typically will affect performance when the engine is cold.
#3. Head Gasket Failure.
Head gaskets contain compression between the cylinder head and engine block, and keep coolant and oil from entering the cylinders.
When head gaskets fail (usually due to overheating or age) all of the systemic flow of compression gases escape between cylinders or into the cooling system.
A blown head gasket most commonly leaks between adjacent cylinders and can include additional signs, such as engine coolant consumption, white exhaust smoke, and overheating.
This common problem is especially common on certain designs known to be heat-stressed, like Subaru boxer engines from the late ‘90s and early 2000s, and vehicles with issues with the cooling system.
#4. Timing Belt or Chain Problems.
The timing belt or chain makes sure that the crankshaft and camshaft rotate at the same rate so that the valves open and close at the right times in relation to the position of the piston.
When timing components wear out, stretch, or break, the timing of the valves is incorrect, and the valves can be left open during a compression stroke.
This results in an immediate (almost) and severe loss of compression across multiple cylinders. Unlike worn components, which worsen over time, timing issues cause performance issues to manifest immediately.
Modern engines with variable valve timing can also experience these issues if their actuators fail.
#5. Worn or Damaged Cylinders.
With time, cylinder walls wear unevenly, becoming oval vs round. This “out-of-round” condition means that piston rings cannot remain in proper contact with the cylinder walls, allowing compression to escape.
Cylinder wear happens slowly over time, but it happens faster when there is poor maintenance, contaminated oil, or excessive heat. Manufacturing advancements, primarily in the hardness of the cylinder material, have resulted in longer life for most modern engines, but all high-mileage vehicles are still subject to wear.
Another cylinder wear issue is scoring, which is when the cylindrical surfaces of the cylinder become damaged by debris or parts resulting in permanent damage affecting proper sealing.
#6. Cracked Cylinder Heads or Block.
Engine overheating is the largest cause of cylinder heads and engine block cracks. Freezing coolant and poor design or manufacturing can also play a role. Cracks create a path for combustion gases to escape, commonly into the cooling system or between cylinders.
Symptoms usually include unexplained coolant loss, bubbles in the radiator, or compression that comes back temporarily after cooling. Cracks in aluminum heads and blocks are more common than iron due to the greater differential in expansion.
While cracks are not as common of a cause of loss of compression as the others we looked at, they tend to be some of the hardest to repair. In most cases, a crack will require component replacement.
#7. Carbon Build-up.
Carbon deposits from incomplete combustion can accumulate on valve seats and prevent a valve from making a seal when it’s trying to compress. Carbon build-up is common in direct injection engines and for drivers who do not allow the engine to reach full operating temperature because of the frequent short trips.
Vehicles that are not driven often with wide-open throttle are also especially prone to carbon build-up, as periods of high-rpm, full-throttle driving can increase exhaust gas temperature enough to clear out the carbon deposits.
While carbon build-up typically does not lead to catastrophic loss of compression, it can lead to a level of compression loss that affects both performance and fuel economy. The deposits develop over time and in mild cases can sometimes be remove with fuel system cleaners or decarbonizing services that do not require significant disassembly.
#8. Cylinder Head Warpage.
Severe overheating can also lead to warped or distorted cylinder heads, especially aluminum heads. The warpage prevents the head from making uniform contact with the head gasket, which leads to small gaps where compression can escape.
Unlike a blown head gasket (where the cylinder heads are typically specific cylinders), cylinder head warpage usually results in varying cylinder compression losses.
Typically, this condition develops after overhead events and gradually becomes worse over time as the head continues to be in a thermally cycled state of distortion. Professional machining or replacement is typically required.
#9. Bent Valves From Timing Failure.
On interference engines (where valves and pistons occupy the same space at different times in the cycle) a broken timing belt or chain will cause catastrophic failure whenever pistons end up colliding with open valves. When this happens, the valves will bend and not seat properly, allowing constant loss of compression.
The failure itself typically affects multiple cylinders at once and makes starts and runs impossible. This is one of the more expensive repairs in the compression loss category, usually due to the need to remove the cylinder head for valve replacement, plus whatever inspections may be necessary for damage to the pistons or cylinder walls.
Testing For Low Compression
Once you have started to suspect some sort of compression issue within any part of your engine, you really would need to measure the actual cylinder pressures to confirm. A compression test would be the most straightforward way to diagnose low compression and determine which cylinders are affected.
A compression test would measure the pressure in each of the cylinders when air is compressed in the compression stroke and give you some valuable diagnostics.
Concrete demo on how to do a compression test on your engine from below- It really is simple procedure using a compression gauge (which you can buy at most auto parts store) and basic tools.
Whether or not you have a professional to do the repairs for you, it is helpful to know how compression testing works so you can have a better understanding when communicating with a mechanic and make a better overall repair choice.
FAQs.
Is it Safe to Drive with Low Compression?
Generally, no, it is not safe to drive your vehicle if it has low compression, for your vehicle’s and perhaps your safety. The risk of further engine damage aside, a vehicle that exhibits compression issues can stall suddenly, surge or hesitate upon acceleration, or lose power unexpectedly when you need it most (such as when merging traffic onto a highway or passing).
The level of danger is directly dependent on how many cylinders are affected and how low the compression is.
If you have “minor compression issues,” you could be able to limp home or to the repair shop. In the case of “significant compression loss” across multiple cylinders, it is not safe to operate a vehicle on public roads.
Will Low Compression Hurt Other Engine Parts?
Yes, low compression can hurt other engine parts if you continue driving with the low compression issue.
When low compression is present, incomplete combustion and dilution of your engine oil with fuel can wash the cylinder walls, thus reducing lubrication. The increased wear described will be on other moving parts in the engine, such as bearings, camshaft, etc.
Most times, low compression creates misfires that can harm catalytic converters due to unburnt fuel entering the exhaust system and harming the complex chemical reactions created once inside the catalytic converters.
Additionally, a few specific low compression causes (such as a blown head gasket) to cause mechanic failure will result in mixing of coolant and oil, thus causing significant damage to the engine over time.
What Compression Numbers are “Low”?
Gasoline engines usually range from 125-180 PSI per cylinder when a healthy engine is involved. All cylinders should be within 10-15% of each other.
Under 100 PSI is typically a significant compression issue, but this will vary depending on what type of engine being tested.
Diesel engines will show significantly higher readings due to higher compression ratios.
Healthy diesel engines will show compression readings from 275-400 PSI per cylinder. Readings below 250 PSI are typically considered low and indicative of a compression issue.
Can Fuel Additives Fix Low Compression?
Fuel additives can only help in some specific situations, when carbon buildup is the only thing affecting compression. Even then, fuel system cleaners won’t fix things unless the carbon buildup is small.
No liquid in a bottle is going to fix mechanical issues like worn rings, damaged valves, or blown head gaskets. For most serious compression issues, only mechanical fixes.