(Updated on June 7, 2021)
An internal combustion engine needs four things to run: air, fuel, spark, and compression. These elements must also be introduced into the combustion chamber at exactly the right time.
If the compression ends up being too low in any of the cylinders, then your vehicle will not be able to drive normally. You will likely notice that your engine performance is significantly reduced, making it difficult to accelerate and drive the car as usual.
If you were to not have compression in all cylinders, then you would not be able to start your car at all.
What is Engine Compression?
An internal combustion engine has a number of pistons connected to a crankshaft. As the crankshaft rotates, these pistons move up and down on different strokes of the combustion cycle. This allows the engine to deliver smooth, constant power.
After the intake stroke, the intake and exhaust valves are closed, effectively sealing off the combustion chamber. Engine compression is the built up pressure in the combustion chamber as the piston travels upward. The volume of the combustion chamber shrinks from its largest volume at bottom dead center (BDC) to its smallest volume at top dead center (TDC).
Top 5 Causes of an Engine Losing Compression
If you notice any symptoms of low car engine compression, the next step is to figure out what caused your engine to lose compression in the first place. Below are the top 5 reasons for low engine compression.
1) Timing Belt or Timing Chain is Bad
The crankshaft and camshaft are attached by a timing belt or timing chain. A timing belt or timing chain allows the valves to open and close at precisely the right time with respect to the motion of the crankshaft.
If you have a bad or loose timing belt, the camshaft won’t allow the intake and exhaust valves to open and close the way they’re supposed to. This can also happen when the timing belt skips a tooth.
If the timing is too far off, you won’t be able to run the engine due to low or no compression. In other words, the intake and exhaust valves are opening at the wrong times.
2) Piston Damage
Engine cylinders contain pistons made of a strong low carbon steel or aluminum alloy. These materials are designed to withstand the extreme pressures and reciprocating forces exerted as the piston travels up and down, thousands of time per minute.
Pistons can be damaged in several ways: cylinder pressures that exceed the maximum pressure the pistons were designed for, debris in the combustion chamber, and premature detonation (also called knock).
Once pistons are damaged, they may not seal as well depending on where the damage has occurred.
3) Piston Rings Damaged
Each piston is surrounded by several piston rings. There are several types of rings on a piston: the compression ring, the wiper ring (also called the second compression ring), and the oil ring. The compression ring is primarily responsible for sealing the combustion chamber, the oil ring wipes excess oil from the cylinder walls, and the wiper ring performs both of these functions simultaneously.
Piston rings can become damaged in the same situations that harm the pistons themselves. Insufficient oil can quickly cause piston ring damage and score the cylinder walls, leaving you with an imperfect seal.
Piston rings are gapped to allow for expansion as they heat up. If the rings are not gapped properly (or excessive heat exceeds the allotted piston gap), the ends of the ring can press together and eventually break, causing additional damage to the piston, cylinder walls, and other components of the engine internals.
When piston rings are damaged, the cylinder will not be able to maintain the same level of compression.
4) Valves Not Fully Seated
All the cylinders in an engine have intake and exhaust valves. The intake valve is where fuel and air flow into before they enter the cylinders. After they’re mixed together, the combustion process takes place and causes exhaust fumes to come out of it.
If a valve is unable to seat properly, the compression stroke will not be adequately sealed off, allowing some of the air-fuel mixture to escape the combustion chamber. This may happen for a variety of reasons, including carbon buildup around the valve seats, a bent valve, worn valve guide seals, or other damage to the valve itself.
5) Bad Head Gasket
The head gasket exists on the cylinder head area. This is another component which helps keep carbon gases trapped in the cylinder.
If the head gasket fails, combustion gases may have an additional place they could leak out of the combustion chamber.
Will an Engine Run With Low Compression?
Many engines will run with low compression, but they often won’t run very well. They certainly won’t make the level of power they were designed to, as not all of the air-fuel mixture is burning like it’s supposed to.
If you have low compression, you may have trouble starting your vehicle. Some engines are even harder to start once they’ve reached operating temperature, such as low compression rotary engines.
How To Check Engine Compression
If you suspect that you have low engine compression, you will need to use a compression gauge to verify this. These gauges can be picked up relatively inexpensively, and you can perform this test yourself in about an hour if you wish. Having a shop perform a compression test for you will cost between $150 and $250.
Performing a compression test is fairly straightforward, but the process differs slightly for each vehicle. For each cylinder, pull one spark plug and screw the compression gauge into the spark plug hole. Next, disable the fuel and ignition systems, then crank the engine for several seconds. Immediately check the compression gauge after you finish cranking.
Each vehicle will have slightly different ideal compression numbers (noted in the factory service manual) but as a rule of thumb, all cylinder pressures should be within about 10% of each other.
If one cylinder is much lower than the others, this may indicate a compression issue on that cylinder. More troubleshooting is needed to determine the exact cause of the problem. A good next step would be a leak down test so you can determine exactly where the gases are escaping.
Some vehicles allow you to disable the fuel system by holding the gas pedal all the way down while you crank the engine. You’ll want to make sure you don’t forget this step so you don’t accidentally flood your engine. To learn this and other specific steps to take for your vehicle, it’s best to reference the factory service manual. Online forums for your make and model may be of some help as well.