Proper compression enables gas and air mixing in engine cylinders, powering operation. But compression issues spell trouble.
Learn what the common causes of low compression in your engine are and the steps needed to diagnose where the exact issue lies.
Top 5 Reasons for Low Compression in a Car Engine
There are many reasons why low compression might exist in a car engine. Sometimes there will be low compression in just one cylinder of the engine and other times, low compression may exist in ALL cylinders.
Generally speaking, if you have low compression in one cylinder, the engine will start but you’ll likely experience misfires and your vehicle will run rough. If you experience no compression in ALL cylinders, your engine simply won’t start.
Here are the top 5 causes of low compression in a typical car engine.
#1 – Holes in Piston
You probably know that the cylinders of an engine have pistons in them. These pistons are typically made from aluminum alloy and are supposed to be able to handle the power of combustion.
However, if there is overheating in the engine, then hot spots will get onto the piston. After a while, these spots will burn holes right through the piston. Once that happens, gases will leak through these holes and cause low compression to take place.
#2 – Leaky Valves
The top of each cylinder contains exhaust valves and intake valves. Air and fuel go into the intake valve for the combustion process. The gases which form from this go out of the exhaust valve.
If these valves become overheated, they could start to leak gas prematurely out of them. Once this happens, you have low compression.
More commonly, the valve seals can over time become worn which allows the gasses to escape, thus causing cylinder compression to go down.
#3 – Worn Timing Belt
There is a timing belt or chain in every engine which connects the camshaft and crankshaft together. If the timing belt were to be broken or damaged, the camshaft would no longer be able to turn anymore.
This means it cannot open or close the exhaust valve or intake valve properly. As a result, the combustion in the cylinders will be ruined and no gases can get released. So, you have low compression because of it.
#4 – Head Gasket Failure
There is a gasket between the area on top of the engine where the cylinder head is connected. If for some reason the head gasket becomes faulty and starts to break, then it will leave a tiny hole in between the cylinder and its head.
This is known as a blown head gasket and causes the gases in the cylinder to leak out of the hole in the gasket. Then you will have low compression and a poor performance. If the head gasket fails between two cylinders, it can cause a compression leak in both.
#5 – Bad Piston Rings
Overheating can cause the rings of the piston to brake or become damaged. This will cause carbon gases to leak through the rings because they are no longer able to seal them inside of the cylinder. As you’re probably aware by now, when there’s this type of leak, low compression results.
How to Fix Low Compression
The first thing you need to do is use a compression gauge and test to see if there is actually low compression in your engine. This process will usually take 45 minutes so make sure you have some free time on your hands.
If you don’t have a compression gauge, then you can either purchase one or take the car to an auto repair shop and have them test the compression for you. If they find there is low compression, the next step is to inspect the cylinder, piston, valves, and gasket to see if any of them are damaged or broken.
From there, you can replace whatever is damaged. However, this will be a lengthy and costly job because it involves taking out the engine. Be prepared for that.
Here’s a good video explaining how to properly perform a compression test:
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