What exactly does a compression test on a car determine?


Edit: thanks for the explanation guys!

In: Engineering

How much compression there is in each cylinder. If there is not enough, that means there is a “leak” somewhere and the gases can’t compress to the amount the engine is tuned for. This can cause misfires, among other issues.

The engine in your car has cylinders that slide up and down to turn a shaft. While doing so, they compress a fuel-air mixture to get highest possible power out of igniting it. If there’s no compression, you have a leaking gasket or bad part somewhere through which air can escape. And without air, fuel and comprssion , you won’t have combustion and your engine doesn’t run

So the idea is that the compression chamber is able to contain the combustion of the air fuel mixture and convert that into the downward force on the piston. If there are leaks in the seals on the intake or exhaust valves or piston rings, the compression isn’t properly “contained” and in turn, the motor won’t operate properly. A compression test determines how well the the compression chamber of the cylinder is sealed, by testing each cylinder individually from it’s threaded spark plug housing and comparing the compression pressures across the board or with pre recorded specs.

The tool used to do this is a simple pressure gauge which threads into the spark plug port. You would remove the spark plug for the cylinder you wish to test, thread the end of the tool into the port, turn the car Ignition over, then check the pressure gauge to see what pressure it read.