If your engine has been running rough, this could point to a number of mechanical or electronic factors. The most common cause of an engine running rough is a problem with one of the cylinders. If you ignore the issue, you'll likely begin to see symptoms such as reduced fuel economy and power output as well as increased emissions. A faulty cylinder will also likely damage the engine if the problem goes ignored.
What happens when a cylinder loses compression?
For the most part, a cylinder malfunctions when the internal pressure cannot be properly regulated. The pressure created by the upward motion of the piston compresses the mixture of fuel and air before the spark plug ignites the gases. If there is a leak or another issue, the pressure may be too low or high. This will throw off the combustion, which can lead to engine misfires. Timing is everything when it comes to the engine running smoothly, and if any cylinders are running out of sync it can lead to a roughly-running motor.
In order to determine which cylinder is not working properly, you can use a compression gauge to get readings of each cylinder's internal pressure. Before you can test the pressure level, you'll need to do a bit of handiwork to prep the engine.
Preparing to perform a compression test
First, you will need to disconnect the wire from the distributor cap or the electrical control module in distributorless engines. Refer to the owner's manual if you are not sure whether your vehicle has a distributor. This will also provide information as to the location of the part you are looking for. Disconnect the wire and position it so the metal end is as far from the spark plugs as possible.
Next, you will need to disconnect the fuel injection system and the fuel pump to prevent gasoline from spraying into the cylinders while you are testing them. Once this is done, fire up the engine and let it run until it stalls on its own. This clears gasoline from the lines and the cylinders to reduce the risk of fuel leaks.
Turn the car off, wait for the engine to cool down and then remove the spark plug wires from the boots, before removing the plugs as well. It is important to keep track of which wire belongs to which plug boot, so you may want to label the parts as you remove them.
Testing the cylinders
To get accurate readings, the engine must be warmed up. You can attach a remote starter to the car battery to run the engine while you test the compression in each cylinder. Simply insert the compression tester into the spark plug hole and run the engine with the remote starter. Make a note of the reading and perform the test again to ensure the numbers are accurate. Repeat this process for each cylinder and then compare your results to the recommended compression ranges for your car's engine, which can be found in the owner's manual.
Pinpointing the problem
Any cylinders that have lower-than-normal compression readings will need to be further inspected to determine what is causing the loss of pressure. It is likely either the valves or the rings. You can add a few drops of motor oil into the problematic cylinders and repeat the test to see if one of the rings is leaking, as the viscous fluid will provide a temporary seal. If the compression reading is still low, chances are the cylinder has a bad valve that will need to be replaced.