How to Determine if Your Car Has a Bad Cylinder
When a car exhibits signs of poor performance, one of the common culprits is a bad cylinder. The symptoms can range from a rough idle to engine misfires and performance issues. By following the steps outlined below, you can perform several tests to identify which cylinder is causing the problem.
Testing for a Bad Cylinder
Ignition System Test
The first step in diagnosing which cylinder is malfunctioning is testing the ignition system. This can be done using a test light with an alligator clip terminal. Connect the clip terminal to a ground bolt on your car's chassis, and then probe each ignition terminal head one at a time. This will ground the coil, effectively killing the spark plug momentarily. If the engine's idle quality and smoothness noticeably change, the cylinder corresponding to that terminal is not firing properly. Conversely, a cylinder that shows no change in performance when the ignition terminal is tested is likely functioning correctly.
Knock Detection
If the engine is making a knocking noise, you can pinpoint the cylinder responsible using the same test light method. When the corresponding cylinder is disconnected or grounded, the knock should cease. This test can help you indentify which cylinder is generating the knocking noise.
Circuitry Considerations for Coil-on-Plug Ignition
Care must be taken when dealing with coil-on-plug ignition systems. If you intend to disconnect the harness, it should be done when the engine is not running. This is to prevent damage to the electronic components and to ensure your safety.
Advanced Diagnostic Tests
In addition to the ignition test, several other diagnostic tests can provide further insight into which cylinder is causing the engine issues.
Power Balance Test
A power balance test involves disconnecting individual ignition cables and observing the engine's idle speed. If the idle speed does not drop when a cable is removed, it suggests that the cylinder was not producing any power to begin with. This test is particularly useful in identifying a non-functioning cylinder.
Compression Test
A compression test measures the pressure of a cylinder as the piston approaches Top Dead Center (TDC). A well-functioning cylinder should produce 100-120 PSI. A damaged cylinder or one with valve problems will show reduced pressure. By comparing multiple cylinders, you can determine which one is weaker and requires attention.
Leak-Down Test
A leak-down test is often performed after a poor compression test. This test determines the extent of any damage and can help locate the source of the leak. At TDC, a cylinder pressurized to 100 PSI with compressed air should maintain 85-90 PSI. If the pressure drops below 60 PSI, a significant leak or damage could be present. The location of the leak can be identified by the source of air escaping, such as the exhaust pipe, throttle body, or dipstick tube.
Practical Methods to Identify the Faulty Cylinder
Spark Plug Wire Unplugging
The simplest way to check if a cylinder is not firing is to unplug each spark plug wire one at a time. If the engine's performance noticeably drops, it indicates that the cylinder is not firing properly.
Exhaust Pipe Temperature Test
This diagnostic method is particularly useful for motorcycles and some cars. Measure the temperature of each exhaust pipe. If the temperatures differ significantly, a problematic cylinder may be causing the issue. Start by checking the spark plugs and carburetors or injectors, as they are often the initial suspects. For cars, similar principles apply, and the exhaust pipes should generally be at similar temperatures.
By performing these diagnostic tests, you can effectively pinpoint the faulty cylinder and take appropriate action to repair or replace it.