My petrol engine has developed a persistent misfire. How can I figure out which cylinder is the culprit in order to hone in on the root cause?
It is easy to tell a misfiring engine as it will transmit unusual vibrations that vary in severity with engine load. A misfire occurs when a cylinder fails to ignite the air/fuel mixture at the right time and could happen due to a wide range of reasons, including lack of spark (a bad coil), lack of fuel delivery (clogged fuel injectors) and improper ignition timing (out-of-spec cam timing).
There are several ways to identify the cylinder(s) responsible for the misfire:
Use a code scanner. Most modern-day vehicles come with an engine control unit (ECU). One of the jobs of the ECU is to store engine faults, and infers the misfiring cylinder based on the readings coming from the engine knock sensors. While this approach is quick and easy, it is not always guaranteed to register the responsible cylinder.
Infer by elimination. The principle behind this approach is quite simple - if a firing cylinder is disabled while the engine is running the vibration "signature" will change, while if a misfiring cylinder is disabled then there will be no discernible change in engine note. This test is best done with the engine running and with appropriate PPE as it will invariably involve disconnecting coils/plug wires one-by-one. After each cylinder is tested, the coil/plug wiring should be reconnected to restore the engine note back to its baseline.
Observe the thermal signature. A misfiring cylinder will not generate the same level of heat as a firing cylinder. This test is best suited to engines with easy-to-access exhaust headers. With the engine cold, start it up and let it run for a few seconds before turning it off. By figuring out which parts of the exhaust header(s) are cold (an IR temperature gauge should work well here), one can hone in on the misfiring cylinder. This test doesn't work well if the engine is run for too long as the heat generated will even out across the headers.