I would 1st have all the plugs changed. If the new plugs cured the problem, I would change the oil/filter and get an oil analysis to determine if there is any long term damage from the Troll gas station. If it still misfires after new plugs, you'll next to bench test each injector and test the coils. How to Diagnose and Fix. A misfire is when a part of the combustion process goes wrong in one of your car’s cylinders. Our mechanic shows you how to diagnose an engine misfire. Visit our instructional blog. First, disconnect the high-voltage wire to the spark plug and remove the spark plug. Then insert the cone of the compression gauge into that hole. Use the starter to turn the engine around 10 to 12 revolutions of the crankshaft. And at the same time, observe the pressure on the dial gauge. 2003 Cadillac DeVille 90,000 mi, Visitor. Car idles rough and check engine light is on and sometimes flashes. Worse when engine warms up but idles rough even when cold. Diagnostic check shows P0300 and 303 codes which indicates a misfiring in the #3 cylinder. I haven't troubleshot it any further. WHAT CAUSES ENGINE MISFIRE, TOP 6 REASONS FOR ENGINE MISFIREIf you have engine misfire and you are trying to figure out what causes engine misfire in this vi Driving with a misfiring engine may not be dangerous right away, but over time it will wear on your engine and you can potentially damage the vehicle. If you lose power while driving on a busy road, this is a safety hazard. If you notice any of the signs or symptoms of a misfiring engine, get them checked out by a professional as soon as possible. A misfire could be caused by not enough fuel getting into your cylinder, leading to an oxygen-heavy explosion. There’s also a slim chance you could have simply filled up your car with a tank of gas that has gone bad. There are a number of components that regulate how much fuel enters a cylinder that makes sure it works correctly. Since engine misfire is a very broad problem, there are many triggers that may cause this to happen. Here are some causes of an engine misfire: A bad ignition coil, faulty spark plug wire, or failed spark plug wire. A faulty fuel injector or problem with the injector’s circuit. An internal engine problem, such as a leaking valve or worn qvrX1J.