A spark tester would also be handy but a known good spark plug will do the job too. Tools: For this, you’ll need insulated pliers, a spark plug boot pulling pliers works best but any insulated pliers do the job. Points (if fitted) and distributor cap plus rotor are OK.At this point we’ll assume you have checked: How To Diagnose Misfiring Engineįrom years of experience on the job, this is the fastest way to isolate and diagnose the root cause of a single-cylinder misfire. Now you have an outline of why spark, fuel, and compression components are important, and you also know that different type misfires may point you in a different diagnostic direction. Random/constant adjacent cylinder misfires Bad crankshaft sensor (later model cars).If the cylinder can’t seal, firing the spark plug won’t ignite the mixture and produce any power to turn the crankshaft, that’s a misfire. Last of all, and least likely to be your problem, is bad compression. Fuel is more than just the gas though, the fuel is a mix of both oxygen and gas, more on this later. Especially in classic cars, using the wrong type of gas can cause it to run poorly and misfire. FuelĪfter spark, gas is the next most common cause. In my experience, spark, or lack of, is the most common of the three to cause an issue, and the problem is usually wet plugs, bad plug wires, or damp/cracked distributor cap. Stay with me here, I’m not going far into the weeds.Īn engine needs three things in order to run smoothly:Īll of these systems are calibrated quite precisely, any variation in the ratio between any of these components can cause a misfire, constantly or randomly over one or more cylinders. Having an outline of why engine misfires will help you diagnose your classic car misfire problem. Your model may not have a distributor cap, either way, check the following components are secure and free from moisture: While you have the cap removed, check it and the rotor arm for burn marks (tracking), and cracking, damaged caps and rotors are also common. You can dry the distributor with tissue paper, compressed air, and when dry spray with WD40. Any moisture here will cause random misfires, across multiple cylinders. Washing a car engine, damp weather, driving through puddles will cause the plug wires and distributor to get wet, it’s a very common cause of misfires.Īs it’s a common cause of misfires, check the distributor cap for dampness. Check that all the plug wires are on securely. ![]()
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