Q: Misfire in Cylinder 3

asked by on December 11, 2016

I was driving one night and on my way home my RPM kept dropping when I came to a stop. And also hard to accelerate. The next day I looked at it and the check engine light was on it was a misfire code. I pulled the spark plug wire and there was no difference in engine sound just to confirm. I replace the spark plug. My next step is to measure the spark plug wire resistance. But I don't have my owner service manual so I do not know what the spec is. I also tried searching the brand but they did not have the spec listed either. The wire is a Belden Premium Wire 5mm diameter, SAE, 12. Part number is BEL 700992. The readings that I am getting are 0.28 ohms. (280 ohms I believe). Is that within spec, or does it need to be replaced? FYI it has also been almost a year since I've changed my spark plugs and wires. Any advice will be greatly appreciated.

My car has 184000 miles.
My car has an automatic transmission.

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Engine misfires can be caused by many different problems and are generally caused by either a spark or fuel issue or in some cases, a combination of both. Spark related problems generally will result from things like ignition coils, crankshaft position sensor, spark plugs, spark plug wires or ignition modules not working properly. When the misfire results from a fuel related issue, this is commonly related to a lean fuel condition (lack of sufficient fuel supply to the motor). Most MAG type spark plug wires have a resistance value of 3,000 to 12,000 ohms per foot. The consistency of the readings between each wire is what determines if the spark plug wire is good or bad. Try measuring the length and resistance of each spark plug wire comparing the resistance values. A bad spark plug wire will have much more resistance than another wire of equal length. This is your indication that it may be time to change plug wires.

Fuel related misfires can be caused by many different things such as low fuel pressure, faulty or dirty fuel injectors, a faulty O2 sensor, a dirty or failing mass air flow sensor, a faulty or dirty idle air control valve or a vacuum or intake leak. When the fuel supplied to the combustion chamber is insufficient, this results in an ignition (spark) that is igniting a less than balanced load of fuel and air. This results in a misfire or an explosion in the cylinder that is much less powerful than the other cylinders. This creates a loss of power that resonates throughout the motor additionally causing other problems with ignition and fuel timing. Due to the number of different things that may cause an engine misfire, the quickest and easiest way to diagnose this type of problem is to hook the vehicle up to a scanning tool which will download any potential fault codes that may have registered in the vehicle’s computer as a result of the poor running condition. I would recommend having an expert from YourMechanic come to your location to diagnose your misfiring problem.

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