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Q: Idle issue

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Hey just had a quick question I have a 2002 Honda civic automatic 1.7L and so I had a question earlier regarding that when I turn it on it would kind of rumble so I seen a hairline crack in the exhaust manifold which i got patches up pretty nice but then it still was the same so I let the car idle and pulled out each ignition coil one by one and there's 4 altogether and so I pulled 3/4 and the Rpm went down as.it should right? And then one of them when I pulled out it didn't.make a difference in the Rpm so then I switched two ignition coil and the problem stayed in the same cylinder could that mean I have a bad spark plug that needs to be replaced?

My car has 290000 miles.
My car has an automatic transmission.

Cylinder misfiring can be caused by: Ignition coil, spark plug, fuel injector, or a wiring issue to the coil or injector. By moving the coil from the misfiring cylinder to another and the misfire staying with the original cylinder, you have basically eliminated the ignition coil. You can try the same thing with the spark plug before buying new ones. If again the misfire stays with the same cylinder, then you have a faulty injector or signal/wiring issue to that coil or injector. Also, mechanical damage with that cylinder such as a damaged valve or valve seat can also cause a misfire. If you need further assistance with your misfiring issue, a certified technician from YourMechanic would be pleased to come out and help you figure out what is going on.

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