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Misfiring even with new parts

So my 2008 accent is my daily driver. I’m nearing the 300,000 mile mark but the last 100,000 miles have been plagued with random multiple misfires. I have changed the cables, crankshaft sensor, plugs, and I’ve been through numerous costly coils (I keep 4 with me). All listed parts were most recently changed at the same time. Why do I misfire on nearly every commute and get stuck operating on 3 cylinders?

It may not be an ignition problem at all. At that mileage I’d do a compression test.

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Might want to check base engine

Compression test, vacuum at idle, for example

The misfires might not necessarily be ignition-related

Could be fuel-related, as well

Does it always misfire on the same cylinder?
You could have a bad injector or any of a number of other causes. Constant coil failures happen when some other malfunction is causing high ignition circuit resistance and subsequent overworking of the coil. That “something” can range from poor engine ground to bad compression. Either can add unwanted resistance to the circuit, although poor compression won’t generally show up first as coil failures.

I’d start by checking for stored fault codes and then checking the engine grounds as well as the compression. A check of the alternator and battery is never a bad idea either. If the alternator is pushing out too much voltage, well, that can cause a coil failure.

Post back.

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The alternator is new, the battery is new, new terminal cables, (even the radiator is new just because it was an easy replacement and took my mind off of the misfiring) I had codes checked and it only reads random multiple misfire. It’s not off of the same cylinder for the misfire, but initially I did have one bad coil (cylinder 1). Now that all the coils are good (they’re new and work in any order when switching them around) I still get these misfires. I have found that I can disconnect the negative terminal for ten minutes and connect it back and the CEL disappears and it will run smooth. The most common instance I get with having a misfire is traveling at interstate speed then incurring a rapid deceleration (rubber-neckers ahead of me) followed by acceleration of any kind (slow and steady or rapid). So basically 70ish mph to 35-40ish mph and upon acceleration, misfire and lose a cylinder until I disconnect the negative terminal for ten minutes.

Additionally, when I FILL my gas tank I have to press the accelerator and hold 2k rpm or more for about 30 seconds otherwise it will stall. Related? Does this link in some way to poor compression?


This one has mixed signals. I’m reading symptoms of fuel starvation. Random misfiring on the highway when the ignition system has been thoroughly gone over would point to the fuel delivery. It’s possible that disconnecting the negative terminal for ten minutes resulting in good operation is a “red herring”. It’s possible that the ten minutes is allowing a vacuum in the fuel tank to dissipate, or even allowing “head pressure” from the fuel to allow the fuel line to refill. Disconnecting the negative terminal is probably resetting the ECU, so I’m wouldn’t consider the CEL light going off as an indication that the problem has been solved, albeit temporarily, only that the OBDII system has been reset.

This one’s tough, but at this point and with all the information provided to now, I’d start looking at the fuel system. It’s sounding more like a weak fuel pump at this point.

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Consider what “misfire” means. The computer is saying the crankshaft isn’t accelerating like it should after the spark plug fires. The means the “bang” that should be happening, isn’t for some reason’. Lack of “bang” can be caused by numerous things, for example

  • No or weak spark
  • Spark ok, but at the wrong time
  • Mixture too lean – too much air or not enough gas in the cylinder prior to combustion
  • Mixture too rich – too much gas or not enough air in the cylinder prior to combustion
  • Mixture not compressed properly – piston ring or valve problem.

Your focus on the ignition system may be ignoring something else which upon testing would be obvious, and confusing your diagnosis.

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At 300k miles you really need to check the compression as mentioned. That really should be a must even on a decent running car that is going through a so-called tune-up or maintenance service.

When compression starts dropping that can lead to incomplete combustion and cause the spark plugs to misfire.

Knowing the compression numbers also helps you decide whether to throw money at it or send it to the boneyard.


Because you are not actually diagnosing the problem. You are just throwing parts and money at it hoping you hit it. It’s either time to get a real diagnosis or get rid of a 300k mile Hyundai money pit.

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Thanks for the ideas, I’ll check compression and take a closer look at the fuel system.

Besides compression, good idea to check the valve clearances too, on a 300K engine. You might just need a simple valve adjustment is all. Not sure how difficult a valve clearnance measurement is on your car, but on my Corolla it is like a 30-45 minutes job, very easy. Adjusting the valves if any are out of clearances can be more complicated, but just checking the clearances is usually a pretty quick thing for a diy’er to do.

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I like your style . . . !

Burnt and/or or misadjusted valves could lead to low compression and misfires

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