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My 2005 Chevy Malibu has a misfire on cylinder 3. The symptoms are:

  • Shaking at idle
  • Knocking sounds as I accelerate, but only under load (sounds fine if I rev it up in neutral)
  • P0303 engine code comes back consistently every time I clear it. Code is for "Cylinder 3 misfire"
  • Acceleration feels sluggish

I have tried to diagnose the issue by ruling out each of the things a cylinder needs in order to fire:

  • Spark

I pulled the spark plug out and cranked the car to check the spark, it made a nice strong blue spark. I then replaced the spark plug, ignition wire, and coil pack anyway, just because I was superstitious, but it made no difference.

  • Fuel

I checked with a multimeter, and the fuel injector is getting electronic pulses. I replaced the fuel injector in case it was clogged, and gas came pouring out of the fuel rail when I did so, so it can't be a fuel issue.

  • Air

I replaced the intake manifold gasket, and I looked down into the intake manifold and saw no obstructions. Anything upstream of that would have to affect all cylinders, so I can rule out air issues.

  • Compression

I checked the compression on the misfiring cylinder, and it held compression perfectly fine.

  • Timing

This one is easy to rule out, timing issues would have to affect all cylinders, and only one cylinder is misfiring.

Nothing I've tried has made any difference. I've ruled out everything that I could think of that would cause a misfire. What am I missing?


Another, related question:

I've driven about 4-5000 miles since it started misfiring (I drive a lot, this poor car gets used and abused), and the knocking sounds have become noticeably worse. Is it possible I've caused any sort of damage to the engine by driving with the misfire, or is it something I could safely ignore for another 10,000-ish miles? It's a V6, so it's still got 5/6 cylinders.

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  • regarding related question - you will destroy a catalytic converter driving a car that is misfiring. Mar 11 at 23:00
  • @BillSullivan Right, but that's just exhaust, right? Worst case I could just cut the cat off, right? Mar 12 at 3:13
  • I would do a mode 6 monitor. You may have a pcm that is flagging the wrong cylinder for a misfire, and a compression test all cylinders. Mar 12 at 9:28
  • @BillSullivan Thank you for the suggestion. All cylinders held compression, and I looked into a mode 6 monitor, but I think it would be above my knowledge level. I was going to take it to a shop and have them do the mode 6 monitor and a leak-down test of all cylinders, but I sort of rethought things and decided it's time for me to just get a new car. It has made it another 3000 miles since I posted the question, though, and it's still running. No idea what the cat looks like though. Mar 24 at 0:42

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