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Mystery Persistent Misfire

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I was given a 2002 automatic with 160,000 miles that's basically running on 4 cylinders at anything less than 2000-3000rpm. Its throwing codes P0305 P0302 P0316 and P0171, so cylinders 2 and 5 seem to be the issue, but I haven't been able to figure out why. The plugs on cylinder 2 and 5 looked pretty dry and are characteristic of a lean condition, but the electrodes and plug wires looked good and had decent spark. So I was thinking fuel injectors, but both injectors tested fine and after swapping them with a couple used but known good ones there was no change. I also checked the coil pack with a multimeter and all the resistance values were within spec. Also checked compression and both cylinders were good. I'm pretty much stuck at this point and not sure what else to check, any ideas? Could a vacuum leak cause a lean condition in a specific cylinder like that? Any other sensors worth investigating? Could it possibly be a timing belt issue and the valve timing not being correct? The previous owner said they had took it to a shop and they said she needed a new pcm, but I haven't found any reports of a bad pcm causing these issues and the car seems to function fine otherwise. Any help/suggestions are appreciated.

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