Thank you Sidemouse and Hatzie. As noted in the original post, I did replace the plugs with the same iridium plugs. Existing plugs were original. I did swap the #3
wire with another and still saw #3
misfire on mode 6 data, only at idle. At 1000rpm or greater it does not misfire. Go figure.
Thanks all and keep the ideas coming.
I just wanted to add, my 2007 does a little stumbling at idle too, it will be running and then like a hiccup... I presume it's a misfire but it's not constant and like you it's only at idle, also my CEL comes on and then goes back off... I have replaced plugs and wires and also the MAF sensor and throttle body, new air filter so if it's in the injectors and so long this is all it does I'm going with the old "if it ain't broke don't fix it."
I have been running tank after tank of premium fuel (93 octane) through it to see if that cleans it up any.
But like I said if that's as bad as it gets I'm not fixing it.
Also for reference I used standard copper core spark plugs gapped to .030
Those will run a good 40,000 miles before needing regapped / replacing.
What I have learned is gap erosion is roughly .005 per 5-10k miles.
I do believe the 'proper' gap for Impalas is .040 but with .030 that's another 10k miles I get out of it before erosion takes its toll.
Granted some folks don't care for that job so Platinums are good for 50k, double plats 75-80k and Iridium's are good for 100k or more.
Gap erosion is strongest with softer metals.
Single platinum still suffers strongest erosion on the soft metal end, thus lasting about 1.5x that of standard plugs.
Double platinum should last 2x as long as standard plugs.
And iridium being the strongest metal, 3-4 times as long.
Those figures are estimates based on my own experience, your mileage may vary.