I pulled these out of the 05 TL I bought a couple months ago. It's barely been driven since I bought it, but I noticed the car had a slightly rough idle when it first started. I have no clue how old these plugs are. I'm assuming they were changed at some time in the past, but who knows. The timing belt was done about 70k miles ago and these plugs look to be older than that. Any guesses how old these might be?

At the same time I changed out whatever junk oil was in the car with Mobile 1 full synthetic and changed the garbage Valvoline oil filter for a Fram Ultra. I changed the transmission oil filter, filter spring, filter housing o-ring, as well as the 3rd and 4th gear transmission pressure switches. The transmission shift so much smoother now!

Maybe it's just me, but those plugs look fairly new; I'd guess no more than twenty to thirty thousand miles.

to me, the shiny steel screams brand new

The electrode and tip look pretty good. But the detective side of me guesses 150K or more. The brown ring on the insulator is pretty thick. That ring gets larger and darker as the plug ages. Mine were about half that large/dark @ 100K miles.

^^^^ You can't tell plug age from the corona. Those plug tips look fine to me. As long as the gap is within spec, they should still be fine.

These Iridium plugs are extremely long lived. I've change plugs twice now at the recommended 105K interval. Both times all the old plugs still had factory gap and I could tell zero difference with the new plugs.

On the other hand, you can buy the plugs on sale for ~$50, so $50 for peace of mind is OK with me.

Did you use anti-seize when you installed the new plugs? What torque spec did you use?

The TL has 178K miles and I've pulled out spark plugs out of cars with 80-100K and the insulator looked way better than these. All 6 were like this with virtually no variance in the color or size of the brown ring. I've never seen NGK plugs that look like these. I ran NGK's on my 3000GT VR4's and Stealth R/T Turboss with upgraded turbos/fuel and I would assume they took a lot more heat and abuse than what the TL is capable of dishing out. That's really why I was throwing it out there to see what everyone else thought. When pulled each one out and they all looked like this I really wondered if they had ever been changed. It seems like a stretch, but I really don't know. Each end of the plug would seem to tell a different story, but I just can't stop looking at all of that brown!

That brown is the corona and it's perfectly normal. The corona is not created by heat. Again, you can't judge plug age by the corona.

Yes, and I torqued them to exactly 13ft/lbs. The car seems to idle a bit more smoothly and it's more noticeable on a cold start. The new plugs are identical with same part number.

So the plugs have 178K on them if I understand correctly? That's great they went that long. Like nfn said, at around $50, I change mine more often just for kicks - probably every 60K or so. I'm not on a schedule with them though.

The corona (had no idea it was called that) might not be a true and fast indicator. But in my experience, the older the plug, the higher and darker it seems to get on the insulator. There does seem to be a correlation. But I'm sure there are exceptions.

I really have no idea how many miles are on them, but I've never seen plugs come out of any car I've ever owned that looked like that. That didn't look very good to me!

I bought the plugs after I purchased the car a few months ago. We just drive the TL locally (short trips) and I've been busy with work/other projects so it took a bit to get to this. The car didn't have any drivability issues so it went on the back burner, but I'm glad I did it.

I'll bet dollars to donuts those plugs have no more than 30,000 miles on them; ain't no freakin' way they've seen 178,000 miles.

Hell, those things probably look better than the ones in my car.....surely they can't be that old.

Either way, good on you for taking the proper steps to take care of your transmission.