A year and a half into ownership of Ford 500 Limited with AWD and CVT tranny; acquired it at 210,000 miles.
Original owner had maintained brakes, coolant, engine oil and not much else; not spark plugs.
So, now spark plugs ...

Following Hanky's advice - thank you, Hanky.
If I was going to inspect plugs, I might as well replace plugs, so got NGK Iridiums from NAPA store in town.
First plug I'd ever seen without a crush washer, BTW; just a 45 degree bevel.
Ford spec for gap is 0.052 - 0.056," these NGK were 0.055 out of the box, so right on.

Took out the front plugs - easy! Surprised that the plug tubes were spotless,
unlike Hondas and Nissans I've occasionally done over the years.
OEM Motorcraft Platinum plug at 231,500 miles


and

Original Motorcraft Platinum plugs showed 0.068" gap after 231,500 miles vs. 0.054" when new.
Have to admit, they were still running okay.


The old and a new here. I put copper antiseize on the new NGK, so the brown color ...


Only replaced the front 3 with the NGK Iridiums, because the back 3 are hard to access;
don't know that I'll do those until I'm back there for some other mechanical reason.
Fired it up and engine caught immediately - seemed to catch quicker than before.

... wanted to share that on our car, Ford V-6 three liter, the original platinum plugs last a lot of miles.

 

Nice to know. Thanks. Mine has 115K miles so I guess I’m safe.

 

Hey I would replace the plugs back their because this will put undue strain on the crankshaft bearings then you will have an off balance motor vibration.
easy to access all you need to do is remove the intake to gain access to the coils.

 

Well, at 242,500 miles now replaced the rear bank of plugs (same NGK plugs). Yeah, a year later ...
Driving it in busy town traffic today it seems to have better pick-up - so success, I'm happy!!

Thanks for your comment, the1driver, somehow it works better with all new same plugs but I didn't detect any motor vibration

 

Surprised the coils still fire the plugs. Nothing will kill the ignition coils faster than worn out plugs.At 100k, it's time to get the plugs changed.

 

I haven't seen one of the tools like this in ages, don't know if any are even available now, but you would install the plug to test in it, apply pressure and firing voltage as compression in a cylinder and you would not believe how a plug that looked so good would not fire correctly under compression. So what I am attempting to tell you is , no matter how good the plug looks , it can still not perform as good as new.
Grant you today's plugs are much better than some past spark plugs, but don't be misled by appearance alone.

Wonder about this,,,,,,,,, manufacturer A says plugs don't require replacement before 100,000 miles then not to be made to look bad manufacturer B says plugs in our vehicles can go 105,000 miles and so on. The downgrade in performance is so gradual that the average driver doesn't notice the decline and thinks I only have 90,000 miles on my plugs, so I'm good for another 10,000 miles. If new plugs are installed , there usually is a noticeaible difference in performance.

It is a good idea to learn what the appearance of the plugs tells you. They can tell you if the engine is running rich, has coolant leaks, running lean lots of info there.
In the case of a "No start", they can tell you if the engine is flooded or not getting any fuel at all.

A little side note, when a vehicle quits running and will not restart , if the plugs are wet, we try to see if the wetness will burn. If not there is a lot of water in the fuel.