The short answer is: hell yes! Seriously, @140K miles, if you're gonna keep the car, change 'em out. Carefully.

 

 

A decent plan, but the problem is that the ones that are likely to seize are the ones you can't test - in the rear bank. It runs MUCH hotter back there and is the reason why the PCV valve was less effective than intended, the rear VCG fails first, the PS hoses harden and crack, as do those coil units on cop equipped cars. Bad air flow against the firewall is the source of a lot of extra maintenance on these cars.

OTOH, there was an SAE white paper I read on iridium plug life and some Japanese engineers said they were seeing up to 200k reliably from quality iridium plugs. In general, if I were you I would not worry too much. What I would plan on doing is replacing all of them when the valve covers start to leak, which on these is pretty much any time now given the age and mileage. I would run them until that is needed and both jobs require the intake removal.

As an aside on plugs, currently I am using the Autolite Iridium. I heard they were good from many on-line posts I researched, and I got them virtually free on a clearance and aggressive rebate. For warm fuzzies I'd prefer the NGK, but for virtually free I thought I'd try the Autolite and in 50k-ish so far, I have no complaints.

 

 

some advice here, when taking the plugs out, make sure the motor is warm and spray throttle body or carb cleaner in the spark plug holes and let it soak a few minutes. When you brake the plug loose, tighten it again and loosen then tighten again and hopefully all will go well or helicoils will be in your future.......I hate aluminum heads!

 

 

 

While it can happen, not sure it is common for plugs to break or strip out heads on our ES model. If we were on the Ford Truck older 4.6L & 5.4L forum - that's a different story.... I have changed plugs several times on a 5.4 Triton (never broke one - knock on wood) but I had the Listle tool handy just in case....

Now I have read a fair amount of discussion on the use / non-use of anti seize on the NGK & Denso plugs.

Lastly, I always use a torque wrench to tighten. I split the final torque figure by 2 and make two passes. Example - 12 ft/lb total torgue. Set the wrench to 6 ft/lb - tighten, then set wrench to 12 ft/lb and tighten.

Below is related to Ford Triton 4.6L & 5.4L ONLY......

 

I've been doing it that way for close to 30 years and never had a problem, aluminum heads on vettes, camaros, mustangs and so on..... a old timer GM service tech taught me this technique back in the day and I'll go with a man's experience/training & proven technique that works over something read in a book but, i understand the concern of aluminum metals being malleable when hot/warm and at the same time this helps break free a old spark plug much easier without breaking the spark plug or stripping the threads, I understand you pov but, I know this works otherwise go at it cold and hope for the best? A helicoil will for sure be needed if it doesn't work while attempting cold removal as to at most a thread chaser will be needed on the warmed aluminum spark plug hole. So, go with your gut instinct......good luck.

 

Technically you are supposed to remove the plugs when the engine is cold but I don't think it matters in practice, at least in my experience. I've never had a Denso plug seize up in the head, you can use anti-seize but there is no benefit the plugs are already nickel coated to prevent this issue.