Hey all, first post here.

Shopping in the $5-8k range for early-2000s Excursions to use as a tow/camping/hauling rig.

Most of what I'm seeing has 130-200k miles.

I'm assuming at that mileage if they're running well with no check engine light, they're not on an original set of plugs, right?

Is it safe to assume that by this mileage, they would've already blown a plug and/or had a repair? Like, if they've lasted this long, don't need to worry...?

Is there any way to assess what my risk of expensive head work when I go to look at one in someone's driveway?

Thanks in advance.

 

Short of pulling a plug to look at it there isn't any way to see if they have been changed yet. And even with a change or two (or more) over the years/miles there can always be the risk of a plug blowout on these earlier short threaded motors. The exact cause(s) have been debated for years now with no clear single item to point to all failures.
For a recently purchased rig with some miles on it I would include a plug/boot change (following the proper procedure) along with a fluid change/flush on ALL the fluids front to back, best way to know what you have and establish a baseline.
If you do experience a plug blowout the repair using a Time-Cert (sp?) can be done at home with the proper kit/tools which can be rented.
If this issue is a major concern you could look for an '04/'05 EX which will have the upgraded heads with more threads for the plugs, blowouts on these newer heads is very, very rare.
Stop on over in the EXcursion King of SUVs forum here for a lot more search help. Excursion - King of SUVs - Ford Truck Enthusiasts Forums

 

It's never safe to assume any vehicle maintenance has been done by a previous owner. Oil has probably been changed here and there but at what interval?

If you are serious about using this truck then plugs, oil, diff fluid, trains fluid, coolant, etc should all be serviced before the truck is put into use.

IMO the only thing a CEL will tell you is that the emissions system is in good shape (until its not of course) and that there are no bad coil packs. Plugs CAN last a frighteningly long time. They will nearly always have degraded performance the longer they go without replacement though.

I had a 4.2 F150 with 160k that was seemingly running fine with no CEL pun purchase. I changed the plugs and wires for the reasons stated above and the plugs were decrepit looking. They were Motorcraft plugs with an older part number that were in it too so they were probably original since the average owner and average independent shop won't usually use OEMs. But there s truly no way to tell if they had never been changed. I think that truck must have gained 50 hp the way if performed after that basic service that had been neglected for so long.

 

Even if the plugs are not original how long have they been in there?
First thing you can do is torque the plugs to make sure none are loose.
Or just replace them right away, they're under $3 each on ebay - Motorcraft SP479.
From my reading the spark plugs blow because they work loose.
Change the plugs at 50k and recheck the torque after another 10k.
Our 2002 6.8L had 108k when it blew #3 (Time-Sert). I checked the torque on the driver's side and they were fine. The entire passenger side bank were slightly loose.
When a plug works loose after more than 100k on early 4 thread heads there are only a couple of threads left and it blows.
The later heads with 8 threads can have the plugs loosen 2 full turns and still have 6 threads left to hold it, but then you can hear exhaust blowing past the plug.
The threads don't seal by tightening like pipe threads, the plug has a seat to seal it against the head casting.