thesavo
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My plugs in mariner looked done after 100k miles. 270k miles must be dark
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the big risk with misfires is deep-six'ing the CATsI bought my 2005 with 99,000 miles on it. One reason it was traded in was due to needing plugs and some other maintenance. It ran fine on the test drive, but would spark knock and buck just a little (hardly noticeable, but I'm a decent mechanic and was paying close attention to everything)on deceleration -- acceleration and pulling was fine. The CEL was on, so I had the dealer pull the codes before buying -- "multiple misfires". So I knew it was the plugs from prior experience with my brother's and nephew's 5.4L Fords (F-150s). Got a good deal on it, and the plugs fixed it. Mine had seen light duty over the last five years according to the Car Fax report -- about 5K a year. I don't put any more than that on it myself.
Change the plugs now or wait until the 100K mark or you start getting the multiple misfires code. Could be a while depending on how much you drive. If one cylinder (plug) starts to go you'll get a misfire code for that cylinder -- but that could be a coil as well. When the plugs are changed make sure the part that goes between the coil and plug is changed also. That usually goes and you will get a bad coil code. Change those and the coils will last longer -- though I don't know if it's that part or the coil at that point. Could be the bad "link" (I forget the correct name) cause the coil to go bad due to higher resistance.
By all means check hoses and belts! I had to change the serpentine belt and when the tranny went shortly after I bought it (just a common failure, nothing I did) the shop that put the rebuilt tranny in changed the cooler hoses due to age and to cover warranty on trans.
I just want to see whats plugs after 270k look like. I'm not even sure how there is an electrode left. I mean they wear out just like a pair of socks. I'd bet ford would like to see them too. Doubt there are many quarter of a million mile plugs in service.
My first 02 4.6 2v blew twice. Put a heli-coil on after and never had a problem since.In short, better to change the plugs if only to catch something junking up or loosing up. The coils are a breeze compared to the plugs.