I have 42,000 miles on my 97 4.6 truck with 3.08 diffrential. At what point will the engine start losing hp and fuel mileage resulting from worn sparkplugs. I don't believe that 100,000 advertising unless the test was on highway only driven vehicle. If these plugs were a little easier to replace I would go ahead and replace them regardless. They sure look tough to get to.

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97 4.6 F150, 1/8 = 10.885 @ 64.10 mph, 88 5.0 Mustang 1/8 = 7.409 @ 93.72 mph

 

The plugs are going to be really fun to replace for sure. A co-worker of mine has 64,000 miles on his 97 4.6 F-150 and it still has a ton of power.
I would not let the plugs go until 100,000 miles either..those are "ideal" and "perfect" driving conditions.
Pull one or two out out and check them..see how they are holding up.
Good luck!

 

Dustoff,

I tried to take one plug out for inspection. There is only one plug with easy access and I pulled so hard on the wire housing that I was afraid it would break if I pulled any harder. Maybe you have to replace the wires at the same time. From what I see here, I will drive my truck anothere 20,000 miles and trade for something a little easier to work on. Ford really made this one tough.

 

Sorry to hear that LarryS...
I have the 5.4L and there are no spark plug wires. I have also noticed what a joy it will be one day to reach some of the plugs!

FORD sure did cram this engine under the hood!

Best of luck!

 

Don't you mean uder the DASH ?

 

FOR SURE! More like under the center arm rest!!!

hahahaha

 

That's one of my main bitiches with ford trucks. In their infinate corperate wisdom they decided that truck owners should be treated just like car owners and they don't need to be doing any of there own general maintance. That and there soft shift transmission along with there POS music system that can't be upgraded but only replaced. THey forgot that some people use there trucks for work and don't have the time to put it into the shop for every little thing!!

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Nastruck1-4.6-377 rears-Superchip-K&N GenII-Dynomax RaceMag With Roush tips-MPC bug shield-40-60 black tint windows-bed liner-class3 drawbar hitch-polk speakers.

 

I wanted my plugs changed so I went out and bought new ones. Husband did the changing. I stuck around for about half of the plugs coming out but when he got to those that are cramed under the dash his language seriously deterated. (he's a former Marine and has a very colorful use of English) Our marriage required me to get away from him for a while Point is, changing those plugs are a real pain, literally! Quite a few of his knuckles had the meat scraped off. Two of my old plugs were showing signs of being bad. I had about 43K on the old plugs. With the old plugs, the truck was running fine. No missing or anything. It did run "smoother" with the new plugs. So I think if I had left the old plugs, that it would have started messing up pretty soon.

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97F150 XLT SC 4x4 Flairside 4.6 V8 5sp Dk Toreador Red, 3.55LS
ARE tonneau cover, p255/70rx16 tires (came w/235), Superchip
Pioneer CD w/4 Polk Speakers & Clarion APA5240amp, skid plates
bed liner, side window deflectors, reese hitch, K&N air filter
40s-2chamber Flowmaster, Bosch +4 spark plugs, rustproofing
Better Built Stainless nerf bars