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Some folks say its easier to remove the front wheels & inner fender splash shield & go at the plugs that way on the ohc 4.0L.
The specified finewire iridium enhanced center electrode with platinum pad ground/sidewire plug design is recommended, along with Motorcraft plug wires, as they're of good quality & designed to take the double work load & limit spark gap erosion our waste spark ignition system puts on them.
Be sure to check plug gap with a wire gauge, but ease up on spark gap & don't forcefully pull the gauge through the gap, so as not to damage the finewire center electrode, or sidewire platinum pad. Properly gapped these plugs will spark at a lower voltage, which helps on cold starts. I get noticeably faster year round starts with them.
Seeing as how spark gap is always opening up, consider setting plug gap at the min side of spec, that way spark gap will stay in spec longer & the closer gap will aid cold starts some & advance spark timing a bit & all that can help on really cold start mornings, when the battery voltage is at its lowest & is struggeling to run the starter to crank the engine, run the fuel pump & make a hot spark to get us going.
I also use a dab of high temp nickel Permatex anti-sieze on the plug threads & torque them to the low side of spec, to allow for the anti-sieze lube effect & not over tighten them.
I also butter on some Permatex silicone grease inside the plug & coil pack wire boots & onto the plugs external creamic insulator, to help prevent flashover on those wet dewey morning starts, or wet rainy days & prevent the boots from sticking/baking onto the plugs ceramics over time.
Some thoughts for consideration, let us know how it goes.
99 Ranger XLT 4dr 4x2 4.0L 5spd Auto 3.55L/S Payload Pkg2 Tow Pkg
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Besides what others have said, set aside an excess amount of time. The plugs in the 4.0L can be a real PITA to get at, especially the right-rear where the A/C - heater stuff is located. Expect to need tools you don't have, and to come away with bloody hands, getting into tight spaces. A tool to extract the plug cap from the plugs is a necessity. When you choose the tool, keep in mind that several of the plugs won't have room to get a straight pull on the cap.
You'll finish the experience with an extra appreciation for the 100,000 mile change interval. Other than that, no problem!
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