The 2005 Nissan Xterra is the second generation of its ilk. Unlike the previous generation and its numerous power-plant options, all second-generation Xterras left the factory with Nissan's excellent VQ-series V-6. The VQ -- an evolution of the VE and VG -- has seen use in practically every car Nissan makes, most notably the Nissan Skyline, 370Z and the JGTC Skyline racecar. Plug replacement is fairly easy as new engines go, so you've got no excuse not to change them every 105,000 miles the way Nissan recommends.
Remove the engine cover from the top of your intake manifold by removing the bolts that secure it. Remove the top from your air cleaner, remove the filter and then remove the bolts that secure the box to the chassis. Remove the air cleaner box and the air tube that connects it to the engine.
Identify the ignition coils; you'll find them on the left and right valve covers. Start at cylinder No. 1, the front-most cylinder on the driver's side valve cover. Unplug the wiring harness from the ignition coil, then remove the single 7 mm bolt that secures the ignition coil to the valve cover. Very gently pull the coil upward, using a slight twisting motion to break it free of the spark plug beneath.
Look into the plug "well" in the valve cover and find the top of the spark plug. Blow a burst of compressed air into the plug well -- you don't want dust, dirt and dead bugs falling into your engine when you remove the plug. Remove the spark plug with a 16 mm spark plug socket on an extension.
Select the spark plug type. Nissan recommends NGK spark plug PLFR5A-11 as the standard replacement, but you may want to consider using a "colder" range plug if you run a lot of miles on the interstate or if you regularly operate the engine at high rpm. Use a "hotter" plug if you frequently turn the engine on and off or live in the arctic circle. The hot-range plug is part number PLFR4A-11, and the colder one is PLFR6A-11.
Check the plug gap on the new plug and re-gap as necessary, using a gapping tool. Nissan wants a 0.043-inch gap, but don't count on your plug having the right gap out of the box. These things get banged around a lot during shipping, and the gap could easily be off by 1/10 inch or more.
Plug the end of the spark plug into the spark plug socket. Lower the new plug down into the plug well, using the extension only, and thread the plug by hand into the cylinder head. Back the plug off slightly if it jams after less than a full rotation.
Torque the spark plug to between 20 and 22 foot-pounds with a torque wrench. Smear some dielectric grease on the inside of the coil's spark plug boot to ease future maintenance, and push the coil down until it snaps onto the plug. Tighten the 7 mm coil bolt hand-tight. Plug the harness back in and repeat the procedure five more times until you've replaced all six plugs. Reinstall the engine cover and air box assembly.
