I don't have alot of experience with cars, but I used to change my own spark plugs on my previous car. On this car, I can't seem to find where the plugs are. On the top of the engine, there is four connectors screwed on to the engine cover. The wires that runs to those connectors are very thin. Could those be the plugs? Also in front of the engine there is another set of four wires that runs from somewhere on the left side of the engine to somewhere on the bottom of the engine. I don't believe these are the ones, but that's all I see. Can someone with more experience help? By the way, if there is any special procedure I should follow to prevent damages to the car while changing the plugs, please advice me. Thank you very much all!
The QG18 has individual coil packs. Undo those four connectors, unscrew the coil pack from the valve cover, and the spark plugs will be underneath the coil packs (still installed). No special procedures required.
Make sure you start the plug by hand so you don't cross thread it. After it's started by hand then use your ratchet to tighten it down. It's aluminum, so don't get crazy tightening it down. Get a feel for how tight they were when you removed them.
^^^Not common but it does happen. We try to soak the seized plug with some penetrant, like PB Blaster, overnight and pray for the best!
If you try forcing the plug loose, it might break! Now the head has to be removed...
There are little rings that slide up and down on the threaded part of the spark plug. They are seals, so if they slip off, put them on the way it came off.
Be sure the Gap is correct. It should say on the box... (.040) or something like that. Use a gap gauge or a set of feeler gages to be sure this is correct. If the gap is .040 you want the .040 gap gauge to drag a little bit, not so much it has to be forced in, but just a little drag.
Before you pull the plug you can blow out any trash/dirt around the spark plug so it doesn't fall into the engine - If you have an air compressor.
You are normally better off using OEM Plugs. For Nissan this should mean NGK.
Anti-Seize is your friend... It will help the plug from seizing in the head like mentioned above. Put a little on the threads and a little on the seal area. High temp (usually a copper color) works better for plugs. See the link below..
One more thing, dielectric grease is your friend. A small tube costs a buck at your local auto parts store. The link above demonstrates.
Change one plug at a time. This way you don't get you ignition timing mixed up. With most modern cars this isn't an issue, they're numbered or the wires are cut to the correct length per cylinder.
If you torque the plugs down when installing, most likely the spec is somewhere between 10-15 ft/lbs for aluminum. You can pick up a Haynes manual for $15 or so to be sure.
The posted link is a good walkthrough.