I will start by saying I am the most novice car guy. The extent of my experience is changing oil and a basic 2" suspension lift. With that said, I do love to tinker. I have a 2003 Jeep Grand Cherokee (WJ). 4.0L I6.
This project started because I kept getting a P0306 fault code, which was a misfire in cylinder #6. After talking to friends and scanning jeepforum I first put some Chevron fuel injector cleaner into my tank and second decided that changing out my spark plugs couldn't hurt and would hopefully be the culprit. Thirdly, I couldn't find any thread on jeepforum that included step by step (with pictures) instructions on how to do the change-out. wjjeeps.com has really good instructions here http://wjjeeps.com/service/sparkplugs.htm
But what I have benefitted much from in the past is someone's documented instructions (it also helps to know that someone else had the same difficulties with something I did...makes me feel less of a dummy).
So here it is:
1. You need the right supplies. Go to the autostore and pick up a small tube of spark plug boot grease and anti-seize thread lubricant. Also you will need a 3/8" drive 6" socket wrench extension and a 5/8" spark plug socket wrench attachment. I didn't realize how vital an actual plug socket wrench is until I realized that inside it is a rubber ring that holds onto the plug as you pull it out. This was HUGE! Finally, I got 6 Champion Platinum Power Spark Plugs (#3034/RC12PEC5).
2. Unattached the negative (black) battery terminal. This will take a 13mm wrench. I like to tuck the terminal around the side of the battery to ensure it doesn't flip up and inadvertently touch the battery and kill me.
3. In order to access the spark plugs I had to remove the coil rail assembly. On the link above to wjjeeps.com there is a great illustration of it. Below is the pic of the actual thing. Before you do anything there are 3 small cords (or hoses) that lay across the rail assembly that you should unclip, followed by 2 larger hoses that run parallel to the assembly that can unclip as well. Move these aside carefully so you don't pull anything apart. It was vital that these get moved so you have room to maneuver. FYI... things get real tricky on the bolt and plug on the innermost side of the assembly (nearest console).
4. Take your 13mm socket wrench and remove the 4 coil mounting bolts. Again the furthest bolt from the front will be the hardest and you will swear the most whenever working in this area of the engine. There is another plastic bracket at the rear here that you can simply pull it up and off the bolts that it is set on; move this towards the left as far as you can.
5. Next you will need to remove the coil connector from the coil assembly rail. This took me at least 20 minutes of complete frustration to disconnect. There is a red slide clip on the back of it that will need to be slid to the opposite side (right). This will take some maneuvering of the assembly as much as possible to access the connector enough to unclip it. Once the red slide tab is slid over (you'll know you did it when it "clicks"). Next push in the second release lock, located below the slide tab. Push in as hard as you can and the connector should slide off. Be patient, it took me forever but I finally got it. This was by far the hardest part of the project.
6. Once the assembly is disconnected pull it out of the car and the spark plugs will be revealed. Take your socket wrench with the 6" extension and plug wrench attachment and begin to remove each of the old spark plugs. Turn hard. As I removed each plug, I marked each one with the number cylinder they came from so I could further check out the #6 misfire my system was reporting. It turns out the #6 (closest to the console) had very little left on the spark part (sorry I don't know the actual term) but compared to the others it looked quite worn.
7. I then wiped a small amount of the anti-seize lubricant on each of the new plugs' threads with my finger. Next I then screwed the new ones into the plug holes. I don't have a torque-wrench so once the plug was tight I turned it an additional 1/4 turn. After all plugs were installed I took the coil rail assembly and lubricated each plug boot with the boot grease. I used some q-tips for this and wiped each boot down thoroughly.
8. Next I reconnected the coil assembly back to the coil connector and pushed the slide tab back to the left (opposite to lock) and fit each boot onto each of the spark plugs and set it back over each bolt hole. Using the 13mm socket wrench I bolted the coil rail assembly back and re-clipped the hoses and cords back to their appropriate clip and cleaned up.
The jeep started great and the check engine light was off, so that's a good sign. I hope these pics and instructions help someone out there. It felt good to take care of my jeep today and hopefully this will help my mpgs and life of the truck.