Yes, you can change the spark plugs in your Jeep. If you are a novice to mechanical work, this job will be very intimidating, but if you walk through it step by step, you can do it and get it right on the first try. It isn't like trying to rebuild an engine. Just uncover the spark plugs, unscrew the old ones, screw in the new ones, and put back the stuff you removed. Let's roll!

What Are Spark Plugs?

Your engine burns gasoline in order to generate power. It sucks in air through the air intake, and it sucks in gasoline through the fuel line from the gas tank. It puts both of these into a hollowed out cylinder in the engine. A piston comes up and squeezes the air and gasoline mixture, compressing it. Then, at just the right moment, the computer tells the spark plug, which is sticking into the cylinder, to fire a spark. When it does, this ignites the gasoline and air, and the mixture explodes. This pushes the piston down. The piston is connected to a big bar called a crank shaft, so when it is pushed down, it makes the shaft turn. This turning is sent out the back of the engine to the transmission, and from there to the transfer case, and from there to the drive shafts.

Spark plugs are the things that stick in the cylinders which fire a spark to burn gasoline.

Why Do I Need to Change My Spark Plugs?

Because as they are used, the metal ends that the spark jumps from are burned. The more they are used, the more this area burns, until finally it no longer makes a good spark for the engine to run smoothly. Sometimes it sparks, and sometimes it pukes out. So, every 30,000 miles, it's a great idea to pull those things out and put in new ones.

Tuning Up Your Jeep

Changing spark plugs is commonly referred to as a tune-up. It used to be that this would involve much more, such as setting timing, setting points, replacing spark plugs, ignition cables, and dorking around with a distributor cap. In my 2004 Jeep Wrangler with a 4.0L engine, everything is run by a computer. So, to tune the thing up, you just need a couple of hours at the most to switch out the spark plugs.

Or you can pay a Jeep stealership's service department to do it for you. But they will charge you considerable cash for the job. Now that I have your attention, consider that spark plugs cost about $20 for a complete replacement, and the tools to pull them out and put in new are a one time purchase of around $35 for the best.

This is no harder than changing your oil. Really. It isn't.

Bolt Size & Torque Specifications

Bolt	Size	Torque
Coil Rail Mount Bolts	13mm	21 ft-lb
Spark Plugs	5/8"	22 ft-lb

Let's Change Some Plugs!

The spark plug coil rail in a 4.0L Wrangler engine from 2004 is sitting on the engine block. Since the engine is an in-line six cylinder model, all of the cylinders are in a straight line. That means that so are the spark plugs. So, there is one line of spark plugs on the passenger side of the engine block. They are covered up by this rail.

As you can see in the picture, the rail is inconveniently located under the air intake tube. You will need a screw driver to loosen the air tube connector from the air box so you can swing this tube out of your way. It is marked in yellow in the picture.

Once you get that out of your way, you can locate the screws that hold that rail onto the engine block, and remove them, one at a time, and place them in a safe location all together where you won't lose them. Without those screws, this engine will never start, so don't lose them!

Here you can see the screw locations along the rail. These are hex bolts, really, and they require a 13mm socket a ratchet wrench to undo them, or you can use a 13mm open or close ended wrench. Putting them back on later, you will need a socket attached to a torque wrench.

The coil rail has, of course, a bunch of cables coming out of the end toward the windshield end of the Jeep, and just to confuse you, Jeep has perhaps put a zip tie on these cables so close to the rail that after you unscrew it, you will not be able to move it around. You'll need a knife or scissors to cut the thing off. It is unnecessary. There should be another one just a few inches farther down that will hold things together. If not, pick up some zip ties for a buck and put on a new one when you are done.

When the coil rail is off, you will see the spark plugs sticking up out of the cylinder heads. You basically put a socket over them and unscrew them. Then you screw in new plugs, and close her back up! More details later in the step by step.

Tools You Will Need

Torque Wrench. You will need a torque wrench to put the spark plugs in. They should be torqued to 22 ft-lbs each, no more, and no less. The coil rail bolts you can put on with any wrench with a short handle and tighten them until they are on good without trying to weld them to the engine by over-tightening.

My long handled 70 lb torque wrench pictured above is crummy for this job. The handle is nice a long, which makes it easier to turn the plugs, but it is so long it hits the hood and makes it hard to turn the spark plug in the very back. Get a short-handled torque wrench for this job - very short. You only need 22 ft-lbs. Don't use one that's cheap with a meter needle. Get one with a turning handle that measures exactly the torque you want.

Ratchet Wrench. You always need this to put a socket on and remove a bolt. Pictured here is my grandfather's 60 year old Snap-On wrench. There's nothing like doing mechanical work with a master mechanic's heirloom tools.

13mm Socket. This will help you remove the coil rail hex bolts that hold it in place.

Flat head screwdriver. For undoing the turnbuckle on the air intake tube.

5/8" Spark Plug Socket. A regular socket is no good for this job. You need a socket that has a rubber or foam insert inside of it. This stablizes the spark plug and prevents it from breaking. One came with my cheapo set of tools from Walmart. But Sears sells a spark plug socket with a wobbly joint in it, meaning you can turn it from any angle. Oh yes, do get the one from Sears. Craftsman tools are schweet! And the wobble joint let's you work around wrenches with long handles and get into places that are otherwise difficult to reach.

Extensions. When I am removing a bolt, usually I use the biggest ass breaker bar I can find so that it will be easy to turn without having my hand down in the engine where it will get cut when the bolt suddenly releases its grip. Sometimes I only use a 3/8" size one, sometimes it needs a 1/2". Get some extenders for your wrench to help you reach down in between hoses and such.

Gap tester. You will need a gap tester to check the gap on your spark plugs. If the gap is too big or too small, the engine will run horribly when you are finished.

Wait! Don't stop reading just because I told you to go buy some tools! These tools might cost a few bucks now, but you can use and reuse them for decades to come. You only buy them once. My tool set is 90% from my grandfather, who has passed on, and my father, who gave me a few things to start me off. Tools are a premium investment - so inexpensive, and usually still cheaper than the service at a stealership. Buy the tools.

Anti-Seize, Di-electric Grease, and Spark Plugs. You will need all three of these from any Autozone store in your area. $20 total.

Champion Truck Plugs. The plugs are Champion truck plugs. Yep, that's right, these are the best plugs that money can buy for a 4.0L Jeep Wrangler. They are pre-gapped to .035 for you. Just screw them in. I tried some that met the FSM's suggested .040 NGK plugs, and the engine ran rough. I also tried some single platinum plugs, and the engine ran rough with them too. Following the advice of lots of folks on JeepForum.com, I too am convinced that these plugs are the best you can get. They work great. I took the picture so you could see the number for the plugs - 4412.

By the way, get 8 plugs, because you will break some. "They break?" Yes, they do. Behold!

It snapped clean in half while I was screwing it in, because I let the socket lean to far one way while torquing it in.

Anti-Seize. This is goo that you rub onto the threads on the plug that make them easier to remove later. Use it. Only one little bag does a whole engine full of plugs. Cover the threads thoroughly, top to bottom, but don't get anywhere else.

Di-Electric Grease. This goes on the end of the plug that sticks out to help its connection inside the boot on the coil rail. I just squirt it into the boots instead of putting it on the plugs, because a greasy plug is difficult to insert, and when they are already installed, there is a small charge in them from the battery.

Compressed Air. A can of compressed air is useful before removing a plug to get any sand, ceramic shards, or anything else away from the hole that will be left. If anything gets in there, your engine will use it to try and kill itself. Keep it clean, and spray it off with air before you work.

Step by Step Guide

Here we go! Hopefully your engine isn't blazing hot right now, because working over a hot stove really is a lot harder and tends to result in burns.

1. Loosen the screw on the silver band around the air tube connection to the air box and remove the tube.

2. Move the air tube out of your way so you can reach the bolts holding in the coil rail.

3. Remove the four bolts holding in the coil rail and store them away somewhere safe.

4. Remove that annoying zip tie I mentioned before if it looks like it will give you trouble.

5. Pull the coil rail off of the spark plugs. The rail has rubber boots on the other side, tubes that go over the ends of the spark plugs to give them electricity from the battery. Try to move the coil rail out of your way without screwing up the cables or sitting it on the battery. Don't touch the battery poles or any red wire ends under the hood.

The spark plugs lie now naked before you, exposed and helpless. They are the white stalks sticking up out of the engine block. This isn't that hard, is it? So far, it's just been some work to push around wires and hoses to get at the screws and some physical labor in pushing around that annoying coil rail. The first time you do it, you are a little scared you will break something, so it goes slower, and there is a higher stress level involved. Relax. You are doing fine. Tell yourself positive things as you make your Jeep bow to your will!

6. Spray compressed air on the spark plugs to get all of the junk away from them so it doesn't fall in the engine. That would be bad. Very bad.

7. Place the spark plug socket over a spark plug, put an extender in it, hook up your breaker bar, and start turning it out of there. Keep the socket seated all the way down to the engine block or the plug will just break and get ceramic everywhere. Keep the extender and the socket straight up and down in line with the angle of the spark plug while turning, also.

8. Pull out the spark plug. Ahh, yes, you can do this after all!

9. Pick up a new spark plug. Don't worry, you bought two spares like I suggested, and your Jeep won't be dead in the water if you break one, because you aren't an idiot who didn't listen to me. Right? You aren't, are you? You bought 8 of them, not just six, only to save $4.00. I know you didn't. Check the gap on the spark plug with the gap tester, and adjust if necessary. If you use Champion 4412, it probably won't be necessary, but check anyway, just to be safe.

10. Spread anti-seize on the threads. Just enough to cover them good. Even if your plugs say they are pre-coated, do this step anyway, or you'll be cussing some serious profanity when you try to get them out in 30,000 miles.

11. Screw in the spark plug by hand. Be careful not to force it to go in unnaturally, or you will end up cross-threading your engine threads, and then you won't be able to get anything to screw into them again until a professional fixes them for you... or you get a new engine. Be careful, thread them in correctly and straight.

12. Torque the spark plug down to 22 ft-lbs.

The ones in back are a bit of a pain to pull out because of the angles and the wires and things in the way. It takes a little struggle, but you can get them. Even I did it. Since you put on anti-seize, you could torque them less, because the anti-seize artificially increases the depth they screw in at the same torque.

13. Repeat this process for the next five spark plugs. Spray air to clean around the plug, pull it out, grease the threads on one, and stick it in. You will have to get creative as you work your way back to the last one using extenders and wobble sockets to pull them out.

14. Squirt a little di-electic grease into each of the six spark plug boots on the coil rail, and put a little on the spark plug ends after they are all in.

15. Stick the coil rail back on, being sure that all six plug ends go into all six boots.

16. Bolt the rail back on, and tighten the bolts a bit with a short handled ratchet wrench. not too much - just enough to hold them.

17. Reattach your air tube and tighten down the screw with the flathead screwdriver.

18. Start your engine.

And you are all done! It is really that easy! You've just tuned up a 4.0L engine on a 2004 Jeep Wrangler! Way to go!

Ruh-Roh!

This is the part of the write-up where I consider some possible mistakes you could make to help you diagnose problems you have after you are done.

Engine won't start. One of the plugs is bad, or perhaps you left one out. You broke something? Hard to say. Before you begin any job like this, be completely done with the last job, so you can isolate the problem to what you are working on now. So, make sure everything works on your Jeep before you begin this job. Then, if the engine won't start, you know for sure it is something that you did with your spark plugs. You can try replacing one of them, or all of them. You can also check the gaps on them with a gap tester, but Champion 4412 truck plugs are pre-gapped, so you should be OK there.

Junk Went into the Engine Block. So, let's say you pull out the old plug, and just that second a bee flies along and flies straight into that hole and down into the engine block where it burns to death and stays. To be honest, I don't know what you can do about something like that. You could try really long, narrow tweezers and hope you get him out. But if something like shards of ceramic goes down in there, you might want to consider not starting your Jeep, and instead having it towed to a mechanic to have them clean it out. This write up assumes you are like me - new to mechanical work - and unable to contemplate disassembling the cylinder heads to clear out something like that. I would not want my engine to run with anything in the cylinder.

I broke a spark plug. The one is easy. Blow the ceramic shards away, remove the broken plug, and replace it with a new one. You always buy 8 or more plugs, because you are going to break one sometime, and you don't want to be stranded in your driveway unable to drive to Autozone for another one. Plugs are cheap, so get extra.

My engine is running rough on my platinum plugs. "Don't go like that." Use Champion truck plug 4412 and you're 4.0L Jeep engine should run smooth as silk. I know the owner's manual says to use NGK plugs. No, they don't work as well. Yes, that is strange, but that's just how it is.

Plugs last longer than 30,000 miles. Yep. Some plugs are supposed to last 90,000 miles or longer. But the problem is that every mile past 30,000 you put on them, the harder they are to remove, and they are a pain in the butt to pull out of there, so do them at 30,000 miles.

Help with Spark Plug Changes Online

Stu's Write Up is on his index page. Just do a find on spark plug and you'll see it. He has designed the site to force everyone through the front door, so I can't link right to the article for you. But it is there, and it is excellent.

4x4xplor.com has a great write up on spark plug changes.

Feedback

Thanks for the great site! I Googled it looking for spark plug information. I have a 2000 TJ with the spark plug rail. It has 73K on it and the plugs were orginal. I changed them last night using tips from your site. Even the use of the truck plugs. I replaced all 6 with welcomed results in accelleration. The job was almost too easy. Thanks again for the helpful tips and information. Keep on, Jeepin on...

Jason Etters (aka Skeeter)
KMAoffroad.com member
Virginia Beach, VA

Hello, I just wanted to say thanks for putting up your web site. I stumbled upon it when searching for some tips on replacing my spark plugs on my Jeep. I have a 2002 Jeep TJ Wrangler X and I can now say that because of your informative and easy to understand instructions I have successfully changed the plugs in my TJ.It seems like it runs better when you do the work yourself,I have tinkered here and there with vehicle repair and I am no expert,but Jeep's are so much fun to own. So anyway thanks again and I have kept this site in my favorites and I am planning on tackling the differential and transfer case service soon! Nice job on the site - my fellow Jeep enthusiast,

-Mario Papaleo

Thank you so much for your post on changing spark plugs on the '04 Jeep Wrangler. I was standing for over a half hour like a dummy looking into the engine bay trying to find the spark plugs and plug wires. I finally gave up and went to my computer and found the information I needed, thanks to you. Great job!

-"Pops"

I just wanted to drop you a line to thank you for posting this site.

Your advice on blowing compressed air when changing plugs saved me this afternoon.

Sure enough... an old plug's ceramic insulator cracked, and a shot of air ... took care of that.

Regards,

Chris Kirby
Aurora, CO