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4.0L Spark Plug Replacement

Please note: This writeup is only for some year 2000 Jeeps with a 4.0L and all 2001 and later 4.0's (it should apply to Cherokees, Grand Cherokees and Wranglers). This is due to a switch over in mid-year 2000 by Jeep from the old distributor based ignition system to a more modern (and cheaper to maintain) distributorless ignition system. To tell if you have the old or new system we need to check if you have a spark coil rail. The spark coil rail is shown viewed from the passenger side in the "Step 2" picture (it's outlined in YELLOW). If you have a spark coil rail, then you have a newer distributorless system, and you can proceed using my writeup below.

Every 30,000 miles I replace the spark plugs in my engines whether they look like they need it or not. spark plugs are one inexpensive way to make sure that you are getting clean combustion in your engine and that your ignition system is working to it's full potential.

A nasty spark plug will give you irregular idle, poor fuel economy, and a myriad of other problems. Another good thing to do while you have the old plugs out is to give them a thorough looking over as they are great indicators of engine conditions.

Briefly, there is one spark plug for each cylinder in your engine (Inline-6 equals 6 spark plugs). A signal from the on board computer (or your distributor in older vehicles) tells the spark plug when to use a jolt of energy (anywhere from 10,000 to 60,000 volts), when it jolts or "fires" it ignites the air/fuel mixture and makes the Jeep engine turnover. A fouled plug can hinder or prevent this spark from fully burning everything in the chamber which in turn will cause carbon build-up, poor fuel economy, smelly fumes, and eventual exhaust system problems.

This project should only take 30-120 minutes depending on how many things you have to remove to get to the plugs and how familiar you are with working under the hood. Follow along in the easy steps below and you should be just fine.

Parts List

Tool/Supply List

The Replacement

Step 1

First, open the hood, get out the shop light, and gather your tools from the list above. Now, start to clear a path under the hood so you can get to the spark plugs. I did this on my Jeep by unhooking the factory air intake tube (the black accordion looking thing 3" in diameter), and gently moving some of the misc. wires, hoses and cables up and out of the way.

Step 1 clear away obstacles

Step 2

After clearing the way, locate the spark coil rail. In the picture below, the spark coil rail is outlined in yellow. There are four (4) bolts on the rail. Go ahead and remove the bolts; you will need the 13mm socket to remove these bolts. You may notice in the picture that I have removed the oil dipstick, it kinda gets in the way and it really helped to remove it.

Step 2 Locate the spark coil rail and remove the bolts

Step 3

Finish removing all four bolts and begin to loosen the spark coil rail by wiggling it slightly up and down and puling towards you. It is pretty easy to remove, so not too much work will be required.

Step 3 remove bolts, loosen the spark rail

Step 4

Go ahead and remove the spark coil rail. Be careful, the rail end towards the rear of the Jeep has a wire connector and you don't want to put too much stress on it. I just swung the rail up and out of the way. Once the spark rail is cleared out, you can now clearly see all 6 spark plugs.

Step 4 Remove the spark rail and find the spark plugs

Step 5

Get out the gap gauge, open the little box for each spark plug. Take the gap gauge and set each plug to .035 inches. If you're using Champion Truck Plugs, they are preset to .035, but I check anyway just in case. Also make sure each new plug has a gasket (actually just a metal bevel-cut washer at the base of the threads) this gasket is pertinent to maintain your compression in the engine combustion chamber.

Step 6

Now is a good time to get the anti-seize compound and a shop rag. Take your 5/8" spark plug socket and remove the old spark plugs one at a time. It doesn't matter which one you remove first. Once you take an old spark plug off, take a new one and smear some anti-seize compound on the threads. When you put the new one in, just put it in finger tight for now. Go ahead and repeat this process for all six cylinders.

Step 7

Take the torque wrench and your 5/8" spark plug socket out. Torque each of the spark plugs down to 26-30 foot pounds. This is per Jeep factory specs, and I went ahead and set it to the average, 28ft/lbs.

Step 8

Put the spark coil rail back on, easing each rubber boot onto each plug. This is crucial to make sure that each boot makes a connection, it may make a click or slight snap into place. Put each of the four coil rail bolts back in finger tight. Take out the torque wrench and the ol' 13mm socket, and set the wrench to 29 Newton/Meters (Nm). This happens to be 21.3893 ft/lbs by the way. Torque down all four bolts, starting with the center 2, then the ends. By torqing the bolts (no matter how unnecessary it may seem) it will ensure that the boot-plug contact is just right.

Step 9

Take all of the wiring/cabling/hoses that you cleared out of the way and put them back to the proper place. Hook your air intake back up and clean up the tools. Keep your old plugs for inspections under better lighting with your manuals handy too. Get in the Jeep and fire it up, let it idle some listen for any unusual sounds and YOU ARE DONE!

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Page created on 2/18/2003
Last updated on 2/12/2004