What are the best spark plugs for my Jeep Wrangler TJ 4.0?

[Jerry Bransford]

This question comes up often, so I figured it's time for a FAQ on the matter.

The later model 4.0 engine (2000 and later) is what is known as a DIS (Distributorless Ignition System) which is far fussier about what spark plugs it likes to run well on. A DIS ignition system also fires the spark plugs twice as often which means a conventional copper-core spark plug only lasts half as long in this type of engine. All 4.0L engines that are 2000 and newer have the DIS ignition system. 1999 and older 4.0L engines aren't nearly as fussy so long as they are the right model plug. 2000 and newer 4.0L engines won't run well for long on single-tipped Platinum plugs, even though some catalogs show them as an acceptable plug. So do avoid any platinum plug that only has platinum on the center electrode.

I always recommend dual-tipped Platinum or Iridium spark plugs which will perform well for at least 100K miles. Iridium tipped plugs will last significantly longer than even the 100K miles dual-tipped platinum plugs will.

Good dual-tipped platinum plugs include the Autolite APP985 and Champion 7412 at https://www.summitracing.com/parts/cpn-7412

A great iridium tipped plug for the 4.0 engine would be the Autolite XP985. A technical trainer at I know at Autolite checked his iridium plugs at 165K miles with his company's electron scanning microscope and they showed no wear yet.

Such plugs last so long that it's a good idea to smear a tiny dab of copper-based antiseize onto their threads before screwing them into the head. Make sure no antiseize gets onto the electrodes (tip).

So while conventional plugs are what the factory installs and they are the cheapest, that doesn't mean they are the best choice for replacement spark plugs.

NOTE: 30,000 miles is the recommended spark plug service interval for conventional (non-platinum/non-iridium) spark plugs due to 2000 and newer 4.0 engines having a waste spark ignition system.

 

[Mike_H]

What makes the platinum plus any better than conventional? Just time of service? I replaced mine with the OEM NGK plus. It runs great, and only cost me 14 bucks for all 6 plus!

 

[Jerry Bransford]

The platinum coating allows the plugs to perform well 3-4X as long as conventional spark plugs. Iridium outlasts platinum by over 100%. How much longer isn't well known yet. An electron scanning microscope saw literally no erosion on the electrodes at 165k miles in a test vehicle owned by Autolite's technical trainer who I know.

The fine-wire electrodes used in better plugs like the Autolite APP985 or XP985 also allow them to fire more reliably in difficult combustion environments.

They're better in many ways not many are aware of.

 

[Chris]

What makes the platinum plus any better than conventional? Just time of service? I replaced mine with the OEM NGK plus. It runs great, and only cost me 14 bucks for all 6 plus!

Those will work just fine. They just won't last as long as the other ones mentioned, but that's no big deal.

 

[derekmac]

I was told (or read....) that you shouldn't adjust the gap on platinum or iridium plugs as they come preset, and can be damaged if you try and change it. Do you know if there's any truth to that?

I need to check the plugs in mine, and I don't want to damage new plugs if I need them.

 

[StG58]

Just throwing it out there, the 2.5 uses the same plugs as the 4.0. Just not as many.

x2 on the Autolite XP985 and the anti-seize on the threads.

 

[Jerry Bransford]

I was told (or read....) that you shouldn't adjust the gap on platinum or iridium plugs as they come preset, and can be damaged if you try and change it. Do you know if there's any truth to that?

The platinum or iridium electrode coating could be damaged if you used a tapered gap setting tool which would abrade the coating as it was slid through the electrodes to widen the gap. Like tapered edges or square loops of wire you force through the gap.

Don't use one like this on platinum or iridium plugs...

Gap setting tools with a notch cut into them that grabs the electrode next to where it is welded to the threaded area can bend the electrode up or down without damaging the coating.

Use one like this on any type plug including platinum or iridium...

 

[lovemachine]

I was always told that you don't have to set the gap on spark plugs anymore. I've been told the same thing by my local dealership.

So is it a good idea to do so? Or does it even make a difference?

 

[Jerry Bransford]

I was always told that you don't have to set the gap on spark plugs anymore. I've been told the same thing by my local dealership.

So is it a good idea to do so? Or does it even make a difference?

Plugs supposedly come pre-gapped but pre-gapped for what? One gap setting is not going to be correct for every single engine it'd be the right plug for. Not to mention it is not uncommon for a boxed spark plug to be accidentally dropped before it's sold which can change its gap setting.

In my book, it's always best to spend 30 seconds on each plug before installing it to make sure its gap is correct. That can save a hard to diagnose problem later.

 

[lowranger]

Great info Jerry! Thanks!

 

[derekmac]

The coating could be damaged if you used a tapered gap setting tool which would abrade the coating as it was slid through the electrodes to widen the gap. Like tapered edges or square loops of wire you force through the gap.

Like this...

View attachment 12607


Gap setting tools with a notch cut into them that grabs the electrode next to where it is welded to the threaded area can bend the electrode up or down without damaging the coating.

Like this...

View attachment 12608

Thank you sir. I only have a gap tool like that round Champion one. Time to get one with the notch I think.

 

[Mike_H]

So...an Update. I posted about the OEM NGK plugs in the second post of this thread. Seems Jerry has left us for the time being, but I'm going to pull the trigger on new plugs. I've been getting misfire codes on my CEL basically since I put the NGK plugs in. I've tried a couple of different things, like swapping plugs and injectors to see if the trouble follows. I've also cleaned the throttle body and IAC (actually removed them). Its worst when the engine is cold, which leads me to think that the plugs may be the culprit. Warm engines ignite easier as the fuel is atomized better. Once I'm warmed up, she runs pretty good, and fairly smooth. I'm thinking my plug don't have enough "zapp" when the engine is cold to get that whole reaction to kick off. A double tipped iridium or something might help get more energy down there where I need it.

Also...Jerry mentioned Copper based anti-Seize. Copper only, or would Aluminum work?

 

[Chris]

So...an Update. I posted about the OEM NGK plugs in the second post of this thread. Seems Jerry has left us for the time being, but I'm going to pull the trigger on new plugs. I've been getting misfire codes on my CEL basically since I put the NGK plugs in. I've tried a couple of different things, like swapping plugs and injectors to see if the trouble follows. I've also cleaned the throttle body and IAC (actually removed them). Its worst when the engine is cold, which leads me to think that the plugs may be the culprit. Warm engines ignite easier as the fuel is atomized better. Once I'm warmed up, she runs pretty good, and fairly smooth. I'm thinking my plug don't have enough "zapp" when the engine is cold to get that whole reaction to kick off. A double tipped iridium or something might help get more energy down there where I need it.

Also...Jerry mentioned Copper based anti-Seize. Copper only, or would Aluminum work?

Not sure if this is helpful, but it's info I found:

The problem with Copper is that it is a high valence metal, so it will dillute a significant lesser valence metal, within time it will weaken the structure of the thread, and you will one day, pull out pieces of the thread with your plug.

Its called Galvanic Corrosion

Copper should only be used on metal to metal threads like the header studs, were there is contact with aluminium steer away from copper, use alu anti seize or graphite based.

Beware of grease based things or oil on your sparkplug, every time there wil be al thin layer of "ceramics" baked on the thread, and the plug will go more unwillingly (the same thing you see as a brown to black enamel on the underside of your pistons).

 

[Mike_H]

Not sure if this is helpful, but it's info I found:

The problem with Copper is that it is a high valence metal, so it will dillute a significant lesser valence metal, within time it will weaken the structure of the thread, and you will one day, pull out pieces of the thread with your plug.

Its called Galvanic Corrosion

Copper should only be used on metal to metal threads like the header studs, were there is contact with aluminium steer away from copper, use alu anti seize or graphite based.

Beware of grease based things or oil on your sparkplug, every time there wil be al thin layer of "ceramics" baked on the thread, and the plug will go more unwillingly (the same thing you see as a brown to black enamel on the underside of your pistons).

Thanks Chris, I read that at lunch time today. Still as clear as mud. Pretty sure for an iron block and steel spark plug threads, galvanic coorosion isn't an issue, so type of anti seize shouldn't matter

 

[Chris]

Thanks Chris, I read that at lunch time today. Still as clear as mud. Pretty sure for an iron block and steel spark plug threads, galvanic coorosion isn't an issue, so type of anti seize shouldn't matter

That was my thought as well. Iron and steel shouldn't be an issue.

I'm curious to see though if this solves your issue.

 

[bobthetj03]

I've been thru 3 sets of NGK ZFR5N plugs throughout the years with no issues. This is the plug my service manual specifies for my 4.0L. Replacement is at 30K intervals, and you will know when it's time, cause idle will start to become a bit rough. I will assume you checked the gap prior to install? Just throwing this out there, but one time I was replacing the plugs on one of my other vehicles, After install, it idled rough. Couldn't figure out what the deal was, so I pulled the plugs back out. Come to find out I damaged one of the porcelain electrodes during the install. Replaced the damaged plug and all was right as rain. Not saying you damaged your plugs, but just thought I'd share my experience.
Oh, and regular silver antisneeze works great, along with a dab of dielectric grease on the end of the plug electrode.

 

[Mike_H]

I've been thru 3 sets of NGK ZFR5N plugs throughout the years with no issues. This is the plug my service manual specifies for my 4.0L. Replacement is at 30K intervals, and you will know when it's time, cause idle will start to become a bit rough. I will assume you checked the gap prior to install? Just throwing this out there, but one time I was replacing the plugs on one of my other vehicles, After install, it idled rough. Couldn't figure out what the deal was, so I pulled the plugs back out. Come to find out I damaged one of the porcelain electrodes during the install. Replaced the damaged plug and all was right as rain. Not saying you damaged your plugs, but just thought I'd share my experience.
Oh, and regular silver antisneeze works great, along with a dab of dielectric grease on the end of the plug electrode.

It's been steadily degrading. The plugs have 7000 or so miles on them. I've cracked porcelain insulators on other engines too. Pretty sure it's not that...and I gapped them prior to install. New plugs will be here today, so I'll check those things when the old one come out too.

 

[Mike_H]

Just a quick update here...been about a month since I installed the autolight XP985 plugs. My CEL had not come back on. I still have a rough idle, so I'm still suspicious of the iac, but the plugs definitely fixed the misfire code I was getting (P0301).

 

[xackley]

A P0301, misfire cylinder 1, could be fuel, air, or spark. Pain to diagnose. Dump a can of SeaFoam or B-12 into a full tank of gas, run it 20 miles and pray.
Let it idle for 15 minutes, pull plug 1 and 2. If 1 is lighter, not enough gas or too much air. If darker or wet, probably missed or weak sparks.
You should also do a compression check even if the Sea Foam cures it. All cylinder readings should be close.

 

[mcoh1030]

Has anyone used the Bosch Platinum Plus 2 Plugs? I've been using them for the last 80k miles. No issues. Pulled one the other day....some carbon deposits....but that's it. Thinking of going for them again, as they are No Gap. Any comment/suggestions on this subject would be greatly appreciated.

Also, when speaking of Plugs, how about the Coil Rail? How long before it should be replaced???