Screwed up changing out spark plugs - multiple cylinder misfires


Staff member
I can't remember changing the spark plugs in my Jeep, so I decided to knock it out today. The old plugs that came out looked terrible. They are NGK plugs, and I wonder if they are the factory plugs (2005 4.0 with 172k Miles on the odometer).

New plugs:
AutoLite Double Platinum Spark Plug: Part No. APP985

Things went well as far as I knew. I used a bit of dielectric grease, applied to the ceramic tip on each plug, and a wisp of antisieze on the threads. Gapped each plug to 0.035 and torqued down to 26 ft lbs.

Upon startup, I was greeting by a flashing check engine light - scanner said it was a cylinder 4 misfire.
I pulled the coil rail off, looked everything over, put the coil rail back on, and started it up. The idle stumbled quite a bit, check engine light came on, and the scanner proclaimed "multiple cylinder misfires".

Could this just be that I botched when applying dielectric grease, and pulling, then putting back on the coil rail smeared the grease causing the spark to fail? The grease was applied directly to the ceramic portion of the spark plug before going into the engine.

I know these plugs have a crush washer - can I remove plugs, inspect, and reuse them, or will I need a new plug (with a fresh washer)?

Here are the old plugs, with a new (properly gapped) one in the center. Some of the old plugs have a gap of 0.085!




Last edited:

First the electrodes in the all of those look like normal wear. I change mine every 30,000 miles. I was closely monitoring in miles per gallon and when I saw it drop and that?s when I change plugs.

Plugs come pre-gapped to the proper spec. If you had to open them, they were miss handled, and I would inspect the ceramic.

From left to right the third one in looks like you have a severe oil leak. Looking at just the top, all of them have enough carbonate oil on top to be a problem.
I would inspect, the boots on the rail and make sure they?re clean and not cracked. Second thing I Will inspect that I didn?t crack the ceramic installing the plugs and look for burn marks to where the ark might be jumping.

Then I would reset the PCM codes so that you lose stored data. This way you could start with a fresh set of data that it?s going to create with the new plugs. Your misfire might be a too rich programming from the old plugs.

What you did with dielectric grease sounds OK. It won?t block the charge. Also, I do not use anti-seize. if I were to use it, I would use it very sparingly. See tips below.

When I tighten the plugs, I tighten them down to snug and then a quarter turn more.

Never look down on anyone unless you are helping them up - Jesse Jackson
Last edited:
What would you clean the coil rail with. As you mentioned cylinder 4 was coated in oil. I've got plenty of brake cleaner but I'm wondering if this isn't a job for electrical connection cleaner.

I may throw a new spark plug on cylinder 4 and clean the coil rail and give it another try.

Sent from my Pixel 6a using mobile app

I?d be afraid the brake cleaner will dissolve the rubber boot. I would just wipe the rubber boots down with dry cloth getting as much of the caked-on grease off. Then a towel of soapy water. Don?t soak it just clean the grease off. Clean inside with a Q-tip.

Make sure when you tighten down the rail you tighten each bilt a little then the next ? just like a tire don?t torque one down then the next? this way, you know each plug should be touching it?s terminal.

When it does start, what does it idle at?

You could sprat sone TB cleaner at the plug while running to see if there is a leak ay the plug

Never look down on anyone unless you are helping them up - Jesse Jackson
Last edited:
I believe that it's fixed now. Here's how it went down:

I cleaned the coil rail boots with electronic parts cleaner, using a soaked Q-tip to get the insides (thanks for the good tip). The Q-tip needed to be changed a couple of times, as it was pretty greasy.
Started back up, and in about 60 seconds the multiple cylinder misfire came back.

I then went to Advance Auto Parts and bought a set of Autolite XP985. This is the more expensive Iridium plug. I pulled out the old Autolite APP985 plugs and took a long look at them. These plug plugs have less than 5 minutes of run time on them:

Cylinder 6 -> 1


The second plug as a noticeable crack in it. I'd bet that the crack would be very hard to see until it has a bit of dirt / stain to it.


6 -> 3


4 -> 1


With the new set of plugs, everything runs great. Looks like I had a bad plug right out of the box.

Props to Advance Auto Parts - they let me return off 6 of the APP985 plugs as defective. I'm now running the XP985 plugs.
If what i see is real 3-1 have deposits.

All of them have caked grease and dirt even where the boot covers them. That?s bad. Sorry to knock you but you have to know.

2 has uneven spray or clocked 180 from the valve

Is the crack because you gapped them? As noted they should be dead on out of the box

When is the last time you cleaned the carbon out of the cylinders and valves???

Id sea foam and then steam. Run some injector cleaner

Ps I?ve never did it but the boots can be changed.

Never look down on anyone unless you are helping them up - Jesse Jackson
Last edited:

I'm with you that plugs 3-1 have some deposits. I haven't cleaned carbon from the cylinders or valves that I can remember. I've got a can of sea foam that I can add to the gas, and I'm happy to run injector cleaner (have any recommendations?).

I think the boots are good now. Here's what they looked like before I cleaned them:


I cleaned up the block with brake clean and hosed down with water (while the plugs were installed and coil rail off), and cleaned the ignition coil rail boots with electronics cleaner and q-tips. I think I got alot of grease on the plugs from the rubber gloves I was wearing as well.

As to the crack - With the old set, I put the gauge on each plug, but didn't have to change any gaps (or pry on the plug). I just verified that each one was set correctly.
With the new set I installed, I just held the plugs up against each other and eyeballed it. I didn't use anti-seize or dielectric grease this time, just trying to keep it simple.

Things are running great now - thanks again for your help.
I am curious about cleaning up my injectors.
Seafoam you syphon into the intake through a vac connection. Only at an amount to nearly stall the engine. Its all on the can.

You do the same with steam. Syphon a small amount of water in a hot engine. The let it sit and dry out the remaining carbon.

Injectors i use dectron or gumout in the tank. However there is a better system you pull the fuel fuse and attach directly to the rail.

Never look down on anyone unless you are helping them up - Jesse Jackson
In the last two weeks I've done two things:

1. Added a can of seafoam to my gas tank, and ran an entire tank of gas through.
2. Used Seafoam spray into the air intake.

The traditional Seafoam can no longer recommends that you add liquid seafoam to your intake - it says that if you want to add to the intake, you should use "Seafoam spray" This is a normal spray bottle with a hooked nozzle. I bought a can of this from Advance Auto Parts, and gave it a try.

You rev the engine to about 2000 rpms, and spray the can into the intake. It takes about 7 minutes to spray the entire contents out, then you are instructed to drive it vigorously. I never saw any smoke out of the tailpipe (which was a little disappointing).

After another 50 miles or so, I ran a borescope down the number 1 cylinder. What do you guys think?


Pretty clean

Run can empty then shut engine off and wait 30 min then drive hard. Thats where you will get the smoke. When running it into the engine you want to nearly choke the engine.

id be more interested in the valves specifically where they seat.
This is where buildup. Caused misfires.

Now every 3 mo drive in 2nd at a speed your rpms go above 3300rpm. This is where the are designed to be free to turn. By turning it gives other areas the buildup spot snd the buildup a chance to burn off.

Never look down on anyone unless you are helping them up - Jesse Jackson
Last edited: