Whoever installed it before might have cross-threaded the plug.

You can check to see if Autozone has a free loaner chaser on their list. But you might want to pay an engine machine shop to help with the actual procedure just in case. They can also put in a helicoil if needed. Call around first.

 

A timsert or helicoil will likely fix the issue and prevent future blow out of the spark plug. If you have a borescope to view the threads would be a big help.

 

From what you're describing, it sounds like the person who installed the previous spark plug torqued it so high that it fully stripped out everything, to the point where the only thing keeping the spark plug in place was the pressure from the rubber boot. Because once you removed the single bolt holding the coil down, the spark plug came up inside the boot, correct?

And it was probably in there good enough until recently when vibrations from the engine caused the last bit of thread holding it place to go, and misfire due to losing contact with the coil.

If so, then I doubt any kind of thread restore will work. it will almost certainly need a helicoil or similar type of thread repair insert placed.

 

Thinking I might pick up a bore scope to get a better idea of what is going on.

I need to see when the plugs were last done per the previous owner. I think they may have been done around 100k and not touched since.

 

Ive installed lots of spark plug helicoils. Fast and easy. Some tips i figured out along the way to help. Load the flutes of the drill bit and tap with grease and turn them slowly when using. This will catch the majority of the aluminum chips. Clean the flutes of both 3 times during drilling and taping and replace the grease. This helps keep the chips out of the new threads. Lastly when done, i use a suction hose on the end of my blow gun to suck up any remaining chips that may have fallen into the cylinder. Good idea also is make sure you turn the engine until both [all] valves in that cylinder are closed while drilling/tapping the spark plug hole. The steel helicoil will be stronger than the original aluminum threads.

 

Make sure the valves are closed.

 

Well I tried a thread chaser, no luck. Although the chaser did seem to go into the threads and could be backed out the plug would not thread in.

Bought a heli coil kit as well. Planning on using that tomorrow. I do have both a compressor and a shop vac so the plan is to use grease on the cutting threads of the reamer, then vacuum, then blow to remove any metal bits.

Two more points.
One, anyone who has done the heli coil, how hard is it to get proper alignment?
And two, how do you install the coil? What I've seen involves screwing it onto an old plug, using red thread locker on the coil threads externally, screwing it in, then unscrewing it and using the provided driver tool to expand out the portion at the top of the coil.

 

It’s not that difficult to get the correct alignment. Look at the other spark plugs. Hold the drill with the bit in it at that same angle as the other plugs. You will be surprised how easy the bit goes thru the old hole especially since most of the threads are already gone. Slow and easy is what has worked for me. The helicoil kit comes with the driver to screw the insert into the head. FOLLOW THE DIRECTIONS. As to thread locker, I’ve never used it on spark plug helicoils and never had a problem. But maybe the kit your buying calls for that. FOLLOW THE DIRECTIONS. Make sure you use the correct length insert. Ie: 1/2 reach. 3/4 reach etc. look at the spark plug thread length that will show you the correct length. After you install the helicoil, back the tool out. Then remove the tab at the bottom of the helicoil. Usually long nose pliers work fine. If that tab falls into the cylinder, not a problem. The helicoil is a ferrous metal. you can remove it with an extending magnet. I’ll attach a picture of the tab im talking about. First time i did this, i practiced on a piece of wood. 1 inch plywood. Couldn’t believe show easy it was. When it’s done you will be surprised haw easy it was. Please post the results.

 

Thanks for the info.

Few notes on my kit. Although its made by Heli-coil its not the traditional version with the tang and the installer tool. The insert has no tang, and it does specify high temp RTV in the threads for installation. Others who've used this kit (marketed as Sav-A-Thread) seem to indicate using an old spark plug to screw the insert in and then backing out the spark plug.

 

Hmmm, that sounds like a better kit than mine. Where did you get it? I’d like to try it. Or if you would have a p/n. Thanks so much.

 

Small update. Installed the coil from the kit tonight. Bit of an aprehensive process since you have 1 shot, but seemed to go well.

Per the kit instructions, the reamer goes in first. It has a thread chaser on the end to go into the existing threads before being followed by a tap that cuts a new set of threads for the coil. After cleaning up the cutting mess with compressed air, a shop vac, and brake cleaner I threaded the insert on to an old plug. Then applied Permatex Red High Temp RTV to the outside threads of the insert. Screwed it in as directed and then reversed direction to pull the old plug. I did not oil the plug threads as some sources say for fear they would be oily tomorrow installing the new plug. I then used the included device on the insert to expand the piece at the top. It looks like some type of punch and is supposed to expand the top threads of the insert outward to bite into the sides of the hole. I had to use a screwdriver as a drift pin to reach down the spark plug well while doing this as the device is too short by itself. Hit it moderately hard, but obviously hard to say what is required. Now to wait for the RTV to cure.

For those wondering this was a "Sav-a-thread" kit branded as HeliCoil and manufactured by Stanley

 

Installed new Ruthenium Spark Plugs in all cylinders today, including the one with the insert. Also replaced the coil that was fused to the old plug. Did a very brief engine test, it ran rough for about 15 seconds, presumably while the residual brake cleaner, grease, etc. left the cylinder and the idle readjusted. Seemed to return to a clean and normal idle after that. I want to give the RTV the overnight hours to be 100% absolutely cured (it had 22 hours before the engine test) before running it more than just a small amount. But it appears that this will work.

 

Alright, this isn't going as well as I thought. Took the car for a slight test drive today around the parking lot. Seemed to still be misfiring. Still had the Cylinder 2 Miss-fire code. Check engine light was flashing. Pulled the coil off the plug to inspect, don't see an obvious problem. Checked resistance between plug and cylinder head, was comparable for cylinder 2 and cylinder 1. Pulled the plug, no damage visible, felt like it might have been barely loose however.
Really at a loss here, any ideas what else could be going on?

 

Do a compression test to see where your baseline is. Valve or ring issue.

Need to measure fuel pressure for leak down check. Might be losing pressure at the rail. Spark has been verified. Now need fuel and compression to see where it is.