Stuck Spark Plug in 3.2L Carrera
 

The reading I've done on this forum hasn't addressed the issue I'm dealing with, hopefully some of you have some insight for me....

I've got an '84 911 Targa which is running just fine. Decided to change out the plugs since I bought the car 4 years ago and don't know when they were last changed. Everything went fine with 5 plugs, but the 6th one is very odd. I can loosen it and back it out 2 or 3 full revolutions (which behave like all the other plugs removed) and then it starts to stick. Today I used a torque wrench set to the same 22 ft-lb used to tighten the plugs and sure enough it "clicked" after a few revolutions and the sticking kicked in.

Thoughts? Cross threaded? Carbon buildup? Can I safely remove this plug without trashing the cylinder head?

When I tighten this plug the car runs just fine, leave it alone and forget about it??

Dwight

I'd try some penetrating oil on it.

+1. Don't apply brute force. Allow the penetrant to gradually soften the carbon so the threads will wipe the carbon off as you remove it. Relatively hard carbon and steel spark plug threads are at work against softer aluminum threads in the cylinder head. Take small steps while allowing the solution to loosen the carbon.

However, penetrant sometimes isn't as effective in softening/removing carbon compared to regular strength carb cleaner (e.g. Berryman's), that can remove carbon from pistons soaked overnight or Chevron Techcron. If you have to resort to this level of soaking, relube the cylinder and change the oil.

Another method of decarbonizing the combustion chamber is to inject a fine stream of water into the intake (raise idle). This will scour the chamber of deposits as well as create a smoke screen in your neighborhood.

Sherwood

As per Wayne & Sherwood.

Take your time. Even if it takes hours, a tread repair job would be much longer.

+1, Never Ever use Brute Force. If you notice the only things that ever requires allot of torque are the rear bearing and maybe the 915 gear box locking nut during a rebuild.

Kroil is a good penetrator too

Aero-Kroil is a fantastic penetrant. Might be hard to find. Good luck.

Tom

In my experience, that is what it feels like when a helicoil is backing out. Leave it alone and forget about it until you have time to mess with it and face the possibility that you might render it inoperable until you take the engine apart.
Good luck getting penetrating oil on the threads of a spark plug while it's in the engine. I wouldn't bother trying.
When you are ready to mess with it, take the car out and get the engine good and hot. Get back there as quick as you can and try getting the plug out while it's still good and hot. If you can get an adjustable impact wrench in there, you might try that, with the power turned way down. The have little battery powered drill/drivers for carpentry work that might work for this. The repeated, low power impacts might break whatever is holding it loose. If that doesn't work, unscrew the plug a turn and start the engine. Don't let it run long and don't rev it up. Then try to get it out.
At this point you are about screwed if it doesn't come out. Plan on the car being out of service while you take the engine out. You may as well go ahead and try brute force at this point, as that's what the machine shop will do to take the plug out. If it comes out and a new one screws right back in, you just won the lottery. If not, you haven't lost a lot.
Still, my money is on a bad helicoil and you would need to win two lotteries to get an old plug out and a new one back in if that's the problem.

I had the same thing happen to me. It would loosen a couple turns then tighten back up. I wasn't sure if it was cross threaded or just carbon build up. I ended up soaking overnight with penetrating oil, tried again still pretty tight. Some recommended heating engine up, so I did that but still pretty tight. I repeated the above process a few times loosening and tightening but eventually had to add a bit more muscle and it finally broke free. It was very stressful as I was afraid of doing damage.

I am pretty sure in the end it was just carbon build up. I recommend moving it back and forth to try and break up the carbon a little at a time.

Good luck.

+3 on the penetrating oil. You probably want to use a can with a red stick tip so you can get some actually on the threads vs. everything else. If I were you I would leave that plug in place and finish buttoning everything else back up and then go for a drive. Get the engine warm and then go back and see if you can get it out while hot.

If the plug is backed out, just a little, that's usually enough gap to allow a good penetrating oil (not WD-40) to migrate into the threads. Yes, use the straw nozzle to pinpoint the target area. And since the seat is recessed into the head, there's no place for the oil to go but downward into the plug hole. Yeah, try driving the car a bit to heat up the head; loosen the spark plug, apply oil again and work it back and forth. You could even run the engine a bit with the plug backed off a turn or two to allow oil better access. It's not going to launch into the engine lid.

Unless your pcar is your primary driver, I suggest being patient.

I always subscribe to do the easy-to-do steps first because they're easiest to do, then proceed to the next option on the difficulty list. Emulating Hans at the factory and proceed to disassemble the engine is pretty impetuous at this point. Your progress will dictate how far you have to/want to go. MHO.

Sherwood

Thank you all for your advice and insight.
Patience, penetrating oil, heat, and repeated cycles of working it are next.

Just to close out this story, the above method worked. When I did get the plug out you could see the carbon buildup in the threads. I cleaned up the threads and ran the old plug in/out a couple times to get as much gunk as possible.

Since any new project requires a new tool purchase, I made a very useful one for this situation. Went to Harbor Freight and got a digital inspection camera (ie- bore-scope). Without this I would not have been able to ensure that the penetrant was actually getting on the threads of the plug.

The new plug went in as easy as the rest of them. The car is back on the road and I'm a happy camper.

Four years is a LONG time between spark plug changes. Either you don't drive it very much, or no one has suggested a 15K mile maintenance interval. Speaking personally, none of the couple of hundred cars and trucks I've bought and sold over the last 55 years have gone more than a week or two of my ownership before I changed the plugs, the oil, the filters, and adjusted the valves ( if they had adjustable valves). You NEVER know what the history is, even if the seller tells you something.

The Cap'n

Congrats :D

Good to hear the repair was a relatively minor procedure. This next suggestion will open up a hornet's nest, but here goes:

Apply a very thin layer of high temp anti-seize to the spark plug threads. Avoid the gasket sealing surfaces.
For example: Loctite Silver High Temperature Anti-Seize temperature range is -297 to 2200 deg F.
Never-Seez® Silver Gray High Temperature Stainless Anti-Seize Lubricant, 16 oz Brush Top Can | Staples®

Spark plug tip temperatures can be around 1200ºF, but the temperature at the seat is typically <400ºF. This is also a common area to sense cylinder head temperature; still way below the max. range of hi-temp AS, thus providing a potential barrier to carbon buildup. Detractors can go to the thread discussing anti-seize on spark plugs for more POVs.

Anal-retentive engine builders can even observe exposed SP threads on not-yet-installed cylinder heads and remove any that could provide a foothold for carbon deposits. Exposed threads are also a more likely shape that can initiate pre-ignition.

Sherwood

You went in through the adjacent spark plug hole to see if pentratant oil was dripping off the stuck plug?

No, not from the inside. I was able to focus the borescope camera on the stuck plug from the outside and position a straw next to the plug threads. This was I could see the penetrating oil soaking the threads.
After I got the camera I realized the "blind" attempts had not worked at all.

Good skills DKM.

DKM, can you please explain again or draw me a picture?

So, you didn't actually see any threads, the spark plug was still in the hole. You just were able to see that the pool of oil at the base of the spark plug was slowly seeping in b/c the level was slowly dropping? Without the boroscope, you would not be able to see this, since the spark plug bolt would block your view of what is happening right where the plug goes into the hole?

What was the straw for ????

this is interesting - suggests that oem 911 plugs are "too long" if the threads are extended into the cyl whereby they can get carbonized to this extant