Today I replaced the spark plugs and the #4 coil (I was getting P0304 code, car was running very badly) in my S40. I had no specific evidence any particular plug was bad, it just seemed reasonable to do it while I was in there; I got this car used about 3 years ago, and I've got no information of when the plugs were last replaced.

Anyhow, all went fine with 4 of the 5 plugs. However when I attempted to remove #3, it didn't loosen up easily, and after a few cranks of the wrench it got even tighter. Not wanting to force it, in case it had been installed cross-threaded, I decided to try to put that plug back in for now, but it wouldn't budge with 25ftlb of force. Unsure of how much force would be safe, I decided to just leave it as it was until I could figure out the best course of action.

I put it all back together, and was quite surprised to find that the car started and ran great. I really expected all sorts of bad things to happen, but apparently the #3 plug is still in far enough to work correctly and the threads are tight enough that the cylinder pressure is holding.

The question though is what to do now? It's crossing my mind to just leave it since it's working, and the worry is that whatever I might try to do next will make matters worse. Will I strip the threads? Break the spark plug? But it really seems like I ought to either get that plug out and put the new one in, or at least get the old one seated back down again.

I googled a bit on this, and saw comments advising things like squirting WD40 into the plug well and running the engine to heat things up (kind of a pain when you have to remove the intake plenum to get at the plug, then put it back to run the engine, then remove it again to try to remove the plug, repeat...).

What's safe to use to loosen the plug? WD40? Liquid Wrench? other?

How much torque is safe to apply to remove the plug without breaking it? Or to put it back down again? I know 20ftlbs is the spec for tightening the plug down against the gasket rings, but how much more can I apply to try to get that plug turning again?

Any advice will be much appreciated.

 

Well, you use whatever force is necessary .

Myself, I wouldn't tighten it back up. Either leave it be or take it out and then chase the threads with a spark plug thread tool. You would need to vacuum out any aluminum filings that fall into the cylinder.

The threads may be galled so you can try some lubricant with a gentle 'tighten a little then loosen more' strategy. Nothing is going to help if it's cross threaded. You may end up having to helicoil the spark plug threads.

 

I agree. I don't see a downside to using some PB Blaster or similar penetrating oil if you want to remove. It may take some force to free up - I'd avoid power tools like an impact wrench but I've used the tighten/loosen trick. Usually its some baked oil or deposits that have encrusted the threads and it can take more force than expected to loosen (ie way more than 25 ft-lbs - when I did the plugs on my 2000 S40 recently, I'd guess it probably took 50-75 ft lbs to remove one of the plugs).

 

thanks guys. sounds like my best course is to forge ahead and carefully coax that plug out.

do I understand correctly that once I get the plug out, I should first try "chasing" the threads with this sort of tool: Performance Tool W83193 - Plug Chase | O'Reilly Auto Parts ? this is meant to clean out and tidy-up the threads if they're not too damaged by the ordeal?

and if the threads are badly damaged, for example if the existing plug had been cross-threaded, then I'll have to go further and use a kit like this: Heli-Coil 5334-14 - Spark Plug Repair Kit | O'Reilly Auto Parts ? this cuts a bigger hole with new threads, which will accept an insert and then the spark plug goes into the good threads in the insert. this is to be avoided if possible of course...

do I have it correct?

 

Yes. Hopefully it's just some galled aluminum threads grating against the steel threads of the spark plug. The chase tool just cleans up any burrs in the aluminum threads. As it says use anti-seize on the threads and on all new spark plugs. Prevents this from happening in the future.

The Heli-coil is the extreme fix. You use it when you have no other choice. I'm not sure if I would even attempt it with the head still on the car.

 

Today I went back in and got that stuck spark plug out. It was extremely tight almost all the way out, and when I got it out it was obvious why... the threads at the bottom were totally smashed. I have no idea how someone got that spark plug installed that way, must have used a power wrench and just forced it in. Photos attached.

Looking down the hole it was obvious the threads in the head were pretty screwed up. I went and got the thread chaser I linked in my last post above, turned out to be $10 well spent. I put some grease on it to catch all the metal shavings and crud, and carefully put it down the hole. It went down pretty smoothly without too much resistance, and it came out covered in both fine and coarse metal debris. I cleaned it up, put more grease on and ran it down again... this time I didn't even need a wrench, finger twisted all the way down. It came out with just a little debris on it, so I called it done. I vacuumed out the hole with a micro-attachment kit on a shop-vac.

After that the new plug went in nice as could be.

 

well done! thanks for the share.