Misfire remains after helicoil?

[highway_star]

I've got an unusual problem here.

This started with a Cylinder 2 misfire code on a 2003 Toyota Corolla. I decided to start the diagnosis by pulling the coil over plug and seeing if anything was amiss. When I pulled the coil out it took the spark plug with it. They were fused together and obviously the spark plug and stripped out of its threads and blown out.

I tried installing a new spark plug but it would not thread. Tried a thread chaser to clean up the threads, still no luck on the spark plug going in. Finally decided a Helicoil type insert was the next option. I went with the "Sav-a-thread" which is made by Stanley, Helicoil branded, and looks like a time sert. Followed the instructions, reaming it out with the tool, cleaning the gunk out of the Cylinder best I could with air, vacuum, and brake clean. Then installed with high temp RTV on the external threads of the insert as per the instructions. Let the RTV cure for 24 hours and installed the plug.

I also replaced the coil over plug that was damaged, and put new plugs in the other cylinders (hedging my bets here as I feared others might have been improperly installed like the one that failed and they were close to due anyway).

I then did a short test run, where it idled poorly for about 10 seconds before sounding more normal. Then today I did a longer test run driving around the lot. It seemed to start fine but rapidly returned to sounding and feeling like it was misfiring. Engine light flashed like it did before and I put it back in the garage.

Investigating the issue, I pulled the coils of cylinder 1 and 2. I checked the resistance between the spark plug wall and the cylinder head with a meter for both, they were comparable values. So I'm tentatively ruling out the RTV impeding a good electrical connection.

I then pulled the spark plug from the cylinder 2 hole with the insert in it. I expected to see some kind of damage to the plug, but it looked fine. The only thing I might say is it seemed to come out perhaps a bit easily, so wondering if it needed to be torqued more to stay in.

I'm at a bit of a loss here as I cannot see a reason for the persistence of the misfire. Does anyone have any thoughts on this?

 

[Trav]

A fuel injector can cause a misfire also.

 

[Astro14]

The use of RTV between the threads and the cylinder head is genuinely surprising. I would worry about a bad ground for the plug. Did you happen to put your multimeter on those threads?

Do they still provide a good ground for the plug?

Check that. Then swap the coils between cylinders one and two. If the misfire follows the coil, it’s time for a new coil.

As @Trav said, it might not be the plug causing your misfire.

 

[highway_star]

The use of RTV between the threads and the cylinder head is genuinely surprising. I would worry about a bad ground for the plug. Did you happen to put your multimeter on those threads?

Do they still provide a good ground for the plug?

Check that. Then swap the coils between cylinders one and two. If the misfire follows the coil, it’s time for a new coil.

As @Trav said, it might not be the plug causing your misfire.

I measured from the sleeve of the plug (ie. above the threads) to the cylinder head with the multi meter. Did the same measurement for cylinder 1 and 2. Both had similar resistance figures (bouncing between 10 and 100 ohms) which is why I'm not leaning towards a continuity issue. There appears to be good ground.

I'm thinking of a coil swap next, although the coil that is on there should be working since it was from a working cylinder originally. The replacement coil is currently on 1, I'll swap that with 2 and see what happens.

I was considering an issue with the injector, etc. but it would be an awful coincidence for that to happen at the same time as the plug is ejected. Unless of course there is a cascading failure mode of some sort in which case it might make sense..

 

[thomasflaherty]

Try a compression test see if the cylinder is in 10% of the other 3.

 

[Hirev]

I would also do a compression test see if to see if you have a low cylinder contribution. Also you could also swap fuel injectors but I would have new injector O rings on hand before doing so.

 

[Mechanician]

It would be quite a coincidence for the injector to crap out at the same time.
Sometimes you can hear a difference in a bad injector with a stethoscope, when compared to the others.
I would lean toward whatever the debris, from the insert installation may have caused first. I assume you already swapped the plug to another cylinder.
Maybe a compression & cylinder leakage tests would be a good idea as suggested.

 

[zzyzzx]

How many miles are on the vehicle? I think you are going to need a new head or at least a visit to a machine shop.

 

[highway_star]

Car has 194k miles.

I did three things. First, I put the cylinder 2 plug back in. Because it felt like it had come out easily, and because the engine ran well at first and got worse I suspected it was too loose. So when I put it back in I torqued to 30 ft/lbs with the thought being that the steel insert should hold up to that and it might keep it in place.
Second, I swapped the coil over plug between cylinder 1 and 2.
Third, I pulled the negative battery cable and let it sit to clear the codes and CEL.

Put everything back together and started it. Ran rough for 15 seconds, got better, then sounded fine. Took it around the lot, still fine. Took it out on the main road, sounded fine. Took it on the highway, sounded fine. Just took it again on the main road, sounds fine. No check engine light, and sounds and feels smooth.

Given that, I'm going to say that injectors, compression, etc. are not a plausible explanation. Either it needed to be tightened a bit more to stay tight. Or it needed to have the newer coil on it for some reason while the old coil was fine for the other cylinder. Or we just needed a reset of the codes etc. to maybe return something to a default trim or spark advance, etc.

But it seems to be working for now, so in absence of any further developments I think that is where we stand.

 

[Exhaustgases]

Finally decided a Helicoil type insert was the next option. I went with the "Sav-a-thread" which is made by Stanley, looks like a time sert.

Misfire remains after Helicoil ? Wrong title, you never used a Helicoil. There is a reason in the aircraft world they don't use a look a like TIME SERT, for spark plug holes. And RTV ?

 

[highway_star]

Misfire remains after Helicoil ? Wrong title, you never used a Helicoil. There is a reason in the aircraft world they don't use a look a like TIME SERT, for spark plug holes. And RTV ?

The packaging, in big bold red letters reads "Helicoil" on it. So one might logically assume that the item contained inside could be referred to as a helicoil. So yes, I think I did use one.

This isn't an aircraft that if it fails to run will fall out of the sky and land on women and children and small pets. Its a 2003 Toyota Corolla with 194k miles. I also don't have someone calibrating my torque wrenches or signing off on every line item of maintenance I'm doing.

The instructions say to use high temp gasket maker on the threads. So I followed said instructions.

 

[Exhaustgases]

Brand name and device used can be two different things. A solid insert no matter who markets it or makes it is not a helicoil type insert.

Aircraft engine or not the same physics and principles apply.

 

[highway_star]

Brand name and device used can be two different things. A solid insert no matter who markets it or makes it is not a helicoil type insert.

Aircraft engine or not the same physics and principles apply.

Seems like Helicoil is a genericized trademark, although I think these are made by the same company.

The same physics may apply, but that doesn't mean aircraft level solutions are required or worthwhile on an economy car with 200k miles thats 20 years old. Economics still applies as well.