Engine misfire causes

[spookybathtub]

All of a sudden, today my engine started running rough, and I got a P0304 trouble code (cylinder 4 misfire). As I work my way through the repair guide, I noticed that spark plug 4 has metal bits caked on the threads. Is this bad?

About two years ago, I had that same plug blow out, and a mechanic fixed it with a helicoil. I worry now that maybe the helicoil is disintegrating, and soon may blow out like last time. I wonder why this would happen twice in the same cylinder. I'd like to fix this myself if possible, but I still have a lot to learn about engines.

Here is a photo of plug 4 (top) and plug 3 (bottom). Full-res photo: https://dl.dropboxusercontent.com/u/33335399/IMG_0237.jpg

[CTScott]

The helicoil may be leaking compression. You can pickup a compression tester from an autoparts store to test that.

[ex-x-fire]

That looks like anti seize on the threads.
The top spark plug in the picture has brown carbon tracking at the base of the porcelain. This is where spark is jumping. I'd change the plugs & coil boots, I like to smear so dielectric grease on the inside/outside of the boots.

[spookybathtub]

Anti-sieze — that makes sense. Hopefully that's what it is. I haven't yet bought a compression tester, but I did find something else:
I tried a spark test, and all plugs spark, but coil 4 has a problem (meaning a known good plug in coil 4 won't spark). When I push the plug into coil 4, it sticks out 1/4" more than it should. It looks like the spring in the coil is damaged, so it won't compress all the way. I just replaced the spring and boot, but the plug still won't go in all the way.

I measured resistance of the coils. Coils 1-3 measure about (3.4, 3.4, 10, 3.4 MΩ) on the four pins. Coil 4 measures (1.6, 1.6, 6.4, 1.6 MΩ). Is the coil shot?

[spookybathtub]

Coil 1 has some black oil on the tip. Coil 2 looks like it's covered in rust powder, and the rubber is slightly brittle. Should I be worried about this? Full-res photo: https://dl.dropboxusercontent.com/u/33335399/IMG_0238.jpg

[CTScott]

With the coil packs, you can't measure resistance, like you can on old fashioned coils, but the fact that one measures significantly different than the other three (and doesn't spark a plug) says that it is likely bad. The coating on #2 is odd in comparison to the others. That is not by any chance the one with the helicoil?

[spookybathtub]

Nope, the helicoil is on number 4. I'm curious to open up the cylinder head cover and see what might be causing that.

[CTScott]

How many miles on the car?

[spookybathtub]

The car has 175k miles. At 120k I replaced all the spark plugs with NGK Laser Iridium (supposed to last 100k miles). The helicoil job was done at 152k, and they put in an aftermarket ignition coil (I won't make that mistake again).

[spookybathtub]

My check engine light came on, again with a misfire on cylinder 4. These plugs have been in for 100k miles so I changed them, even though the gaps are good and they don't look burnt. I found a lot of oil in coils 1 and 2. That same slow leak from 3 years ago has gotten worse. So I'm going to replace the valve cover gasket, and hopefully that takes care of it.