Misfire--valve cover gaskets?
 

A few weeks ago, I put a bottle of Red Line fuel system cleaner in the tank as a once in a while kind of a thing (maybe once or twice a year). I usually use Techron, but wanted to try this stuff out as the company is very highly respected for top notch products. Car ran fine for the rest of day for about 100-150 miles. Eventually, a few days later it started misfiring (P0300 random misfire and P1316, excess emissions from misfire) at idle when cold, but would smooth out once warm and the codes would disappear. If the misfire was random, I figured it was a fuel issue and the car didn't like the fuel system cleaner, so I just filled up with pure fresh gas.

Car ran decently for a couple more days, and then the misfire returned worse than ever. However, this time, it was P0307 and P1316. I did the usual from my last time with P0307 and P1316. I pulled the coil and noticed the boot was cracked. So I swapped in a working coil, and the car was smoother, but still misfiring. I pulled the spark plug, and the threads were covered in a dark black goo. It was unclear to me if this was burnt oil, or if it was somehow the fuel system additive that hadn't combusted properly. I swapped in a new spark plug just to see what would happen, and the car ran fine after that and didn't show any pending codes. It also started up much quicker.

Here's what I'm wondering:
- Perhaps I have a fouled spark plug gasket--how else would the substance (oil or additive) have gotten to the spark plug threads? Time to change valve cover gaskets? I was pretty sure they got replaced when the head was off, but I'll have to look at the invoice.
- I think I'm going to check the other spark plugs to see if I can find the same thing. Is it unwise to uninstall and reinstall the same spark plug? I don't really mind buying new ones, but I'd want to know if that's necessary even if the others are fine.
- Maybe this is a combo issue of some sort--a gasket on its last legs being exacerbated by an unusual substance in the fuel.

Probably fuel filter needs changing
 

Not sure when your fuel filter was last changed, but generally crap from the tank is stopped at the fuel filter before it reaches your engine, but.... age gets to everything and leaks happen. Fuel filter is a cheap part and it’s an easy DIY.

As to the spark plugs- removal, inspection, bit of a clean, put back in is perfectly acceptable. Modern iridium plugs last 100,000 easy ... without “crud” build up.

Always a bit of a coincidence as you just put different Red Line additive in. Hopefully no harm done.

Yeah, fuel filter was changed within the last 1500 miles (a lot of fuel system stuff was done when I had misfire issues last year). Injectors are freshly refurbished and cleaned, too. Also within last 1500 miles: MAF cleaned, Throttle body cleaned, the little PCV hole thing that gets plugged was cleaned, air filter replaced, intake hose replaced, ignition modules checked and cleaned, ECM and ABS control module repaired.

Interestingly, when I first replaced the coil, I forgot to plug it in (d'oh), and the car ran about the same, signaling to me that spark is definitely the issue. The buildup on the electrode, though, didn't actually look too bad. I think it may have been worse that what I could see with the naked eye, perhaps.

“Crispy” spark plug cables?
 

When I did change my spark plugs, I had to bend the cables out of the way. They’re skinny and “ crispy” and now old and brittle. Maybe worth checking them.

yes, and if they short out with each other the coil pack is often ruined.

Will do. Always scared of those--when I did forget to connect the coil plug, I got a new code for the circuit not being complete, which is a good sign for that. Also the cables were check around 1000 miles ago, but at this age, they can go at any time. Will check back in this weekend.

I installed a brand new fuel filter last week. wires for the coils are old but seem intact, that will be on the list if I can't sort this through conventional means.

I saw the tsb on the pcv cleaning, pics are kinda vague though, will have to look into it.