On June 30th, we took Grace out to celebrate the birthday of the Corvette itself. It was a hot day, but she performed well—until she didn’t. On the way home, I had to brake fairly suddenly when an old woman in a Camry pulled out in front of me. Immediately afterward (coincidence?), we started to get a rough idle and an occasional backfire.

The last time we had a similar issue (back in 2013), the reason was that the clamp following the MAF had come almost completely detached, allowing a stream of hot and uncontrolled air into an engine that was believing what the MAF was telling it. I pulled the car over and took a quick look. That wasn’t true this time—nothing was loose.

I nursed the car back home (about ten miles). Acceleration was fine, and the car was smoother at higher RPMs. However the rough idle and backfiring under load at low RPMs continued.

The first thing I did was check for codes—there were none. I then proceeded up the intake chain. I cleaned and re-oiled the Blackwing air filter (at 50,375 miles, it was due). The idle got a little bit better, but the car blew a code—the generic P0300 random/multiple cylinder misfire code.

Next, I cleaned the MAF (likely the first time that’s been done). The idle again got a little bit better, and the code went away, but the idle was still much rougher than expected. I cleaned the front of the throttle body. The idle once again got a little bit better, but the car gave a new code—the more specific P0305 cylinder 5 misfire code.

I carefully pulled the #5 wire, which disintegrated in my hands as I removed it. Hmmm … I ordered new plugs and wires. Next, I tried to remove the #5 spark plug. At this point, I discovered that my tools were striking sub-optimal for the task at hand, with my old Craftsman spark plug socket essentially useless and all my ratchet lengths incorrect. I ordered a modern swivel spark plug socket and a longer teardrop ratchet and asked for help from Guru member Rex Ruby. Before he came over, I sprayed a ton of PB Blaster on the plugs.

It took us about an hour to remove the plugs and wires, with Rex doing most of the hard work. As expected, #7 was the most challenging, but none were particularly easy. Despite all the PB Blaster, removing the plugs took all my strength. One of the other wires was also bad, but the rest looked pretty good. The plugs looked worn, but not too bad.

We checked the gap on the new plugs (they were all fine), put some anti-seize on, and re-installed the plugs and wires. With everything buttoned back, I started the car. After the PB Blaster had burnt off (a notable amount of smoke), the idle settled down to a much smoother note. It's still rougher than I remember, but now I'm starting to wonder if I remembered it as being smoother than it actually was.