#LeakdownTest

Sajeev,

I recently bought a 2002 Miata, manual transmission, in silver. The prior owners appear to have been followers of the sans souci school of maintenance, so I have been wrenching on it quite a bit. It has only one problem I can’t solve: a persistent P0301 code, showing a misfire in Cylinder 1. The actual experience of driving the car is fine — I don’t feel a miss or a drop in power. That light, though… it isn’t constant. It doesn’t come on instantly; if I clear it, I might get a few minutes of light-free driving, or an hour, or a day. But after that, the CEL goes blinky-blinky.

Logic suggests that the misfire can only come from spark, air, or fuel, so I went at each as follows:

• Spark: I’ve replaced the spark plugs (NGKs), plug wires (ditto) and boots. I swapped ignition coils to see if the code would move; it didn’t. The ignition wiring harness has some broken protective tape, but I don’t see any broken wiring.
• Air: I’ve replaced the air filter and checked for vacuum leaks.
• Fuel: I replaced the fuel filter, and, to make sure I didn’t have a fuel injector problem, I swapped the injectors between cylinder 1 and cylinder 4. I spilled a bunch of fuel, but the code stayed put.

I’ve been busy fixing other issues (leaking valve cover gasket, cracked radiator, soft top made of cheesecloth) on this Miata, but the P0301 has me stumped. I checked compression, and it’s good across all four cylinders. Forums mostly argue in favor of ignition problems, but I feel like I’ve covered that area.

The big question is this: Do you think new driving shoes will fix the problem?