Fix NA (95-97) Miata P0303 – Misfire

TL;DR: Coil pack

This is for a 128k NA Miata. I was getting little misstep/bog down at idle, but when driving at higher RPMs the car seemed to be driving fine. This lasted for a couple days, then I lost what felt like nearly all power accelerating and slight missteps during higher RPM cruising.

Then finally, after a hard acceleration up to highway speeds, the car threw a flashing check engine light and read as P0303, misfire on cylinder 3.

First step, checking for spark from the plug wires. With the car running, I pulled each plug wire from the spark plug and placed the end near a valve cover bolt. Checking to see/hear if there was a spark. All sparked except cylinder 3.

Pulled the wires from a different car, and also pulled the spark plugs. All plugs looked good except cylinder 3 being fairly fouled. Which might mean this has been happening on some scale for a while, which makes me wonder if this could’ve been the source of my intermittent P0420 code this whole time.

Cylinder 1 vs 3 spark plugs

Tried running the car again. Ran correctly for about 15 seconds then the misstep came back. And again no spark on cylinder 3.

Onto the coil packs. Miata coil packs varied between years, so neither of my other Miatas had usable coil packs I could test with. So I switched the coil packs arounds. Switching the (for my year) 3 pin connectors then switching the plug wires to match the new coil packs.

Instead of 4 1 2 3 order, it’s now 2 3 4 1. So if it was the coil pack, I should be getting no spark on plug wire 1, but getting spark on 3 now. Which is exactly what happened in my case.

$83 Amazon purchase later, I got a new Delphi set of coil packs and replaced them. Amazon was nearly half the price of any other place including eBay, which were selling used ones for the same price and higher.

And no more misfire.

During replacing them, I found the working coil pack was not connected to the mounting bracket whatsoever, and the 1 bolt that was used to connect the bracket was completely stripped by a wrench. So I’m going to guess the previous owner replaced the 1 coil pack, did a poor job of it, and didn’t want to spend the money or do the work to replace the other coil pack with it.

Stripped bracket bolt

I also have not gotten my P0420 code for the past week of driving it, so maybe this also fixed that.