2002 Miata P0304

I’ve owned this car for nearly 15 years, and it has 53,000 miles on it now. It lives in a garage, in Northern California, where it’s rarely very hot and never very cold. It’s in great condition. I’ve replaced the timing belt a while ago and the shocks because it was time, also the accessory belts. Over the years I’ve had to replace both oxygen sensors after they reported in with a check engine light code. In the past 8 months I have had a recurring check engine light with a P0304 code (misfire on cylinder 4). All the jobs I’ve done are listed here.

  1. Cleaned every electric ground I could find
  2. Added a bottle of injector cleaner to tank of gas
  3. Swapped coil packs (there are 2, one for 1 and 4, 1 for 2 and 3). One coil sits on top of the sparkplug of 4 so no wires involved. No change at all.
  4. Replaced all spark plugs (no change)
  5. Removed and had injector of cylinder 4 professionally cleaned. They reported it was dripping and the internal filter was dirty. No change.
  6. Replaced fuel filter. It was dirty. No change, but it took 100 miles for the light to go on again.
  7. Discovered belt driving alternator was sort of loose, tightened it up.
  8. More injector cleaner (Techron).

After each step I reset the check engine light and then drive it. The light returns within 25 to 40 miles except for the time after the fuel filter replacement.

I have not yet checked the compression because I don’t have a reliable tester. It seems so unlikely that it’s compression with only 53,000 miles on the engine and I really never drive it that hard.

My next thought is that there’s enough junk in the fuel tank that I should replace the fuel filter again, because the last time I did it there was really a lot of black powdery stuff that came out of the old filter. But I admit I’m floundering.

How 'bout the coil boots?



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Thanks for the idea, I’ll take a close look at them tomorrow morning.

I pulled off the #4 coil pack, separated the spark plug boot, which looked brand new, and checked the spring conductor. All seemed fine, no signs of cracks or tracking. I cleaned the spring with contact cleaner, cleaned the contacts at the connection of the coil pack, put it all together again. I was really hoping you were right but so far no change. I reset the CEL yesterday morning and drove about 30 miles, so far no CEL, but it’s too early to know much.

Is it possible to place a spare plug where the number 4 coil usually goes, and visually test for a robust spark at the tip of the plug? If you had contaminated gas you’d more likely get misfires on all cylinders (p0300) rather than just one. But I suppose it’s possible some grit got into just the one injector. You could swap the number 4 injector with one of the other ones to determine if the misfire moves to the other cylinder. If I had this symptom and was confident there was no compression problem, those are the tests I’d start with.

A couple months ago my truck’s engine completely stopped, for no apparent reason. When I did a spark test, there was a visible spark, but it was reddish, weak and not normal in appearance from the usual robust bluish-white I’m used to seeing. The problem turned out to be the ignition points were not making a good low ohm connection when closed, which was preventing the coil from getting fully charged. Fixing that problem returned the engine to excellent running condition, pronto. I think if you can do a visible spark test you’ll get some clues.

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Another good idea that I’ll check out in a day or 2. Obviously this isn’t my daily driver (I am a scooter rider through East Bay traffic). When I swapped the coils and shut off the CEL, it did come back on after a while, still P0304, so I eliminated the coil, but I’ll check anything. Also, the same coil serves 1 and 4 and fires both every revolution, so I would expect 1 and 4 to miss if the coil is weak.