Help Needed: Cylinder 4 Misfire (No Spark) on 2007 MX-5 Miata NC

Maintenance/Repairs
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Hi everyone,

We’re trying to troubleshoot an issue with a 2007 Mazda MX-5 Miata NC (6-speed manual, around 160k miles, 4 cylinders 2L), and we’d really appreciate some advice from folks with experience.

The problem: Cylinder 4 is misfiring due to spark not working properly.

What we’ve tried so far:

  • Replaced the spark plug
  • Replaced the ignition coil
  • Rewired the ignition coil wiring (the misfire briefly stopped after this, but it came back shortly after)

At this point, we’re open to suggestions, any insights or ideas are welcome. Thanks in advance!

2

Have you checked compression? Swapped fuel injectors?

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3

The driver in the computer that grounds the coil for #4 cylinder may have failed.

Tester

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4

We’ll check the compression later, but we haven’t swapped the fuel injectors (my partner checked them and seemed fine). thanks for the ideas!

5

Oh, we’ll look into that! We’re not familiar with the electrical/computer side, do you have any tips on how to confirm this? Thx

6

Go to the ECU and probe across the terminals that lead to #4. Start the car and read if the ECU is sending a pulse to the #4 coil. A DVM that reads pulse frequency or PWM should work for that. Also check the resistance from each of the 2 wires that go to the coil at the ECU and at the Coil. You say you re-wired #4 but what does that mean exactly? New wires from the ECU to the coil? Or what?

Those 2 tests should tell you if it is the ECU or the wiring

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7

Great! Thank you so much for that tip. I’m not totally sure what he did, but I assume he replaced or something.

8

I just feel a need to ask/clarify. The “misfire” description that comes with the codes (likely P0304 in your case) gives a bad impression because it makes people think only of spark (the “fire”). Did anyone find that the spark was not, in fact, working properly? If so, then how?

The P03xx codes should come with the label of “miscombustion” - as in, that there wasn’t an appropriate “explosion” of a fuel air mixture in whatever cylinder. That can happen for multiple reasons. Probably the most common is spark problems (plugs/wires/coils). But it will also happen due to other things like poor compression or lack of fuel. (Thus the suggestions to check compression and swap fuel injectors).

Short story, don’t get hung up on the “fire” part of the “misfire” codes. It’s “miscombustion” and the combustion part requires more than just the “fire” (spark). Proper combustion in a cylinder requires an appropriate air/fuel mix + compression + spark + correct timing. If the error code is only one cylinder, you can move worrying about air or timing to the back seat of potential issues. But not compression or fuel delivery.

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9

I have a 2002 Miata, and for a few years had recurring warnings to “check engine”. There was a trouble code of P0303. After chasing after plugs, cables, grounds, injector clogs and who knows what else, it turned out to be a too tight clearance on the exhaust valve (at #3 piston, of course). I replaced the shim with one that would create the correct clearance, and no more problem.

10

congratulations on fixing your misfire by adjusting valve lash

many people incorrectly assume valves are only a problem if they’re noisy

11

Did you finally run a compression test to find the issue?? maybe a cylinder balance test/leak down test??

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Neither. I just decided to plunge in and took off the valve cover, then started checking clearances. I found a very helpful Youtube video on TheCarPassionChannel. The link is: Valve adjust

Bob

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