I own a Mazda MX5 mk2 1.6L.

I had severe problems with engine misfiring, caused by the valve cover seal, to leak. The oil that was leaking was concentrated near the spark plugs, and periodically it grounded them.

First I had the seal changed. The leak stopped, and the repair was a success. However, due to everything being covered by the oil, the misfires continued, which I considered normal.

Both the spark plugs and the plug wires, were in a mess.

Today I changed the spark plugs, to new NGK ZFR5F-11, with the gap set to 1.1mm, and the plug wires.

The engine now run very smoothly with no sign of misfiring. However I noticed a SEVERE loss of power, both at low and high RPM.

Before the plug change, sometime the engine worked without misfiring and was delivering the proper power, so I can compare before and after in a few hours of time difference between the test drives. This leads me to be heavily biased to blaming the spark plug change.

What may I have done wrong?

(1.1mm is what Mazda recommends, ZFR5F-11 are considered a very good option for this car, they are long-reach, as were the previous ones, and firing order is definitely set correctly.)

1 Answer 1


Several things to check:

  • Ensure all of your spark plug boots are fully seated. Also, use dielectric grease in the boots to ensure you can pull them off easily the next time.
  • Ensure you have the wires going to the correct spark plug. It's easy to switch two of these out, which will cause a major power decrease.
  • Pull a single spark plug out and ensure it is still gapped correctly. Since these are not stock spark plugs, they may be a bit longer than you'd expect. If you noticed that the gap has decreased, this may be a sign the piston has ran into the spark plug and closed the gap. If this has happened, either get shorter spark plugs, or you could also try a second washer/seal on the spark plug, which will pull it back slightly, out of the way of the piston.
  • Pull the battery power and allow the car to sit for 30 minutes. This will reset the ECM and bring the fuel map back to stock and allow the computer to relearn for the new plugs and wires.
  • Check to ensure you didn't dislodge any vacuum lines or other connections when doing the plug/wire change.
  • Check to ensure your intake tract didn't get dislodged somehow during the plug/wire change. If it's now getting unmetered air into the system, it could be wreaking havoc with your power.

Hopefully one of these things will help with your issue.

  • Will repeatedly (meaning 2-3 times...) removing and remounting the plugs cause any harm to their sealing rings? Or the will be ok? Jul 4, 2015 at 16:14
  • This will not cause an issue with your plugs. Jul 4, 2015 at 16:19
  • 3
    The solution was to reset the ECU by unpluging the battery. Thank you Jul 6, 2015 at 16:15
  • I love it when a plan comes together. Glad I could help. Jul 6, 2015 at 18:54

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