So I have a 07 mazdaspeed 3 with @ 94,000 miles on it. The.check engine.light came on and after hooking it up to the sensor it said there was a "engine misfire in the 4th plug" first I changed just the 4th spark plug to no avail. Then took it to a mazda dealership and had them diagnose it which they did and then changed the 4th plug again, as well as changing all 4 coils. It is still doing the same thing... When I give it gas around 3,000 RPM's it will begine to sputter and backfire, in any and all gears. Any insight as to what may be the issue?
Thank you for your time and attention in this matter.
Chad

 

I'm thinking it might be time for a compression and/or cylinder leak-down test.