Changed out the spark plugs on the wife's Mazda 5 (2007) and now it seems to have a "miss" in 2nd gear, just kind of a lilt that acts like it's not getting gas. On changing the plugs, I did have trouble with the 4th plug, misthreaded it, and had to re-tap the cylinder, but it seems seated fine (as do all the others).

NOW, as this was my first time under the hood (beyond changing air filters and headlights), I did "poke around" just seeing how it all went together, fiddled with the fuel injectors...nothing drastic, just kind of a "get to know ya" session. I'm ABSOLUTELY positive I put everything back together the way I found it, but dang if I can explain the miss. (And yes, I realize now that was a great big old can of stupid I opened). Any ideas? Could it just be a bad "new" plug. The car starts fine, idles fine, and has plenty of get up when taking off, but when it hits about 25-30 mph there's a bad hesitation.

 

First off, are they OEM spark plugs or aftermarket. I know that certain kind of brand likes OEM, though sometimes, you may think that there are better than OEM aftermarket such as Bosche Platinum, more often than not, using the OEM brand seems to be the best choice. Worst case, you can put the old spark plugs back on to see if the problem goes away. Often, it isn't the issue of spark plugs, it's almost something else that causes the issue. It sounds to me like the one you were referring to, you might have accidentally either not seated the cable correctly or damaged it. Worst case, by rethreading, you also may be experiencing a loss of compression, hope not but best thing to do is to connect a diagnostics tool to see what errors are triggering. It sounds like to me, a simple DIY job turned out to be a nightmare.

My recommendation is to initially call a mechanic that deals with Mazda and other Japanese cars. Tell them exactly what you did and ask them to tell you what may be wrong and get 2nd or 3rd opinions from various other mechanics. Get a estimate to fix it from several people.

Good luck!