I "seafoamed" my engine through the brake booster line with 0 issues. Not sure what the problem could be, did you throw the old plugs away? I went with NGK plugs 1 step colder than stock.

 

I seafoamed mine through the vacuum line on the intake...that way it gets to all 4 cylinders evenly.

I have the original plugs...I put them back in...and it acted the same. Vince is on it now.....

 

Maybe there was knock previously that the ECU had corrected for? I don't know if these ECUs do that for certain, but others do. If that happened, the new plugs would require some re-learning. Like 2 or 3 restarts, or 100 miles, or something. I'm just guessing, but that kind of thing can happen.