How long will these last on our cars? Will they cause you to misfire if they are bad? I'm about to change my spark plugs again and reset my power tuner back to all stock 1.5v file. I was running really rich. My LTFTs were at -19 to -25 cruising around. I think I may have fouled out my plugs, because the car is running like shit. At certain RPMs the car will bog. What do you think?