Secret No. 5: A misfire at high speed doesn’t necessarily mean the ignition is breaking down.
Your mechanic might say you have “major electrical component failure” when in fact a misfire could be the result of simple corrosion. “When you hear a misfire, the first thing that pops into a lot of people’s heads is maybe the ignition is breaking down,” Hanks says. “I had a guy come into my shop who had already been to four shops previously. He’d gotten a full tuneup, caps, rotor, fuel system and lots of diagnostic charges.” In the end, all the guy had was a bad ground. Check the battery, or any jumper grounds between engines, and make sure you have a solid ground connection. Terminal corrosion, a loose cable or corrosion in the wire can all result in similar problems. Another culprit could be spark plugs. Check to make sure they’re the proper heat range and type. Shady mechanics are notorious for using cheap plugs.