Ok.... so my '03 S'crew with a new 5.4 has developed a miss or shudder at highway speed under load. We were towing the boat on a mountain pass and the engine seemed to have a shudder after assending the pass (on the flat at the top). I put new MSD cops on the engine when installed, but I am still running the same injectors after 320,000km's. I never got an CE light or code. The cats are also the orig, is it possible the cats are restricing the exhaust (I put a set of headers on during the engine install as well)? With no codes popping up, I'm at a loss.

 

What plugs did you use? Failure of injectors is fairly rare and most likely that is not the reason for your missfire. It's more likely a coil (msd coils arent really better then stock motorcraft) a plug or a poor connection in the boot. Its a low grade missfire sounds like and typically they are caused by weak coils..