In the past two weeks, my check engine light came on indicating a misfire on cylinder 3 (according to my ODBII) so I changed out all the plugs. this indeed resulted in a smoother idle. so I thought I'd change the cap and rotor today while I had some free time.......well, now the engine misses -- can't even drive it now because it misses so badly. OBDII code reader says P1310 (misfire on at least one cylinder - TWC damage) and P0301 (misfire on cylinder 1). So what gives! it was running fine and now after a simple cap and rotor change she misfires.

I did notice that the plate on which the rotor is bolted to is missing on retainer nut - so only two of three bolts are used to hold down the rotor. Could this cause an imbalance and misfire?? it wasn't happening before?

Of course I put the old parts back but she still misfires now. I guess the old saying 'if it ain't broke, don't fix it' applies here.