Paul1212

I have a 99 mustang gt with the following: K&N Intake, Throttle Body, Upper Intake Plenum, UPR O/R X pipe, Magnaflow Magnapacks, and now Steeda UD pulleys with SFI. I installed the pulleys yesterday and as soon as I did I took it out for a drive and on my way back home my Service engine soon came on and is Giving me Code P0300 & P0303 which is random misfire and cylinder 3 misfire. Did the pulleys cause this or is it a coincidence and maybe a coil pack went bad at the same time? I just changed my spark plugs about 2 months ago to NGK iridium. HELP PLEASE!!!

Fryguy22

im guessing that a coil pack went bad. I was having the same problem a few months ago.

Paul1212

No one knows???

MollyMustang

I don't know if it's just coincidence or not, but my car did the same thing. I got a engine misfire right after i installed my stedda UDP set and a zex kit. The car didn't even have 40,000 miles on it yet. It was the coil pack for cylinder #5. Before you go and replace anything i would first try to reset the computer by disconnecting the battery for a few minutes and then pull the code again if it still doesn't work. Good luck.

MollyMustang

Forgot to say that mine was a flashing check engine light that eventually stayed solid.