I just started having this problem. I installed a new alternator three weeks ago and everything appeared fine until today. The Jeep fires up ok, then after 4 minutes the volt meter spikes to 19+. I put a volt meter to it at both the alternator and the battery and it was in the 14 range.
I know I am in desperate need of a tune up as I am getting back fires under load.
Ok I'm a newb but I read somewhere that the voltage spike may be caused by a bad ignition coil. Does anyone know if that is true and if a new coil may solve my back fire issue as well?