[Double Venom]

Conman,
If I may, assuming your engine is set up right, IF you are running an electric fan, ESPECIALY DUAL FANS, they act lilke small generators and will feed 12v's back down the system especailly with an MSD box and will allow the enrine to run on until the fan(s) slow down enough to drop voltage. If this is the car this is where you would put the diode in, the feed lines to the fan (s). Very common problem with week grounds in these cars.
DV