Ijust bought a 1991Honda civic cx it was sitting for three months out in the cold and snow Without starting it turn on if i jump the car but while its running (not driving) the car idles but stayon

a car will run without a battery once its started getting 12 volt power from the alternator

"while its running (not driving) the car idles but stays on" most cars are on if its idling and not driving.....whats the problem exactly?!

You could A. have a faulty alternator or B. a short going from the alternator to your battery or C the ground for your battery could be damaged. Where the ground wire links into the terminal connector. First thing you should check is your battery, then the ground from your battery to your alternator. Lastly move to your alternator.