I did some (rookie) welding two weeks ago and started the car and it wouldn't stop--the old vacuum leak problem. I tore apart the passenger cabin looking for the vacuum line I thought I had singed while welding. I wasted an hour doing so before I checked the vacuum reading at the black plastic diaphragm in the engine compartment.
Thats when I realized a lot of the rubber fittings that coonect the plastic tubing get hard in frigid weather (15 - 20) and they don't fit as well...and several of them can lead to a big vacuum leak. It had nothing to do with the welding. I forgot that the weather had turned frigid recently.