I recently bought a 2013 Jaguar XF AWD. The car starts up just fine, no smoke or anything that emits from the exhaust. However today I turned the eco start/stop feature on today while waiting at a light and I noticed a plum of white smoke coming from my exhaust. When I turned the feature off the smoke dissipated. It later came back when I had the AC on full and went away as soon as I turned the AC off. Is this a serious problem?

 

White smoke in the exhaust is usually a sign of some sort of water vapour being burned. See if you're able to replicate the problem with the vehicle parked and see if the smoke smells like coolant (sweet) - you may have head gasket issues.