smoking out a groundhog?

have had problems over the years with groundhogs feasting on our garden , over the years have taken care of many with the 12 gauge , only problem with that solution is the time , had heard years back about smoking them with the exhaust from a tractor , tryed it last night with a piece of flex pipe from the tractor to the hole covering the hole with a blanket to keep most of the gass in the hole . woundering how long to leave the tractor running to gas whats in the hole , have also used smoke bombs but not sure how effective they are , also one time after missing with the 12 gauge poured a half gallon of old chainsaw gas down the hole and lite it , don't like that idea , of dumping oil in to the ground , and thought this morning of dumping burning wood / charcoal down the holes

woundering what others have done to rid thier gardens of groundhogs