I've heard of smoke bombs and flooding with water for this but never exhaust from an engine...lol
Unfortunately the network of tunnels is typically VERY extensive so success may be ineffective or temporary at best. I read somewhere that groundhogs inhabit the same area for many generations and it's not unusual for a family to be there for 100+ years. That's alot of tunnel.
The best humane tactic is to use a Haveahart or similar box trap. The most effective, permanent method is a properly sized Conibear type spring trap.....quick and deadly.
http://www.nwtrappers.com/catalog/traps/vt/vtc.asp
I also think that these animals are of varying sizes. Here in the Northeast adults are typically the size of a small to medium sized dog...with short legs
