My 03 GMC Durtymax 2500 4X4 Extended Cab smokes like a bonfire with
wet grass at idle.  It cleans up at cruise and gets great mileage
otherwise. But let it idle for a few minutes and you may think the
swat team is smoking out a bad guy.  Anyone ever heard of this?  If so
your help is appreciated.  I have had it back to dealer twice and they
have not offered any suggestion.

Thanks,
Matt