I just got a 2002 GMC Sierra. When it is cold it sputters on acceleration and back fires a little, but at idle it runs perfect. When it gets warm it clears and runs great. It has two glass packs that the previous owner installed which are being removed today and stock exhaust being put back on but does anyone have an idea why it might be running this way. Like I said within 5-10 mins it clears and have no other issues.