Smoke coming out of the tailpipe on startup

When I start my truck up there is a large cloud of blueish white smoke that smells like fuel. It blows out huge clouds when it is really cold and a really small cloud or no smoke at all when it is warmer. I replaced the valve seals because the smoke was smelling like oil, but now it only smells of fuel. A couple guys I know said it could be that the temperature sensor is bad. It is not telling the computer what the temperature is so the truck is dumping more fuel in than needed. The tailpipe tip is blacker then usual. I was just wondering if they are correct or if it is something else? (The oil on the dip stick is clean so I assume the gaskets are still good).