2004 chevrolet silverado ext cab 4.3 v-6 auot trans. the truck has recently got a problem. most of the time it runs great and then some times it will not start. It will spin over but no start.Leave the truck set sometimes a hour sometimes a day and it will run like it has never had a problem. I have replaced crank sensor,ignition control module,coil,pick up coil,cap rotor,wires,plugs,. Checked it for spark and it has got it. Checked it for fuel and it is there. Hasnt never set a egine code... Can anybody help??????

 

tried egr valve?