J and K AUTOMOTIVE TOWING AND REPAIR 2/23/2011have you fuel system pressure tested also you mention it backfire :
misfiring cylinder possibly due to fouled spark plug, ignition timing incorrect (leads swapped around). The reason for the backfire is unburnt fuel accumulates in the very hot exhaust system and ignites there
HouseCallAuto 2/23/2011Cracked upper intake manifolds are very common on this engine. This situation can also give you the backfire you mentioned. The reason is that some of the cylinders are getting coolant in them while the car is parked. It would depend on the size of the crack / opening as to whether you will have either a no start or a hard start cold. This theory can be verified by a good mechanic checking the car and mention about checking the upper intake manifold for a coolant breach by the EGR tube. There CAN be other causes for a no start when cold. Verify fuel pressure is at specification. check the fuel pressure regulator for leakage into the vacuum hose. And of course, check for any fault codes that might have set.