There are a few things that can cause this condition. First is a low fuel pressure condition. Full fuel pressure is needed under load, check fuel pressure chart I have attached. It may be the fuel pump. Second is the ignition system. If the ignition system is worn it can misfire causing the backfire you describe. Check your spark plugs and ignition coils, replace as needed. It is possible that it is the ignition module also. I have attached info to make sure your plug wire routing is correct. Also, really look at the coils, the bottom. Sometimes they start to leak definitely showing that they are bad. That is attached also. Also, the module operation is attached. It is very possible that the module is bad as it controls timing.
Three twin tower ignition coils are combined into a single coil pack unit. This unit is mounted to the ignition module. Each coil provides the spark for two spark plugs at once simultaneously (waste spark distribution). However, all three coil packs must be replaced as a unit if one of the coils is defective (Type I coil pack). On Type II coil packs, the coils can be replaced separately. Between the coil pack and the module there are six spade-type terminals. Three of which are connected together by a common wire from the module, suppling all three coil packs with +12.0 volts. The other three terminals are individually connected to the module. The module will only connect one coil at a time, in the correct order by suppling and removing the primary circuit ground to each coil at the proper time.
NOTE: It is important to identify which type (Type I or Type II) of coil pack is used on the vehicle you are working on. Even though they operate in the same manner, the replacement procedures are different.
The C3I ignition module monitors the camshaft sensor and the crankshaft sensor signals. This information is transmitted to the ECM so that the correct spark and ignition timing can be maintained during all driving conditions. During cranking, it monitors the cam signal or "Sync Pulse" to begin the ignition firing sequence. Below 400 rpm, the module controls spark advance by triggering each of the three coil packs at a pre-determined interval based on engine speed only. Above 400 rpm, the ECM controls the spark timing (EST) and compensates for all driving conditions. The C3I module must receive a cam signal then a crank signal in than order to enable the engine to start. The C3I module is not repairable. When a module is replaced, the coils must be transferred to the new module.