will the fuel pres reg keep fuel from inj

Backfiring is caused by unburnt fuel in the combustion chamber that when the spark fails then begins to work again suddenly, as if you turn it off then right back on, the fuel explodes during the exhaust stroke..

The fuel pressure regulator will not keep fuel from the injectors. The fuel pump sends high pressure fuel to the fuel rail. The regulator lowers the pressure to a set amount so the correct amount of fuel is sent to the combustion chamber. A leaking fuel pressure regulator will lean out the fuel mixture and that sometimes will give you the check engine light and hotter operating temperature.

if you are having backfiring problem, it means the spark is happening at the wrong time as davidh25 states. Most likely this is caused by a bad cap and rotor which are regular maint. items so i would do that first. is the check engine light on? the distributor in the vortec 5.7 is known for going bad as well. distributor has the cam sensor in it, and that has to synchronize with the crank sensor for correct spark and injector timing. you didn't by chance have the dist. out before this problem did you, they are hard to get back in time correctly. the fuel pressure regulator can go bad and cause a huge fuel leak directly into the intake through vacuum port. You are sure you have fuel pressure like 65psi when you turn the key on? do you have injector pulse?

On these systems the regulator CAN keep fuel from the injector. It can allow all or most of the fuel to bypass back in the tank or leak it out into the upper intake area. If you can get a fuel pressure reading and do a leakdown test that would tell alot.