You said you have the 5th injector mounted next to the throttle body? On the intake pipe or manifold? That injector better be gushing out a gallon of fuel. Remmember the fuel that injector is putting out is being divided into 4 cylinders, so lets say its putting out 40 psi of added fuel, each cylinder is getting only 10psi from it. And i doubt that the fueil is being evenly distributed to all four cylinders, thats why each cylinder has its own injector. If more fuel pressure is needed, it has to come from those.

And i doubt your map sensor is liking the fuel that is getting all over it too.