I read this thread wrong, you said if the problem is not the pump lol.. I wouldn't add another injector before you tune the car or upgrade the existing ones, a hobbs switch would hold the injector open, dumping fuel into the engine, the injector may also burn out due to the continuous duty. Woody on this forum added injectors with a megasquirt, which would be a better idea.. but I'd personally rather try to run the engine on standalone than add injectors at that point.