is this a recent problem i mean has the car ran before now? and how much boost are you running? from my general idea and im sure that if im wrong i will be corrected if your running too much boost then that could very well cause your car not to start and by taking off the valve cover it might be letting non compressed air into the engine.. and im totally admitting that i could be wrong.. but try disconnecting your downpipe from your intake manifold temporarily to see if you can start it that way with the valve cover on. thats my simi educated guess and as for it running rich.. a turbo requires a bit more fuel but that could also be your problem..