well dis connect the pipe off the back of the standard gauge in dash and take the other end of it off at the inlet manifold and blow air thru it and make sure it's clear, then if it is put a t piece on it at the back of the gauge in the dash plug it back in then run the other side of the t piece to your new gready gauge. you will have two boost gauges then and you will be able to see if the original gauge is faulty or not!. also you could plug the end of the vacum hose when you have it off at the inlet manifold then send air thru it from behind the standard gauge in car and listen for air leaks this mite explain faulty gauge reading!