Think about this, when you use boost to control the wastegate and turn the controller all the way up you end up with equal pressure on both sides of the diaphragm. This equal pressure in theory should make no difference in the way the diaphragm controls the wastegate, so the diaphragm is doing nothing only the spring pressure is acting on the valve. If you don't have anything hooked to the wastegate you have roughly 14.7 psi on each side, atmosphere pressure. Then the gate is only operating on spring pressure also. So in theory there should be no difference if you have no pressure or have 100 psi on each side. Your wastegate is just operating on spring pressure. The net on each side of the diaphragm is zero. The exhaust pushes on the gate piston and will overcome the spring pressure. Your gate is opening with exhaust pressure overcoming the spring pressure. You cannot keep the gate closed all the time using boost as a controller. So in theory, if you have a 15 psi spring, once backpressure is 15 psi higher then boost then the gate will open. But even at the lower back pressure using a spring to hold something closed the gate will start to bleed off pressure even before the 15 psi differential is reached. If you want to keep the gate closed all the time you have to put exhaust pressure to the top of the gate not boost.
So in the end your gate is still working just it is set at a much higher pressure level.
Food for thought...
Just because its a 15 lb spring doesnt mean that its going to open the valve at 15psi of back pressure. The spring in the wastegate is compressed considerably, therefore the spring is preloaded, so to say that the gate will open at 15psi is slightly inaccurate, whats needed to know is seat pressure, or the pressure it takes to begin to lift the valve off the valve seat in the gate... which... really isnt given out in wastegate instructions or tuning. I supposed the proper way to see what seat pressure is would be to weld a pipe shut, put an air fitting in the pipe, and slowely increase the pressure on the wastegate valve and see at what point it begins to open.... might do that before i do my next turbo set up so i know what im working with.
I bring this up because we spent some considerable time tuning a truck with max powers on it, had some trouble with the manual regulator, as well as the AMS500 controller, no matter what we did the gate would surge and stay open or closed and not very under control.
So we put regulated boost to the top... held the gate open and maxed out the EBP reading on IDS... no matter how little, or how much pressure. As long as there was pressure on the top of the gate it held it shut
Moved to the side port put pressure to it, now the gate would blow open early and then not shut fast enough, making a crazy dyno graph reading, as well as making the back pressure surge between 80psi absolute and 55psi absolute, as well as making the turbos act funny.
Tried regulated to the top, full boost to the bottom, about the same results
tried regulated to the bottom full to the top about the same happened, nothing worked and the gate surged like crazy
Got a wild hair to just unhook all pressure to the wastegate top and bottom... magic... it makes 76 psi absolute of back pressure on the nose, and stays there at 55 psi of boost. Nice clean graph on the dyno, ebp graph follows nicely as well. BTW this is with a "15 lb spring"
but in short, jeremy is right, your gate is probably doing something to relieve back pressure right now, putting pressure to the top of the gate to hold it close will probably reveal that the gate is doing something afterall.
