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mine does the same thing until my operating temp gets around 145-147 degree range. From what I have read that is normal, it is trying to warm itself up.
Heating the cab also pulls some of the heat the motor is trying to keep. I read in another thread that if you set your dials to MAX AC and full hot, it will recirculate the cabin air causing less heat to be taken out of the cooling system. The AC unit will not kick on because of the temperature.
when the hissing starts does it go away if you roll on the throttle? Like I said mine does the same thing, it hisses and no boost until it hits about 145. If I roll on the throttle the hissing will stop and I will then see some boost pressure as well, as soon as I go back to normal throttle the hiss will come back until I hit the temp that is required to open the valve. I believe computer uses air temp and operating temp to determine if it should be closed or not. Someone can correct me if I am wrong but I think the sensor for the air box reads the air temp and if it is removed the EBPV will no longer be engaged, although it might throw a code, I haven't tried it. Once someone else chimes in and says this is correct you can try it and see if your hissing goes away. Or watch your operating temp and see if the hissing is still occurring when you hit 150 degrees.
Last edited by JBeaz; 01-21-2013 at 05:31 AM.
Reason: added a little more