Just curious....

[391dman]

I know the loud hissing noise at first start idle is normal...but why does it go away if I tap the brake? I thought that loud hissing was the flap bypassing the turbo...? Does that sound come from a vacuum pump or something??

[excursionMark]

nope its a valve in the exhaust that aids in heating up the engine and the reason it goes off when u tap the brake is because it assumes your gonna drive and doing that with a valve closing up some of ur exhaust would make your truck less than pleasurable to drive ... said less than eloquently .. hope u get the point lol

[391dman]

Interesting. Thanks for the info!

[Dietoremain]

yup. same thing here. sometimes it takes a minute to open like today i drove down the street a bit til it opened(sounds like a huge turbine or somethin lol) when you put it under load enough it opens.