Just killed the dpf and of course I have the LMM hiss. My question is: Why does it go away and comeback sometimes? Over the last weekend crusin', I would come up to a stop (idling LMM hiss style) and the truck would go into LB7 idle mode (no hiss) for 3-5 seconds and go back to LMM idle mode. I know that the turbo vanes are like 80% closed at idle which gives me the hiss, but do they occassionally open up and cycle themselves or what? Just wondering if anyone knows the why in all this?

I think it is just the ECM resetting thing occasionally and taking "inventory" of where everything is set. It's normal.

If you are standing at the exh tip when it happens, you understand why they have the vanes closed at idle. When it opens, you hear the turbo spool DOWN. When they close again, you hear the turbo spool UP. This is why the VVTs are sooooo driveable.

set the vane postion to 10% AT IDLE if you dont want the hiss but yes they do cycle every once and a while

Just curious, guys. Thanks. Love to learn on these DMAX's.

I dont think its just a Lmm thing,, my LBZ sounds like a freekin harrier jump jet usually just after i start it up with the remote,, can hear the hiss come on and then go off,, gives me the visual image of the directional things on a harrier adjusting themselves lol... makes me grin everytime...

I also have the LMM hiss, and it does dip and get deeper and stop hissing every now and again. I have noticed that it is turbo boost pressure on mine it base lines at .4 -.6 LBS at idle and when it looses the hiss it is at 0.0 -.1 LBS it does not last long. My guess is that it is a verable vane turbo and it adjusts the vane and it is correction that causes it (that is my guess)