I have an 03 QC 4x4 LB with the HO 305 and 48RE auto, 161,000km, truck is bone stock other than a mech. boost gauge.

I have read the posts about the 48 shifting pretty firm but I think this is more than that. Having not even barely ridden in another CTD I can’t say what is normal or not for what I am hearing and feeling – so I need your help.

Happens mostly under moderate or higher load, and not all the time, but often enough. The 3-4 shift is quite hard but the thing that concerns me is the noise. It’s like a somewhat loud air whoosh and a dull thud under the passenger floor pan (at least from my perspective as driver). The closest thing I can alike it to is it sounds like a I just ran over a 12" sealed cardboard box on the passenger side . A pretty load "Wahhflump" during the shift. It sounds a bit like turbo stall, but much loader and coupled with the dull thunk. I have no mods on the truck at all, so I wouldn’t think I could hear the turbo stall even if it did (plus I didn’t think you could stall a turbo with an open throttle).

The shift seams normal other than it’s for sure one of the firm ones, the RPM’s seam normal, a big drop (say 2500 down to 1700 or so) followed immediately by a small 200RPM or so drop for the TC lockup. Not precisely sure on the RPM drops other than they are much bigger than the 1-2 or 2-3 shifts.

I thought it might be the exhaust pipe hitting the underside of the cab due to the torque on the motor mounts generate by the firm shift (the truck does launch forward pretty good on the 3-4 shift even when towing a 4000lbs trailer). But just casually looking under the cab there seams to be a far bit of clearance between exhaust and cab and no obvious contact point, plus it’s not a hard clunk and I dont' "feel the sound", must hear it. And a physical contact wouldn’t explain the sound of exploding air.

Upon writing this, I’m also thinking it sounds like a very muffled backfire coming from the mid section of the truck.

If you have any idea’s please let me know.

Thanks.

 

The whoosh is called Turbo Bark!! This is and will be the cause of Turbo Failure in the future for you. All of the air that is already pressurized in the Hot/Cold/Intercooler piping is rushing back against the Impellar fins of the turbo when the 3 to 4 shift occures. Automatics do not have a BOV/Pressure Releif Valve to allow the boost to be releived atmouspheric. The gearing of the tranny and the rear end going from a lower gear to the higher 4th gear is lugging the engine (slowing the rpm's down and causing the boost to drop allowing this back pressure to slam back against the turbo)!!!!! Not good!!!!

 

Do most auto's do this then?

 

I agree that it is turbo bark. Mine never did it with the stock turbo but I never made more than 30psi with mine. I have learned to drive so it doesn't do it now but I couldn't really tell you what I'm doing different.....