This week I got my first chance to really test how much the install of a adjustable boost elbow has helped my truck. The reason I installed it was to be able to keep my speed up on long steep hills with a heavy load; without developing too high of EGT's. So with my road bike tied down in the bed and the camp trailer in tow and a weeks worth of supplies we headed over several mountain passes.
Most of the time every thing worked fine. We would hit a steep incline and the boost crept up to 28-30 psi and the EGTs went up to about 1150-1200F and held while and the speed stayed at a steady 60 mph. Just what I had hoped for! However a few times things got really funny.
For instance when I came up behind a slow moving semi, I had to slow down and wait for a passing lane. When I got to one we were going about 50 mph and I got on the gas a bit to pass. The boost pressure started bouncing rapidly between maybe 10 and 23 psi. The truck would start shuddering with the noise and vibration definitely coming from the turbo charger area. The noise had a tinny sound, like a piece of sheet metal slapping around. As I got off the gas, It would go away.
Under steady throttle it works fine, just like it should. But unsteady throttle under load it sounds like maybe the waste gate starts slamming open and closed (?) Any thoughts?

 

Thats compressor surge. Real hard on thrust bearings and the shaft, makes turbos disappear. Drop out of OD when that happens, get the RPM's up you'll be fine.

 

Yup.. My smaller S300 would do that at anything under the 1500 Rpm mark with a hard load. With the fuel plate its getting way more fuel than it used to and the turbo can't make boost without surging at that speed.