Have two issues with my 08 X GSR that I'm gonna to have checked out at the dealership.

1st. Overboosting

At first, I thought it was a cold weather issue, but I've been able to recreate it at much milder temperatures than when I first noticed it. Checked hoses for cracks, checked also to see if the boost pill was OEM (it is). Figure it's either the WG actuator or one of the solenoids. Have P0234 CEL (turbocharger overboost/wastegate malfunction) load hitting 245 (car is all stock except for my FMIC and UICP).

2nd. Rear diff/AYC noise.

I hear a noise coming from my rear end, only at WOT, kind of a constant thumping sound. It changes frequency depending on speed. It's not during gear shifts, but when I'm at full throttle power.

Any ideas of what I'm looking at before I go in to the dealer?

 

Not sure about your overboosting, but I'd drain the fluids out of your diff to see if there's metal chunks in there or something. Metal shavings are fine, it's chunks you need to worry about.

 

Is the thumping kinda like a vibration that coincides with RPM and speed? Are you lowered?

 

Good call, I'll have that done. Should be changed soon anyways, since I bought the car used and I don't know when it was last done.

Exactly that. Not lowered. Car is bone stock except for the FMIC.

 

Huh, not sure then. Good luck and update the thread when you find anything out.

 

Update. Overboost seems to have been caused by the wastegate actuator sticking. Applying pressure to the actuator and lots of penetrating fluid seem to have cleared that up. The actuator is supposed to move at ~14.5 psi, but it took ~16psi to move it the first time we tried. After applying pressure a few times and using the penetrating fluid, the actuator now moves at the normal pressure, and also moves much more smoothly than it did before. I'm now unable to cause the car to hit overboost fuel cut, and the CEL won't come back on. Will see if it stays that way.

Rear diff is another story. Found lots of shavings and tiny metal chunks in the fluid, looks like both brass and steel. Will run with new fluid for a couple weeks to see what happens, but diff will most likely need to be changed or rebuilt.