Originally Posted by Al C.
I have an open RO with GM for the same problem.. Not enough vacuum to close them at low RMP's. GM engineer as tried to fix but they havent been able to. They have tried bigger vacuum boxes, it helps but doesnt close them completely. Car has been in the shop for 3 weeks now. Do you have an automatic? GM said it only happens with auto's. Last I heard they were going to order a new complete exhaust system from a 6speed manual to see if that would fix it, from the manifold to exhaust tips.
Mines a 6 speed manual and does it too.