I assume no one has had issues with this recently since not responses and search brings this stuff up from back around 2002.

Anyhow, for whatever it is worth, I have narrowed it down to the check valve vacuum controller (located on the radiator shroud). The solenoid appears to have jammed open allowing vacuum to be appled to the check valve 100% of the time. Exhaust backflows to the air pump in much the same manner as if the check valve were stuck open. I have disconnected the vacuum controller, plugged the vacuum line and removed the relay for the air pump. the local dealer ordered the controller ($85) and said this is the first one of those they have ever had to order.

BTW, I tested the relay (late 98 relays are in the main fuse box as relay #2, a Hella brand typical bosch type relay) applying current to pin 86 and ground to pin 85 gives off the audible click of the contact plate and continuity is achieved across the #30 and #87 contacts.