My bad. I seem to be full of misinformation. I've looked at so many diagrams in the last few days, I'm getting my wires crossed. There is no additional valve between the air pump and the air injection valve. The switchover valve is the device that routes vacuum to the injection valve to open it up. So, if an air injection valve is stuck open, perhaps it would be possible for air to passively get drawn into the exhaust stream, but it would have to get sucked through the non-running pump mechanism, unless as I mentioned before, there is a break in the air supply lines between the pump and the injection valves.

Brett

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1997 S600 sedan