Yes, it is the air injection valve. But, I don't like this as a possible cause to our problems either. It's not enough for the valve to be stuck open in my opinion. For air to enter the exhaust system and lean out the exhaust, more things have to happen at the same time. The clutch on the air pump has to be energized so the pump turns, and the switchover valve (another valve between the air pump and the air valves) has to be opened. Now air can be pumped through the opened air injection valve. Perhaps it is possible for air to be drawn into the exhaust if you have a break in the air supply line between the switchover valve and the air injection valves, and I suppose this should be ruled out as a problem, too. But, overall I just don't think that is it. Just my thoughts.
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QUESTION: How does one test for proper Air Valve operation? More to the point, how does one remove the valve from the engine (my R129 Repair DVD is unavailable)?
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I don't think you need to check operation. The car does it for you regularly. Well, it checks that air injection occurs when commanded. The computer cannot specifically identify a stuck open valve. It would be inseparable diagnostically from all the other vacuum leaks causing lean running, as you surmize. I think you're right that you'd have to remove it from the cylinder head and apply a vacuum to it and see if you can blow through it. And check that you can't blow through it when the vacuum is removed. I haven't looked closely enough to see how it is removed. I will look.
Brett