I'm just getting into logging/tuning etc. and have a question about the O2 sensor position and AIR injection. I've got a '91 3.1 but the question may apply to any set up. The AIR pump injects air into the exhaust manifold ahead of the O2 sensor so does this not 'corrupt' what the sensor sees? The ecm doesn't control the AIR pump so doesn't know when it's injecting air so doesn't it always have to play catch up trying to adjust for the extra air? It seems like it would be better for the air to be injected after the sensor or directly into the cat. The cat has what looks like an input tube but it's sealed off.

 

The AIR system only injects air upstream of the O2 sensor on a cold engine. It's stopped long before the ECM begins using the sensor and only injects into the cat, if at all.

 

The ECM does control the AIR pump in the '90-'92 3.1l f-body's. The pump air is only sent to the exhaust port when the engine is cold, under 61* F (in the AZTY calibration). So during the Summer it is always in divert mode (dump to atmo).

The ECM won't go closed loop until the engine is at 144* F. Which is well above the divert enable threshold for the AIR system.

The vacuum line to the AIR pump valve is to be sure that it goes into divert mode on decel. Otherwise it will cause backfires in the exhaust.

RBob.