Interesting that someone should have disconnected the AIS plug. Unless the AIS valve and plumbing has been removed completely and been blanked off a the exhaust ports it's a pretty pointless exercise.

Unplugging the AIS connector does not disable the AIS as such and certainly won't stop the backfiring on the overrun - it'll make it worse. All that unplugging it does is leave the AIS valve permanently open allowing air into the exhaust ports downstream of the valves all the time. It cannot close unless there is power to it.

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