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If it is efi, it just became a primary requirement. Now you can’t have it fail to the detriment of safety.
Should not be...
Let's define terms. Open loop mode is where the ECU is calibrated to give your mixture based upon operating variables - sensor inputs for engine speed, air flow, etc. Open loop is not tuned as you operate.
Closed loop starts with and open loop mode programmed to be a safe for all engines, then it learns your engine and adjusts both the base programming and adjusts mixture in real time to keep the mixture right on the desired level, usually right at stoichometric.
Most of the EFI systems for airplanes are open loop - with them, the fuel dose based on operating variables is established and programmed using the EGO and EHT, but is not dependent upon the EGO in normal use.
Now if you run closed loop, the ECU is adjusting fuel dose to get a certain mixture during less than WOT with feedback from the EGO. These systems usually usually go open loop at WOT. They should have programming to identify EGO faults and revert to open loop mode, not a limp home mode. This is why carrying over the factory ECU can be a bad scheme.
Some ECU systems that are closed loop on EGO inputs were not programmed to identify EGO faults. What they should do is recognize the faults and revert to open loop programming. If they do not identify EGO faults, they chase the mixture to either dead rich or dead lean trying to bring the EGO back to neutral. Bad.... Most automotive systems now have EGO fault detection, but they may not immediately recognize the fault and get into open loop programming. I really do not like the idea of trusting the programming to recognize the fault or of it taking 500 engine turns before changing over, particularly if I am at 50 feet on takeoff or just pushed on the throttle for a downdraft on short final...
And the EGO in open loop mode is just a gage helping you know your mixture...
Billski