Sounds like you were able to get your IDLE CO mixture dialed in fairly well. Closed loop 45% - % 60 fluctuating is a well adjusted reading.

I would suspect your EHA valve might be bad. Mine did the same thing. I got the idle mixture adjusted, then when I rev to 2500rpm the duty cycle slowly maxed out / went to zero. The EHA is primarily what affects mixture above 2500rpm. A good functioning EHA will ideally keep the mixture in the same 45% - 60% range that you have at idle. I just swapped a used EHA into my car and it fixed that problem immediately. You can adjust the EHA, but it is pretty pointless. The stock setting should not be messed with.

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1991 300E - 212K and rising fast...