1991 p30 7.4 code 43

[Donald Keeney II]

I have been chasing a code 43 in 1991 chevy p30 with tbi 454. I have replaced knock sensor checked wiring to ecm all good! Even get voltage jump at ecm if tapping on block. Replaced ignition module, ecm, knock sensor. Sets code within 30 sec of startup. I'm at a loss and where to go next! Timing set right fired right up no miss or noises at all