Because the hego isnt responding to the computer in closed loop its unable to correct the A/F ratio, so its being forced into open loop to run off the default tables would be my guess.
Hi ive got a 90 fox mustang 5.0 that i chucked a turbo on late last year. It didnt seem to be 100% before i turbo'd it and surely was 100% worse with the turbo on. Tried just about everything and like you left the IAC to last, not realising that these things have huge amounts of carbon going...
