I've got two EEC-IV vehicles that do this. (My 89 turbo sho and my 88 Bronco). Both tuned with the Quarterhorse, have siemens deka 60lb injectors, larger than stock mafs, and both don't have oxygens sensors and are always running in open loop. For the first 5 or so minutes after start, they lean out to around 20:1 at part throttle between 2000-4000 rpm. Once they warm up its fine and behave normally. Not sure what the issue and haven't had much luck figuring it out.