Roger that on the 450-500mv being normal, but that is for criuse - part throttle conditions. Under load, the O2 readings should be up around the 750-850mv range. The EFI folks refer to this as PE or Power Enrichment mode. You need the richened mixture under load to keep exh-temps reasonable, as well as to make good power. To carb folks that's main jet territory. Since you'll probably be using the one-wire (non-heated) sensor, get it as close to the heat as possible. It needs to be hot to work. More than two feet from the exhaust manifold and it will be marginal IMO. If you can't, there is a more expensive 4-wire heated sensor available from GM. The two white wires are for the 12v heater circuit amd the others are signal and signal ground. Application for the heated sensor is 91-92 GMC Syclone & Typhoon Vin "Z" Turbo 4.3 Trucks. There was a NON-TURBO VIN "Z" 4.3 engine that used a single wire sensor, so inspect it when they bring it out. GM & Napa have them. Purists will tell you that the costlier aftermarket wide-band systems are needed, but I disagree. Narrow band will get you in the ball park. mv
