Were was your o2 sensor on your d100
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IDK about 1G pickups, but there might be an upstream and downstream sensor, like on later models. An upstream sensor should be somewhere between the exhaust manifold and catalytic converter..again, on later models, there is one for each cylinder bank-maybe the same on 1G V-8s. Downstream sensors are after the catalytic converter.
I believe I saw in your other post you have stock exhaust manifolds. Start looking on them down to the catalytic converter for either something with wires or see if there's been some part of the exhaust replaced. Sometimes people would screw in a pipe plug when eliminating sensors.