i just bought a 91 w150 4x4 w/a 318 auto trans.dose anyone know the location of the oxeygen sensor. not in manifolds or y-pipe anywhere i can see. just a tube off back of each manifold that goes behind motor somewhere. its running rich so i would like to find this and replace it.not sure if original y-pipe. previous owner might have eliminated sensor???

I have an 88 D100, and I'm not sure if it applies, but mine was behind the Y-pipe, a few inches back. Since have removed cat and installed duals so I can't remember exactly how far back.

did you reinstall the oxygen sensor with new exhaust or is yours not computer controlled anymore? i plan on putting headers and duals on but would like to keep the fuel injection so i at least need to find the oxygen sensor wire!!!

From what I understand, the oxygen sensor is right where Bama said it was. And I have been told that when you take off the cat, all you have to do is tap the pipe, and put the sensor in the hole.

Yep, all I did was tap one of the two pipes (true duals) and inserted the sensor. I went ahead and installed a new sensor for new measure, and yes, sadly, my truck if still comp. controlled fuel injection.

That's the way to do it, for sure.

thanks guys. i found it on manifold just out of sight.