I bought my 91 p/u with 20,000 miles on a crate engine and at first I didn't think about the power I just assumed it didn't have much any way but the hesitation and power loss has gotten worse it seems or I have just started noticing it so I checked the codes and it's thr owing an o2 sensor code and I got to looking and it dosent have an o2 sensor at all on the exhaust. The place where most o2 sensors are where the pipes come together at the header theres not a whole at all for an o2 sensor and I have no idea where the wiring comes from the computer to go to it. Any help would be appreciated. Thanks

 

You might have a little parts mix going on. A desmogged 22r header won't have an o2 bung at all. You can have one welded on. There are 2 versions of sensor, make sure you get the one with a longer wire.

 

Do you know where the wiring goes into the computer at or where it should be and will my truck need 2 sensors to run right one at header and one behind exhaust?

 

Behind cat I mean

 

I've got an 85 so my knowledge is limited but as far as I've seen, our trucks only need one pre-cat sensor.

My truck has the o2 connector harness running along the driver's side inner fender, just behind the coil/igniter. That's an 85, though. My ecu might be different than yours, but here's a shot of my fsm: Ox is almost right in the middle, bottom row

 

Mine wasn't there does anybody know where they hook into the computer for the later model efi

 

Your model (91) should have a single sensor, in the intermediate pipe (between the manifold and cat). It's wire exits the main engine harness between the upper and lower intake manifold and runs over the top of the transmission.

The local Toyota dealer can provide you with the pin map for the computer if you need or want to rewire it completely, at which point it doesn't matter where you run it as long as it is pre cat.

 

The second O2 sensor (downstream of the Catalytic converter) is on California-spec vehicles only. (It monitors the effectiveness of the Cat.) If you had a California ECU, missing that sensor should through code 27.

I assume your "crate" engine is EFI (if carbureted, you have the wrong ECU). Without an O2 sensor, it would never reach closed loop operation, and would run terribly. I also assume it had a CEL from when you bought it; did you ask about that?