We have a 1991 4Runner 22re 4x4. Just bought the truck the previous owner says he thought it was running rich. We replaced the AFM, TPS catalytic converter and O2 sensor because of multiple codes. We now only have have code 25 (lean). We did not have this prior to parts replacement. There is black soot coming out the exhaust and we can smell fuel. O2 sensor is Denso 234-4056. After engine is warmed up, OX to E1 port reads about .2 volts and keeps dropping slowly. Revving engine does not affect output voltage. We removed the O2 sensor and it is very black after only a few minutes of running. Tested sensor at the sensor connector and verified .2 volts and dropping. Tested O2 sensor on bench by applying heat with propane torch. Could not get voltage above .4. I should see a rapid .1 to .9 swing as heat is applied and removed. Also, VF to E1 voltage shows 0. Replaced O2 sensor again and had the same results. Wiring has been rung out. Tested cold start injector and it is not statically leaking. How can the O2 sensor say I am lean when it is actually rich??

 

Welcome to YotaTech. Kudos on such careful testing (and reporting).

First, I'd look for an exhaust leak. The air leaking into the exhaust causes the O2 sensor to (correctly) see "lean," and it commands the ECM to enrichen. The ECM enrichens as far as it can, but the O2 sensor is still seeing leaked air,and continues to report "lean," even though the engine is running VERY rich.

I'd check for 12v on the O2 sensor heater circuit. If the heater isn't working, you might not be getting to closed loop during the time of your testing.

The manual suggests racing the engine (2500 rpm for 90 seconds, then during the measurement) to assure the sensor is hot enough. Mine will stay in closed loop at idle (at least for a while), but the manual gives that instruction for a reason.