I have a 91 toyota 4x4 deluxe with 22re 5 speed in it. When I first purchased it the engine light was on, after looking around I noticed the o2 sensor in the exhaust pre cat had been broken off, so someone just unplugged it. I ordered the sensor and installed it plugged it in and the enigine light was gone. I noticed that my mpg went from almost 17 mpg down to 13 mpg when I plugged it in. I unplugged it to get better mpg but it has very little power in 3rd, 4th, and 5th unless the pedal is all the way to the floor. Any ideas?

 

This is more spitballing than actual experience....

I'm pretty sure that without the O2 sensor the ECU is attempting to run in 'safe mode' which has a pretty low performance fuel/acceleration curve. When you plug in your o2 sensor, the ECU is now getting a signal that it needs to alter the fuel/air ratio. Since you're using more fuel it could mean a few things. Off the top of my head, you're getting more power, that costs fuel. Should it cost 4mpg? I'm not sure. Another explanation is you could have a plugged cat/leaky exhaust, and the O2 sensor is picking up more oxygen than it should be picking up. (edit: I just noticed you said your o2 sensor was pre-cat, so ignore my second theory). it is possible the replacement 02 sensor is bad, you might want to try to get a hold of a known good spare to test with before buying a new one.

Beyond those ideas, the ECU has a whole host of emissions controls that depend on the o2 sensor and makes changes to the running characteristics accordingly.

I'm sorry to say that there isn't likely an 'easy' fix, you're probably going to have to really dig through your truck to get it straightened out.

 

What will happen if I just cut the cat off?

 

Your car is going to be really loud and you won't pass smog... wait. Do you have smog? sorry i live in california.

When was the last time a basic tune up was done. Plugs, wires, dizzy

 

First of all, reset the ECU.
Unplug the battery and turn the key.
Plug in the battery and start as normal. This is the quickest way to reset the ECU.

Drive it around. A couple trips around will register a code, if any. Some check engine codes need to be registered twice during separate trips to give you a check engine light.

Check the code. Use the paperclip trick. Plug a piece of wire or paperclip in the TE1 and E1 terminals of the diagnostics port (little grey box under the hood, near the fuse box). Turn the key to on. Count the flashes. Check the FSM (field service manual) online to see exactly how to do this and what the codes are.

Check and diagnose from there. Best of luck. It's really hard to troubleshoot a vehicle over the internet.



P.S: It's a 22re, it made 110HP from the factory 21 years ago, it's gonna be slow and gutless.