I have a 91 4Runner that began running a bit sluggish and seemed to skip a little accelerating from a stop. Timing belt was changed not to long ago. I am getting codes 12 and 25. Wondering if I should try an new 02 sensor or if I am on the wrong track. Home on a break from the military overseas and hoping to get this thing back to normal shortly so I will have transportation.

 

Start with the code 12, w/o the RPM signal the computer doesn't know to fire the injectors so your code 25 might be a phantom problem.

 

http://personal.utulsa.edu/~nathan-b...85diagnosi.pdf

Code 12 is an electrical issue or a bad distributor. I had my distributor go bad, when it does the truck won't run. Must be on its way out. Or, it could be your timing is way off, that could possibly also throw a code 25 (lean air/fuel ratio).

Code 25 is usually just the O2 sensor. If you want to be sure, grab a multimeter and test it per the FSM specs. Just unplug the sensor and measure the resistance (ohms). If it doesn't match up, replace it.

If that is not your issue, post again. I've done just about every diagnosis possible for a code 25 (chased mine for 5 months). We'll figure it out.

 

So I am measuring the resistance at the o2 sensor? I have used a multimeter before so I should be able to pull this off. Are there more detailed directions on where/how to measure?

 

http://personal.utulsa.edu/~nathan-b...12onvehicl.pdf

Page Ig-23 Step #5. Start with inspecting the sensor coils are in spec resistance, then make sure they are properly adjusted (Air gap), then make sure the plug is clean and has a firm connection (clean the male spade, and tighten the female side).

Once you have proper ignition signals you can move onto the oxygen sensor.
http://personal.utulsa.edu/~nathan-b...01heatedox.pdf

 

Save the link below to your favorites. It's the entire Factory Service Manual for a 93 pickup, but it works on 4Runners and Pickups of different years (the engines didn't change, only the interior has discrepancies).

http://personal.utulsa.edu/~nathan-buchanan/93fsm/

Here's what you are looking for about the O2 sensor:

http://personal.utulsa.edu/~nathan-b...01heatedox.pdf

Note that this only tests the heater circuit. There's other driving tests you can do under the MFI section under Diagnosis, but I've found they are hard to do, wouldn't work for me when I had a code 25.

 

Thanks for all this great information. I got called back overseas temporarily but I'm back now and ready to dig into this. I will keep you posted.

 

Wow, what a journey this has been. Thanks to everyone for their help. I spent so much time and money to no avail on one of my BMW's I decided I didn't have time to get too involved with the 4Runner in the limited time I had. Spent about $1000 on BMW parts that I did myself none of which fixed the problem. Took the Toyota in and all it was was a couple of bad plug wired that had melted on this metal tube thing that was beside them. The 4runner runs better than when I got it. Such a big difference. I actually believe this may have been building since I got it seeing how smooth it accelerates now. I wish I would have spent my time on this and took the BMW to the garage now.