Toyota Repair: 1994 Toyota 4 runner 3.0 EFI, vacuum leak, head gaskets

Question
Good morning
The engine started to act like it has a vacuum leak when it is hot and is idleing.
When I did the check on the air valve adjustment screw on the intake when I screwed the adjustment in with the engine hot it tried to quit. I am unable to locate the air valve on the engine to do the repairs. Can you give me some direction?
Also,after I started looking for the vacuum leak I got a check engine light for code 25. Is there a check I can do on the O2 sensor?
The engine has 125000 miles and the sensor has not been changed.
Do you have any other suggestions on things to check.
The head gaskets gave out and the valves and heads were redone about 10000 miles ago. This is the first problem I have had with it since that work.
Thank you for any directions you can give me.

Answer
The code 25 indicates a lean condition and in most cases the problem is not the sensor but the cause of the engine misfire, this could be caused by a vacuum leak or a malfuctioning cylinder, try to isolate which cylinder is causing the problem, this can also be caused by a tight valve.