It has two, one in front of the catalytic converter and one behind it. The one in front (closest to the engine) affects gas mileage and performance. The second one monitors the condition of the catalytic converter.
You can shift to 4wd (4 hi) at any time below 80 km/h.
2, Before & after the catalytic converter.
Here is a web site that I saw that will help you on removing and replacing the starter on a 1999 Toyota 4Runner: http://www.4crawler.com/4x4/CheapTricks/Starter.shtml
Stop idiling? Do you mean surge? It's likely to be either the Mass air flow sensor or one of the O2 sensors.
According to fueleconomy.gov: A 2WD Toyota 4Runner can get 16 MPG City and 19 MPG Highway. A 4WD Toyota 4Runner can get 14 MPG City and 17 MPG Highway.
The curb weight of a 4Runner is around 4600 lbs. Need the year and if it's a 4wd to be more specific.
Remove it and bring to an auto parts store that is equipped to test it for continuity, amperage draw, drag, etc.
mine is not a sr5, which should be the same, has 1
There are 4 of them. 2 Up streams and 2 down streams.
Limited 4WD - 4L 236hp @5200 RPM V6 Limited 4WD - 4.7L 260hp @5400 RPM V8
The throttle position sensor on a 1999 Toyota 4Runner is located by the throttle body. It will be mounted near the throttle cable.
check your owners book