In addition to IAC valves as a whole, another issue you might notice with one that fails or causes your 1994 Lexus LS400’s idle to jump is a leak in their corresponding air lines, typically made of rubber.
If these develop tiny gaps or cracks or lose a seal, excess air makes its way into your LS400’s fuel-air mixture and throws off fuel atomization, affecting your RPMs in turn.
As such, visually inspect air hoses to and from your idling control, and replace any that demonstrate holes.
