Your 99 does not have a sensor near the back of the head. Any leak before the throttle body is irrelevant..you can undo the 3 inch hose on your airbox and the engine will never know..any leak that varies idle speeds must come from downstream of the actual throttle plate or be a computer controlled variation

 

tried checking for vacuum leaks again and still can't find anything. i have no idea what's causing the rough idle lol.

 

Ok, assuming your manifolds are not the cause and already did tune up, cleaned IAC, adjusted throttle cable, replaced TPS....
Next step would be cleaning throttle body, especially where plate sits, measure TPS output to verify its correct position at idle.

(​​​​​​CPS and Cam are a possibility too, since they are involved in engine running management, although I think they would be less likely with your symptoms)

 

Bad exhaust manifold? I recently had a Dorman exhaust manifold that was welded very poorly internally and caused uneven and poor airflow. Just a thought.

 

I cleaned the throttle body and replaced the gasket, still no change.

how can I measure TPS output? with a multimeter?

thanks for the help!

 

Yep

Can't access the notes I saved somewhere, but if you do a search on the TPS testing procedure, you will find it for sure.

 

SOLVED: turned out to be the upstream O2 sensor. put in a new NTK one and the problem is fixed. thanks to everyone for the advice!

 

Glad you fixed it
How did you come to find the O2 being the cause?

Thanks for getting back to the thread
Closure is good