Originally Posted by Carcrazygts2

The ac will cause a surge from 600-800 only. If its going above 1200 rpms, you could possibly have 2 issues. One could be a vacuum leak. Most commonly from the large PCV elbow on the back of the upper intake manifold. The second thing would be a dirty throttle body, and it leads to a sticking throttle body, and a sticking idle air control valve.

Originally Posted by Rayna

I have a 98 f150 that when stopped lets say at a red light or just idling the RPMS rev up and then drop and then rev up and then drop...what is wrong!?!? Please help and if I let off the break a little bit I nudge forward more than I would
Like to.

Originally Posted by 1997FordF150XLT4.6V8

I have a feeling you have the same problem my 97 with the 4.6L had. It would also surge while in park or at a stop light. Picked up an a/c recharge kit from autozone and she took the whole bottle and never had a problem since. I believe the kit was around 37 bucks. Good luck -Zach

Originally Posted by Carcrazygts2

The ac will cause a surge from 600-800 only. If its going above 1200 rpms, you could possibly have 2 issues. One could be a vacuum leak. Most commonly from the large PCV elbow on the back of the upper intake manifold. The second thing would be a dirty throttle body, and it leads to a sticking throttle body, and a sticking idle air control valve.

Originally Posted by 1997FordF150XLT4.6V8

I have a feeling you have the same problem my 97 with the 4.6L had. It would also surge while in park or at a stop light. Picked up an a/c recharge kit from autozone and she took the whole bottle and never had a problem since. I believe the kit was around 37 bucks. Good luck -Zach