2001 Ranger XLT 4.0, Auto Tansmission 140,000 miles
Runs/idles fine in P, N,D, 2,1. When in reverse the idle drops to about 200-450 and it surges and drops low when my foot is on the brake. If I reverse up a small hill or add additional load, it stalls. It runs perfectly fine when cold, the low idle/stalling only comes when the engine is warm.
I have replaced : MAF, IAC, intake manifold seals, DPFE, EGR solenoid, coolant temp sensor, cam position sensor, spark plugs, coil pack
I have found no vacuum leaks in multiple tries with propane.
I have removed and replaced (not with a different one) the transmission position sensor.
I have tested the EGR system following these steps. https://easyautodiagnostics.com/ford...e-egr-system-1 Everything checked out fine, except I was not able to record a vacuum being pulled from the solenoid (Test 6) even when I had replaced the solenoid with two different ones (one new and one from a parts truck that runs great).

I'm not sure if it is related, but I hear what sounds like air hiss with each engine revolution on the passenger side of the engine bay. I have tried to pinpoint with a metal rod in my ear to no avail. The sound is barely noticeable at idle in park, neutral or a forward drive gear, but it gets a lot louder while the engine revs low in reverse.

Any ideas would be very much appreciated.
Thank you

 

After some more messing around, I have found a solution. I figured I would put it here in case someone else has similar problems.

I replaced the transmission valve body with a kit from Transmission Hard Parts, and that fixed the issues with reverse. I am not sure if it was the solenoids, the valve body, valves, plate ect. that needed replacing, since I did everything together. My old gaskets looked to be in fine shape. It came with the upgraded shift kit from Superior, which does provide notably quicker shifts.
It wasn't too bad of a job for an amateur like me - it was mostly just messy.

I was a bit skeptical going into it because I was not having the classic symptoms that others described. Everything in forward gears and neutral was fine, it only happened when warm.

In this process I also got the Blue Driver code reader. If you have a smartphone to connect to, it is the best code reader I have used (not that I have used a bunch). It reads all types of codes - ABS, transmission, airbag, engine... - and the interface on your phone is easy to use. At $99 it seems like a good buy.