Truck is a 2003 , Screw , 5.4 engine with about 78,000 miles on it . Problem is when you take it out of Drive and go to Park or Neutral , the engine rpm drops to about 400 , then surges to about 700 , back to about 500 , then about 650 and then settles out . Problem doesn`t exist when the AC is on or the temp control is on Automatic . Also happens if you take your foot off the gas quickly at any load when coming to a stop . Going from the Automatic temp mode to vent causes the same problem .
Just checked and no codes are showing .... tried Seafoam in a tank of gas , then Chevron Fuel system cleaner in the next tank ... no go .
Starting to drive me crazy .... any and all suggestions will be appreciated !

Bob

 

I've had this exact same problem on my Mustang. The first thing I did was check for vacuum leaks. None found. Then I started swapped sensors with known good parts (MAF sensor, ThrottlePositionSensor). No change.

I noticed that I can make the "bouncing idle" problem go away for a while by restarting the car. This makes me believe that the PCM is getting a bad baseline value for air or fuel at initial startup and that the PCM is trying to compensate, hence the crazy idle. Right now I'm thinking of taking a look at the underhood fuel pressure sensor/regulator on the Mustang. It pulls its reference vacuum from the manifold and compares it to fuel pressure. I just have to figure out how to test that part.

So, long story short: I don't know the cause, but I feel your pain. If you recently changed spark plugs/coilpacks or did something to jolt a ground wire, you might take a look there.

 

I'd start by cleaning your throttle body, and possibly sea-foam the engine. I think the TB cleaning would help a lot in this case.

 

Fuel Filter probably needs to be changed.....