First off, I'm new to this board. I've read through some older and very helpful posts regarding idle.

My truck is a 93 F-150 302 5.0L 4x4. 150,000mi.
Start up runs fine in all gears. After driving and the engine warms up, when put in Neutral or Park the idle fluxuates between 1000rpm and 500rpm. It doesn't stall but just constantly goes up and down until it is put back in gear. In Drive and Reverse it runs fine.

I have already replaced the TPS, IAC, and O2 sensor.

Any ideas on what to check next?

 

Dirty/faulty EGR, vacuum leak, dirty throttle body, coolant sensor...

You should start off by trying to pull any codes. After that, get a good manual, like Haynes, and a decent multimeter (and maybe a bottle of propane to find vacuum leaks) and go to town. Run a search on (+surging +idle) on this board, you'll get lots of pointers on where to start. I like this post on the subject:

surging idle

Keep us informed of your progress, and I'm sure other members of the board can guide you in the right direction. Good luck...

 

I just wanted to post a follow up to the surging problem sicne it is fixed. Hopefully it might help someone.


The good news is, the surging on my truck has stopped but I'm not certain what caused it to stop as I have replaced and adjusted several components.
Here's the history.
Truck was stalling and the computer gave a code for the O2 sensor which was replaced. After that the surging began.
The computer was not giving any problem codes for the surging. I replaced (in this order) the O2 sensor, throttle position sensor, and idle air control valve, coolant temp. sensor. Truck ran better but still had surging. Drove it for a week with the surging then for the heck of it replaced the plugs, wires, rotor, and dist. cap. That helped some but there was still slight surging and it was driving me nuts because I couldn't find the cause of it. I kept driving for several months with the surging since it only occured in Park and Nuetral it didn't really affect the performance but I still wanted it fix it.
One day truck started running like crap and one day it would not start. Turned out to be the fuel pressure regulator. Pressure was too high; getting too much gas. Replaced that and the surging was very minor and over the next couple days it stopped and hasn't surged since. I'm not sure if it was the fuel pressure regulator but the surging stopped soon after replacing it.

 

Dano1,
I'd have to lay odds on it being the FPR to begin with...I just went through this myself.
Go read through my posts..."'87 F150 rough idle, dies in gear".
It was a mess and took me like 2 weeks to get to the source...but she now runs better than she has since I first bought it thanks to MemOrex and Popa Tim's help.
Rory/Roddey