All of a sudden the other day i noticed that my idle is running lower then normal. it is running about 600rpms. I surges once in a while but basically it just sits low. I was thinking my TPS might be going bad.
I am 99% sure the TPS has nothing to do with it, your idle screw might just be loose. Just turn the idle screw some to raise the idle. (If you dont know what an idle screw is its a screw by the throttle lever on the TB that rotates the lever by pushing on it, so the more the screw is sticking out the more air gets through the TB => same as pressing gas pedal => higher idle)
The idle speed is controlled by the computer - it attempts to control to a preset 672 rpms with the IAB solenoid which is capable of letting more/less air bypass the throttle plate in response to what the computer senses and tells it to do. The screw referred to above is a throttle stop, not an idle adjust screw. Ford never intended for the idle speed to be changed or adjusted.
First thing I'd do is pull the diagnostic codes and see if the computer is telling you what the problem might be. If you don't have a scanner/don't know how, go to Autozone or PepBoys and they'll usually pull them for you. Just a guess - more than likely, your IAB solenoid and/or throttle body is clogged up/dirty. I'd pull them both off and give them a thorough cleaning. Then reassemble, disconnect the battery for a while to clear any memory the computer has of trying to cope with the issue, and restart the engine. See what you've got then.
If you decide to simply alter the position of the throttle stop by turning the screw, that will alter your idle-tps setting. You should check it after you turn the screw. It should be between .85V-.95V with the engine warm and the ignition on/engine off. That's measured between the black and green wire of the tps. However, I wouldn't just turn the screw. As often as not that can make the problem/symptoms even worse.
Look behind the driver's side Strut tower - between the back of it and the firewall. The access plug for diagnostics connects to the wiring harness in that vicinity.
I am using the stock tach so yes it prob is off. But i still have a problem. The idle drops just enough that the computer carches it and surges up again. I just need to find the service port so tomorrow i can check it for codes.
Mine idles at about 900rpms because I used the throttlestop to be that way, otherwise it will surge and idle really rough, but then again that have to do with the cam.