Is there a way to adjust the idle on a 1991 5.0? The idle control seems to be working, cold starts idle higher and then come down as the truck warms up, and the engine purrs like a kitten. I'm just curious if there is a way to make an adjustment.

 

Sounds like its working properly. Why would you want to adjust it?

 

X2 ^^^

No, the idle screw is factory set and not to be adjusted in the field.
If you mess with it you can cause a lot of problems and it it is very hard to get it back to factory setting.

 

There is and then again there aint. You not suppose to move the idle screw, but Ford Racing had instructions on how to adjust the idle, if you have mods, that involves the idle screw.

I had to adjust my idle like Ford Racing said because I put on bigger heads and a small cam. So it would idle properly.

 

I have some mild work done to the engine, that is why I am wanting to screw with the idle. When I got the truck it was idling very weak and sometimes cutting off, i am thinking someone may have "made adjustments" to try to make it run better. Has anyone seen that some throttle bodies have holes in the throttle plates and some dont? I have the one with the holes. I am assuming this is the right setup for my year - anyone know for sure?

 

If your truck is idling so low its cutting off, you can try this.

Its just best not to mess with the idle screw unless you've just tryed everything else.

 

I am usually the dissenting opinion on the idle screw on these boards. I disagree it is some sacred not-to-be touched thing that only the factory can set. I say this because I owned a 1986 T-bird Turbo Coupe that had what Ford called a "base idle" setting of 400 rpm right on the emissions sticker and you acheived this by disconnecting the IAC valve and then adjusting the screw. It even gave you the procedure right there on the sticker. So I have always referred to it as the "base idle" screw. It is my opinion that this was the original intent by Ford but too many folks monkeyed with it to correct other problems that Ford decided it would be better if the owners did not mess with it (and in most cases they are probably right).

So I say it's your truck do what you want. If your truck won't idle with the IAC disconnected, then jack up the idle screw until it will, probably should be set at about 500 rpm. Then hook your IAC back up and go from there. This, of course, assumes you do not have other problems that are causing the idle to be abnormally low.

 

it's been a while since i messed with fuel injection on these trucks but if there is a TPS (throttle position sensor) then you have to hook up a multimeter and read the voltage coming out of the sensor (should be less that 5v) then adjust your idle and oval out the holes on the TPS so it can be turned the same amount that you adjusted the throttle screw (so that the voltage reading before was the same as after you adjusted it)

 

You shouldn't have to "oval out the holes" on the TPS, the holes are slotted so it can be adjusted.

 

The problem with adjusting the screws is when you try to drop the idle down. The screws are set just above the throttle plates sticking closed and that is what they are trying to avoid.
The voltage from the TPS to the computer is not a very critical value. It can be anywhere from 0.6-1.0 volts. When the engine is started each time the computer looks at the this valve from the TPS and takes it as Closed Throttle value.
This is why you should never touch the throttle while starting the engine.

The computer only has three steps for the TPS CT, PT and FT.
These are the only values in the computer program the computer has to work with.

The other values are done by math in the computer to find these two other steps.
Part Throttle is triggered @ 0.04 volts above Closed Throttle.
Full Throttle is triggered @ 2.71 volts above Closed Throttle.

Whoever wrote the Ford Racing thing above it sounds like he does not know how the computer works.
1. The computer relearns the TPS value each time it is started. No need to disconnect the battery. Only needed for changing out the other sensors.
2. No need to make the holes bigger to set to that voltage as the valve the computer needs is larger than that.
3. All you gain by doing it his way is upping the idle speed if it dies from modification.

For more information on the TPS go to the link below:
Fuel Injection Technical Library » Throttle Position Sensor (TPS)