I have a question that I hope someone can answer. I just replaced the MAF sensor a week ago and it took care of the stuttering and stalling issues. My question is about the adjustment screw on the throttle body, I messed with it a while back trying to raise the idle a little bit when the A/C was on and now that I have replaced the MAF and replaced the plugs, wires and coils it runs good but I know the idle should be around 850rpms but it is at about 650ish now and will drop lower after idling for a few minutes. Is there a set point for this screw to be? I know it relates to the position of the throttle plate when it is fully closed.

 

That screw is not for adjusting the idle. Its to tune the TPS sensor and yes it has a specific point, been a long time since I fiddled with one but I believe you need to run a volt meter on the tps sensor to get it back in to calibration. Can't remeber the proper voltage at 0% throttle. But youll need to reset that first.

The IAC valve controls the idle, there is no "adjustment" the computer does all the work providing the IAC and TPS are functioning. Most likely you need to clean or replace your IAC.

 

I just went out and checked for any codes and it showed P0420 and P0171 and both the STFT & LTFT are both sky high (+32 and +25) .and the scanner is showing the throttle at 18% at idle.

 

The screw on the throttle linkage is called the anti-diesel screw, and its pretty easy to set

First you have to warm up the engine
Fuel injected engines can't use an idle screw, they must use an Air Valve, Ford calls theirs the IAC(idle air control) Valve
The computer operates this valve
It looks like a small Can, and will be on the throttle body, it controls air flowing thru a passage that bypasses the throttle plate, so this is air that came thru the MAF sensor, not outside air

You need to locate your IAC Valve, it will have 2 wires connected.
After engine is warmed up and idling, unplug those 2 wires
IAC Valve will close with no power, and idle should drop down to about 500rpms, engine barely running
If idle stays high the turn screw counter clockwise until engine is at about 500rpms or about to stall
Screw is set
Plug back in IAC Valve wires and idle should go up to 700-800, computer is now in control

Yes this screw can be used to adjust TPS(throttle position sensor) voltage, but TPS should have slotted bolt holes for that adjustment.
18% throttle position doesn't really mean anything, the computer "learns" TPS voltage over time, under 25% with "foot off the gas pedal" should be OK


The computer should bump up the idle, open the IAC Valve a bit more, then you turn on the AC
Pin 41 on the computer is connected to 12volt line on AC Compressor so if Compressor gets power computer bumps up idle
If thats not happening then there is a wiring issue, maybe computer issue but more likely to be a wiring issue.

OR.......................IAC Valve is not working like is should.
When you start a cold engine computer should use IAC Valve to set higher cold engine idle, 1,100rpms, then as engine.coolant warms up idle RPMs will start to go down, warm engine idle with manual trans should be about 700rpm, 800rpm with automatic


Your Fuel trims are way too high, so you most likely have a vacuum leak, which will be a problem when adjusting that screw, it assumes no vacuum leak.
With engine warm and idling, unplug IAC Valve, then disconnect PCV hose from intake an put your finger over the intakes port, do the same with power brake vacuum hose
Then do each vacuum hose one at a time.
If RPMs drop when port is plugged then that hose/system has the leak

 

Thank You RonD, it was a massive vacuum leak just not where you said to look. some how the tube going from the filter assembly to the throttle body came loose at the filter, I guess when I replaced theMAF I didn't tighten down the clamp enough and it worked its way off. I did try to adjust the throttle plate going by what you said, with the engine warm I unplugged the IAC and it dropped to about 650rpm's with the screw allthe way out, plugged it back in and the rpm's would go up to 800rpm's, not bad but the throttle would hang up slightly so I adjusted it in just enough to open it a little, now it idles at around 780rpm's. Both fuel trims are now back down to 0 to +3 when idling and the truck has a little more power. Thanks again.

 

Good work

Thank for posting the update and FIX