My 1989 F-150 xlt lariat with the 351 windsor needed new fuel injectors. Installed them, and everything. Trucks runs great now, but the idle speed is very high. The screw on the throttle body which id imagine is to adjust idle speed with does not have a head on it, and is set to what seems to be the highest position by previous owner. Is adjusting this screw the only thing i need to do to lower idle? How can i turn the screw without a head, and without damaging the thread? Thanks

 

That is a throttle stock screw to prevent the blades from completely closing and binding in the bores. It is not an idle speed adjustment although it will have some effect on it if it has been altered from the factory position. Idle speed is controlled by the computer via the IAC valve but that assumes the throttle blades close to the correct position. Sounds like your screw has been messed with so you need to back it out until you see about 0.9-0.99v on the TPS sensor, or swap out the whole TB assembly with one that has not been altered.

 

What you have to do in order to be able to adjust the screw is up to you. Drill it out, replace TB, whatever. After you have a functional screw, disconnect IAC and use the screw to set the idle speed to 400-500 RPM. Then turn off motor, reconnect IAC, and then disconnect the negative battery cable for 10 minutes to reset computer. It may take up to a week for ECU to relearn idle after this (idle may fluctuate during this time).

Probably a good idea to clean the IAC beforehand and you might also check TPS adjustment after this procedure.

 

None of that is necessary the, the only things that matter are a closed throttle TPS voltage in the right range and a functional IAC valve. The computer takes a reading of the TPS every time the motor is started and then as long as the IAC works correctly it will set and control the correct idle speed of about 680rpm... no learning necessary.

 

Thanks for the help!

 

We've had this argument before. Your way assumes the TPS sensor is adjusted correctly, which we do not know to be the case.

Hell, japple, try both methods and let us know which one worked better for you. Then maybe me and Paul won't have to argue over this anymore. :-)