Answer
Nov 25, 2025 - 10:54 PM
On a 1990 Ford F-250 5.8L (351W) you can’t really adjust the idle screw like on the old carb setups — it’s fuel injected with an IAC (Idle Air Control) valve doing the work. If the idle is too high or too low, there’s usually a reason and the computer is fighting it.
Here’s the right way to go at it:
- Check for vacuum leaks — Any leak after the throttle body will make the idle go nuts. Cracked hoses, intake gasket, PCV hose — super common.
- Make sure the throttle plate isn’t being held open — Sometimes the throttle cable is too tight, or the throttle body is just full of carbon.
Give the throttle body a good cleaning — that alone fixes half the idle problems. - Check/clean the IAC valve — If the IAC is dirty or sticking, the idle gets rough or low.
Pull it off and spray it out with throttle body cleaner. - Set base idle ONLY if needed — There is a throttle stop screw, but it’s not meant for adjusting normal idle.
You only touch it after cleaning everything and making sure the IAC is working — and even then, a tiny tweak just gets the throttle blade back where the computer expects it. - Make sure timing is correct — If the timing is off, idle will be trash no matter what you do.
If the idle changed suddenly after some work, something’s wrong — fix the cause, don’t crank on the screw right away. If you want a clean step-by-step with the diagrams (IAC, vacuum routing, base idle procedure), I recommend grabbing this 1988-1991 Ford F-Series (F-100, F-150, F-250, F-350) Service & Repair Manual Software, which is compatible with your '90 F-250 5.8L V8 and could show the exact process.
