High rev

[Low]

Hello ranger ppl of the internet.
I have a question. When I start my truck with foot on the clutch and brake the trucks rpms go up to 2-3 thou then comes back down after a couple seconds. Once the truck is warmed up after me driving it for a while it stops doing this.
Any idea what this could be?
Any response is appreciated!

 

[rusty ol ranger]

Idle air control is most likely culprit.

RPMS should ride high on a cold start but not that high

 

[RonD]

All fuel injected engines use an air inlet valve to set idle and to control engine RPMs for emissions reasons

Ford calls theirs the IAC(idle air control) Valve
Its a 2 wire solenoid
With key on the computer opens IAC Valve all the way for cold start
When engine starts it should REV to 1,500-2,000rpms, when cold
Then computer starts to close the IAC Valve to match Coolant Temp it gets from the ECT sensor
Cold engine should idle at 1,000-1,200rpm
Warm engine 650-800rpm, depends on 4cyl or V6 and manual or automatic transmission

If engine is already warmed up, then computer may start closing IAC Valve before startup so it won't REV as high as when cold

Ford uses a true Solenoid IAC valve, Motorcraft or Hitachi brands are the only true solenoid valves available
3rd Party valves are solenoid/stepper which can work but will have odd results, wandering idles and high REVs

To test if there is an Air Leak/Vacuum leak
Warm up engine, and let it idle
Unplug the 2 wire connector on the IAC Valve
RPMs should drop, engine may even die, either is good, it means no air leaks

If RPMs do not drop then there is an air leak, IAC Valve/computer can not control minimum idle, the air leak is doing that

 

[Low]

Yall spoil me! Big thanks for the quick and detailed response guys.
I'll check it out after work today.
I Can't tell yall how much I appreciate this forum!!
Be well yall

 

[FEandGoingBroke]

In my experience. It means to me that your brake booster is momentarily losing vacuum as you depress the pedal and the engine is not calculating that vacuum loss and the RPMs are rising until the idle air control motor adjusts the idle to normal parameters