Truck sounds like it has a minor cam in it, but the RPMs dont move? Cleaned IAC , MAF, Throttle Body, PCV valve, checked and regapped plugs. I feel like I've done everything . Its not really a problem but its just bugging the hell out of me. Oh ya, truck is a 99 Ranger XLT 4x4 4.0 OHV. Did the head gaskets maybe 25k ago and intake and injectors probably about 10k ago. Also on rare occasions ill get a bank 2 lean code. Maybe fuel pump not pumping enough?

 

After engine is warmed up unplug the 2 wires on IAC Valve
RPMs should drop down to 500 or so, and be steady, engine may even stall, either is good it means no vacuum leaks

If you still get the varying RPMs you have a leak somewhere, probably lower intake bank 2

A single cylinder or bank 2 leak can cause exaggerated up and down RPMs at idle because computer is trying to compensate for the up and down RPMs by adjusting the IAC Valve, it just can't do it fast enough to smooth it out

 

Hey Ron, I have surging up and down idle too.... along with hesitation / surging during acceleration... installed part to clear up CIL...and threw other parts at it ...still exists...
BUT IN my situation it idles great for the first minute upon start up before it starts surging ---- my understanding is that the PCM doesn't read / work with sensors at startup- runs on presets, but then does after a minute...so it would seem it points to a sensor and not a vacuum leak in my situation....yes ?

 

You should start your own thread and include YEAR and engine size
Forum etiquette is pretty basic, you post in someone else thread to help them solve their problem, you shouldn't post in their thread to solve yours, you can ask if they solved their problem if they never responded back with the solution

Computer runs Choke mode on Cold start, it uses all the sensors except the O2s because they must be heated above 650degF before they can work

Yes, computer does use air/fuel mix tables in memory until O2s are working, but these tables are modified by Long Term Fuel Trims(LTFT) at the time of cold start to take into account the engine's aging systems, all engines get older and have small air leaks and lower compression, along with lower fuel pressure, the LTFT compensates for these factors

wandering idle can be dirty PCV valve, stuck and loosens up when warmer, to failing IAC Valve

Please DO NOT respond here, just start a new thread with year, engine and symptoms