Throttle falls very slow

hi I have a 2001 ford e250 motor is a 5.4 , I have a disability but I can still drive , if the van is in park and rev up the motor ,the throttle falls very slow kinda lags as it falls, if your in the mud or snow and get up on the throttle it does the same thing even going down the road and you let off the gas it drops the throttle slow, Ive changed the egr, throttle position senser, the ideal air control, ive cleaned the throttle body, im at a loss of what to look for to fix this problem, please help

As a test find the IAC again…I see you replaced it already.

See if it is working properly. Start the engine and rev it from under the hood. If the idle drops slowly…try rapping on the IAC with the handle of a large screw driver. It may be sticking and if the idle drops quicker by rapping it…you know it’s the IAC.

If this corrects the problem, you may have gotten a bad IAC from the parts house. If you replaced it with a used one from another vehicle…buy a good one.

Also be sure that there is not carbon and ash built up inside where the IAC is mounted.

Yosemite

Do you mean the idle rpm is too low? Yes, that can be frustrating to diagnose what the cause is.

Does the engine run ok otherwise, no missing on acceleration or at road speeds? If you have some missing going on, you might have a problem with the crank sensor. The ECM can’t control the idle speed if it doesn’t know what the rpm is.

Another idea, the ECM may be confused what the engine coolant temp is. Or the engine coolant temp may not be what it should be.

Even one more idea, a poster here recently had a problem with the EGR metering valve, forget what they call it. Four letters as I recall. Anyway, this part seems to be a problem w/Fords, so maybe the EGR is getting activated when it shouldn’t. That would slow down the idle speed, and replacing the EGR itself wouldn’t fix it.