Stumbling Rpms

[mustang4lyfe]

So I've been having this problem for a while now and im getting very frustrated because I don't know what it is. I have a 2000 mustang gt with about 70k miles. It has full exhaust except headers(off road no cats), I also have a sct tuner, CAI, tb and intake plenum, and steeda underdrive pullies. Also my pcv valve works properly and I have replaced my IACV. My issue is when I rev my car up to anything of significance when the rpms are decending they will dip low to around 500 and than jump back up to normal as if its stumbling and almost suffacating or something. It almsot seems like my motor wants to die. If i watch the motor while this happens it shakes and seems like it isnt firing right or like its struggling to stay running, this has been going on for a while what could my problem be? Please help I'm sick of this problem!!!
thanks

 

[mustangjoe02]

i have this same problem but i have mac longtubes as well. replaced o2s egr checked vacuum leaks with smoke machine replaced iac replaced fuel filter. no idea

 

[mustang4lyfe]

hmm i feel like alot of poeple have this problem but I really have no idea what it could be. Could a small exhaust leak cause this?

 

[trinity_gt]

If you disconnect the IAC harness (or plug the air line to it), what does the engine idle at?

It's possible the base bypass air is not correctly set on the TB. With the IAC disabled, you might try making sure the idle speed is at least 500 by adjusting the throttle stop screw as needed.

 

[mustang4lyfe]

im confused by wat u mean?

 

[trinity_gt]

im confused by wat u mean?

The IAC is used to bypass air around the closed throttle plate to control idle speed. If the IAC is closed completely, the engine will need to draw all of its idle air through the throttle plate. If the plate is not properly adjusted you'll have issues: Too tight and not enough air will enter which can lead to issues like you're seeing, too loose and too much air gets by leading to "floaty" idle RPM and hanging.

You can adjust this with the throttle stop screw on the TB but to know what's right you need to ensure no air gets by the IAC while adjusting it. You can block the air line from the intake duct to the IAC or unplug the IAC. Start the engine and see where it idles. If it refuses to idle or is very low, adjust the stop screw to open the throttle until you get, say, 500RPM. Once you have that and the throttle stop screw nut cinched down you can re-enable the IAC (open up the air inlet or plug the connector in...) and see how the engine idle is.

When you adjust the screw it might take a bit for the PCM to re-learn the closed loop IAC parameters so be patient...