Stumbling Rpms

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

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 said:

im confused by wat u mean?

Click to expand...

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...