99 Mustang Gt Idle Issues

Drive it around for a while to see if you get any codes, then report back with your findings...could be various things...

One of the more common reasons for a low idle is a vacuum leak between the MAF and throttle body butterfly valve. The leak could also be in the IAC bypass line.

Another typical cause of slow idle is cylinder balance related. IE, one or more cylinders are not pulling their weight in relationship to the others. Other causes could be low fuel pressure, ignition misfire, bad IAC value, bad/dirty MAF.

Since the MAF has been cleaned and the IAC replaced, let's assume these are OK.

Suggest starting with a through review of the battery and battery cables. Does the battery hold a charge? Are the battery cables clean and tight?

Double check that the fuel pressure sensor intake vacuum reference line is connected and leak free. Check the line for raw gas.

Check the throttle body return spring. Are there two? Does the throttle work smoothly without binding?

Now assuming the problem is still on going, the easiest way to trouble shoot this is with and ODB2 scanner. Monitor the IAC duty percentage. It should be between 40-60 percent at idle. Bump the throttle and note how it reacts. What you are looking for is "hunting" outside the normal range. IE, the IAC duty percent starts out at a low value and then adjusts up. This could indicate there is a blockage in the butterfly bleed through holes are the idle stop is set too low.

If you do not have an ODB2 scanner, consider performing a cylinder compression test.