Mike Fowler

2002 Mustang GT 110,000 miles Automatic(yeah I know). My problem is, maybe its normal, When the car is in drive the car idles at 500 rpms and feels like it wants to stall. Never does though. I had the floating idle 500 to 1000 to 300 and so forth. I replaced the IAC this took care of the floating idle. I have also replaced the EGR valve, EGR Valve pressure sensor and the DGFE solenoid. Any ideas of what could be causing this? Or is this normal? Thanks

Shampue

Don't be hating on the automatic! I enjoy driving a 5/6 speed but sometimes the laziness of the automatic is just heaven

cliffyk

Since replacing the IAC and other components have you disconnected the battery for 5-7 minutes, to clear the KAM (Keep Alive Memory) and force the PCM to re-learn the idle air and fuel trim values?

Also check the throttle stop screw setting? The adjusting screw tip, and the throttle plate bell-crank tab on which the screw sits, wear over time and miles which can cause the throttle plate to colse more tightly than it should. When this happens there is too little bypass air for the PCM to control the idle even with the IAC opened fully.

Here is how to adjust the stop screw...

Mike Fowler

I did disconnect the negative cable. When you adjust the idle screw are you adjusting it at idle when the car is in park or this adjusts it when the car is in drive and you are at a full stop but still in drive? The car in park idles at I would say 800 rpms. Thanks for the help.

ctgreddy

adjust it in park is fine, i would try and up it 100rpms, idling at 600 in drive will be fine that's my car old gt auto used to idle at.

cliffyk

There is no "idle screw", the throttle plate stop screw on the TB is used to preset the amount that the plate is held open, to provide a fixed and appropriate amount of air bleed through the TB when the throttle is "closed". This expected air flow is a scalar value in the tune identified as ITHBMA (Idle THrottle Body Mass Air), Typical values are 0.39 to 0.45 lb/m.

Setting the throttle stop screw as described in the link I provided above (click here) ensures that the reality of that airflow matches what the tune told the PCM to expect.

Properly set, the air flow through the closed TB will be just barely enough to allow the engine to idle. The PCM then opens the IAC as needed to allow more air flow, enough to keep the engine idling at the speed(s)¹ set in the tune.

When additional engine load is applied (generator, power steering, AC or putting the tranny in drive on a slushbox) the PCM open the IAC further to allow more air in, and maintain the idle speed.

Here is how to test the IAC--just because it's new does not 100% guaranty it is good.


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¹ - Modifying those scalar values in the tune is the only way to properly change the idle speed--using the stop screw to do so will just screw (non pun intended) things up. For most 4.6L 2V GTs the idle speed in neutral is set to 720 rpm, with the base idle in drive being 560 rpm. In drive there are a number of "adder" values that will be applied by the PCM, these are based on ECT, IAT, etc.

hpwrestler220

Car should be idling at 750 i believe.

cliffyk

It depends on the strategy/boxcode, I have seen the programmed idle speed in neutral vary from 600 to 850 rpm in the various tunes I have worked with--here are a few:

• 625 in FMAG0.RMR0;
• 640 in RYBE2.YDH1;
• 640 in RZAO1,BMD0;
• 680 in CRAJ0.AOL3;
• 680 in RYAF1.MDE3;
• 705 in RVAF1.KSG0;
• 720 in RVAFA.NUX2;
• 736 in FBFG2.RKN2;
• 800 in FBLF1.VDX1;

Mike Fowler

So there is no way to adjust the idle, to idle higher, when the car is in drive. The idle is perfect when the car is in park. Its only when the car is in drive(idles at 500 rpms) that the car feels like its going to stall. IAC checked out perfectly. Thank You.

cliffyk

The idle speed in drive can be set in the tune, however there should be no need to change it--something else is wrong.

Have you checked the throttle stop screw setting yet?

As the screw tip and bell crank tab wear the throttle is allowed to close further than it should and the PCM can no longer raise the idle suffiently using the IAC...