03 Gt Will Not Idle Under Load Or When A/c Is Turned On

Ken Binnion · Aug 19, 2017

Fellow members,

I need help or advise, here's what my issues and current repairs are;

my 03 GT will not idle under load or when a/c is turned on. when a/c is turned on and at idle it will idle erratically. I can drive it without a/c all day long with no issues but once a/c is turned on it goes to crap. will not idle at stops, will not stay running at idle, runs erratically. Turn a/c off and it takes a few minutes to reset itself. so here are the specs and some of the attempted fixes;

total engine rebuilt 1 1/2 ago, Comp Cam extreme energy stage 1 cams installed.

car ran good until 1 month ago, no issues.

4.6L automatic

changed IAC solenoid

Changed MAF, MAP and TPS

Cleaned purge solenoid located inside of passenger side front fender.

checked all vacuum connections, no issues.

installed a air/oil separator.

Cold air intake

had a mechanic dial in TPS and reflashed ECU. Mechanic could not find issue.

have performance chip installed by BAMA tunes. I have it set to factory setting right now.

please help. KLB

wmburns · Aug 19, 2017

The PCM maintains two Idle/IAC trim settings. AC on and AC off.

IF the idle is fine with the AC off then the most logical conclusion is something is preventing the PCM from learning the correct AC on idle trim value. What could cause this?

AC low on Freon or short cycling which is preventing the PCM from learning. The AC needs to run long enough to allow the PCM to learn.
There's something wrong with the battery and or charging system where the PCM is dropping power. This is making the PCM "forget" the learned idle trim values.
loose or dirty battery cables. Bad Alternator diode
Or the IAC isn't working at all and it just happens to be correct with the AC off.
Or the PCM is not getting a signal that let's the PCM "know" that the AC is on or off.

Don't cut corners on the battery and charging system maintenance.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

To confirm IF the AC is cycling watch the center section of the AC compressor. When the clutch is engaged the center section will turn.

One method to work around this is to jump the AC low pressure switch that will "force" the AC to run. The idea is to allow the AC to run long enough for the PCM to re-learn.

Of course the other method is to fix the AC so that it no longer short cycles.

Note, it may be difficult to prevent the AC from short cycling IF the outside temperature are very low.

Do not run the AC with the low pressure switch jumped for long periods of time. This could cause the inside coil to freeze over.

Ken Binnion · Aug 19, 2017

WMburns,

ill check the battery. the battery terminals are new and clean. I have a higher amp alternator so I'm sure that's not it. I will check my a/c charge. the AIC is new but its not a motorcraft IAC, I purchased it at Advance auto parts.KLB

imp · Aug 20, 2017

Clearly, Burns knows whereof he speaks. I will be needing his input regarding some really imponderable issues, if he's willing. imp

Sluggie24 · Aug 20, 2017

Did you replace the IAC with a Ford part or aftermarket? Aftermarket parts can be hit or miss sometimes.

When the AC is on does it work properly? Is there any unusually loud noise from the compressor?

Have you tried dis-connecting the AC clutch and turning on the AC to see if the PCM raises the idle point?

Ken Binnion · Aug 20, 2017

Sluggie24 said:
Did you replace the IAC with a Ford part or aftermarket? Aftermarket parts can be hit or miss sometimes.

When the AC is on does it work properly? Is there any unusually loud noise from the compressor?

Have you tried disconnecting the AC clutch and turning on the AC to see if the PCM raises the idle point?

Sluggie24,

I never thought about testing the A/C system. When the A/C system is off I have no issues. The IAC is a aftermarket module. Advance auto parts was nice enough to allow me to swap out 2 IAC's. I'm not so sure its the AIC itself but the PMC that controls receiving erroneous signals. ill be checking not only the A?C system and the charging system as WMburns recommended. KLB

Ken Binnion · Aug 21, 2017

wmburns said:
The PCM maintains two Idle/IAC trim settings. AC on and AC off.

IF the idle is fine with the AC off then the most logical conclusion is something is preventing the PCM from learning the correct AC on idle trim value. What could cause this?

AC low on Freon or short cycling which is preventing the PCM from learning. The AC needs to run long enough to allow the PCM to learn.

There's something wrong with the battery and or charging system where the PCM is dropping power. This is making the PCM "forget" the learned idle trim values.

loose or dirty battery cables. Bad Alternator diode

Or the IAC isn't working at all and it just happens to be correct with the AC off.

Don't cut corners on the battery and charging system maintenance.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

WMburns,

I had the battery and alternator checked and they are good. I unplugged the AC compressor and I'm still stalling when the engine is under load. when I have the mustang under load and then put it in park the engine takes a few seconds to correct its idle. while its correcting its idle the rpms will fluctuate up and down until it levels out at 900rpm. In park it idles fine.

I'm going to replace my coil packs with new accel packs and replace the iac with a motorcraft iac. KLB

rebus20 · Aug 21, 2017

My 03 is doing something similar. At idle with the ac on it will drop to 500rpms and almost stall every 10 seconds or so. I cleaned the maf and iac, charged the ac and cleaned the battery terminals to no avail. Very annoying. I havent checked the charging system but I have no lights on and it starts fine even if sitting a week.

Ken Binnion · Sep 23, 2017

ok everyone, its been awhile since my last post, what with Hurricane Irma and all I have jumped back into trying to figure out what is wrong with my Mustang. So here's the latest:

I changed the coil packs and spark plugs.
I can only drive my Stang with the A/C on or off. I can not turn on or off the A/C while the Stang is running, if I do then the issues arise, wont idle and will stall. so I have to drive the Mustang with the A/C on at all times. If I change the settings then I have to disconnect the battery and allow the ECU to reset itself. I don't mind driving my Stang with the A/C on all the time but I would like to fix the underlying issue. So my question to all is this; what could be causing this issue.

KLB

wmburns · Sep 24, 2017

It would be handy to know if the PCM is getting a good "AC on" and "AC off" signal. A good ODB2 scanner can read the AC PID. I could see how this could cause issues if the PCM were unaware of the actual running state of the AC.

Here's some information on an affordable Windows based ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Ken Binnion · Sep 24, 2017

WMburns,

thanks for the help. ill get it checked out. thanks again.

KLB

03 Gt Will Not Idle Under Load Or When A/c Is Turned On

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