99 Gt Low Idle With Air Conditioning On

[Mark Pierce]

I have searched threads and replace several things related to the idle control om my 99 G. The idle issue with the air conditioning off has been fixed but whenever the air conditioning or the defrost is on it will struggle to stay running at a stop. If I let off the break at a stop the idle will recover and to make it even stranger, the harder I press on the break peddle the faster the car will die at a stop light.


Ideas?

 

[wmburns]

The PCM maintains two IAC/idle trim settings. AC on and AC off. For the PCM to relearn new IAC/idle trim settings the AC needs to run long enough for the PCM to learn. IF the AC is short cycling (for what ever reason) then the PCM can't learn.

What are some other things that might prevent the PCM from re-leaning new IAC/idle? How about unstable charging system?

However it seems like to me that your problem is the hydro-boost is drawing too much load.

 

[Mark Pierce]

The PCM maintains two IAC/idle trim settings. AC on and AC off. For the PCM to relearn new IAC/idle trim settings the AC needs to run long enough for the PCM to learn. IF the AC is short cycling (for what ever reason) then the PCM can't learn.

What are some other things that might prevent the PCM from re-leaning new IAC/idle? How about unstable charging system?

However it seems like to me that your problem is the hydro-boost is drawing too much load.

I put a alternator on recently so I'm thinking the charging system is ok. What is the hydro-boost system?

 

[General karthief]

I put a alternator on recently so I'm thinking the charging system is ok. What is the hydro-boost system?

Only thing I can say is never 'think' anything on a car is ok, those are the things that trip us up, check and make 'sure'.
Hydro boost is your brake assist, runs off your power steering pump.
I think
Or it may be a hydraulically driven supercharger
Nope, definitely brake booster, right?

 

[Chuck Gladfelter]

FWIW my IAC has gone bad on two of the Mustangs I've recently owned.

 

[wmburns]

FWIIW, If the IAC really is bad then it stands to reason that the idle would be bad all of the time. Not just when the AC is on.

Here's some more information on how to trouble shoot idle/IAC problems.

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

But to correctly trouble shoot this problem it would be handy to know the IAC duty cycle. 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/

With this one would be able to quickly answer the question about the IAC duty cycle. Further answer the question about IF the IAC duty cycle changes in response to motor load changes.