Engine Car stalls when going to a stop with the AC on

[michaelgt]

Hi everybody I have a 1989 Mustang GT 5.0 t5 manual transmission. My mods are K&N air filter with Factory air box C&L 76 mass air meter professional products throttle body 65 Holley systemax upper and lower intake manifold 24 injectors 1.7 rocker arms ported stock cylinder heads BBK shorty headers with the stock H pipe Dynomax super turbo mufflers Cat-Back exhaust.. had this for quite awhile... like the title says when I come to a stop with the AC only on it wants to die sometimes it dies and sometimes it doesn't but when I don't have the AC on no problem at all...the things I've done new idle control valve new spark plugs and wires I've checked the freon in the AC system it's a 40 PSI I've clean the mass air meter done the basic idle reset put the TPS sensor to .97 put a new VSS sensor fuel pressure is at 42 with a stock fuel pressure regulator I even tried putting the idle higher to 900 it works for a little bit and then within 3 days the problem comes back I even cleaned all the grounds check the vacuum leaks everything looks pretty good can you guys help me out thanks in advance

 

[michaelgt]

Also I've checked the alternator I have a 130 amps at idle its 80 amps I've also clean the EGR and the throttle body

 

[General karthief]

Help me create the "Surging Idle Checklist"

Updated 26-Oct-2019 to clarify creeping idle RPM increase as the car is being driven. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of...

forums.stangnet.com

Go through this checklist starting at the top, do not skip around, the next step depends on the result from the previous one. Even if you have replaced something already. It works, ask me how I know.

 

[Mustang5L5]

Perform the idle reset/relearn procedure

 

[michaelgt]

I've done the checklist and I did the basic idle reset and it still does it should I just put up the idle

 

[Mustang5L5]

Clean the 10 pin connectors.

Also, have you pulled codes to verify no VSS code?

 

[michaelgt]

I've clean the 10-point connectors and no codes

 

[Mustang5L5]

Well this is a good one then.

Stalls only with ac? No other load?

 

[jrichker]

I've clean the 10-point connectors and no codes

No code 11 (two flashes when the computer dumps the codes)? A perfectly functioning system will always get a code 11, it is the everything is OK code. The 11 code is computer passed its internal self test.
If you don't get an 11, you have computer or wiring problems.
Please check and repost.

 

[michaelgt]

Yes it just stalls with the AC on what the headlight and fog lights and windshield wipers and the emergency it doesn't stall and it has a Code 11 which I already know that the system is okay

 

[General karthief]

If the computer will give you the ok code (11) then hook up a vacuum gauge to see where your vacuum is at.
also what is your timing set at

 

[michaelgt]

I'll need to buy a vacuum gauge and my timing is set at 12

 

[michaelgt]

If the computer will give you the ok code (11) then hook up a vacuum gauge to see where your vacuum is at.
also what is your timing set at

How would checking my vacuum fix my problem and how do you check the vacuum with a gauge?

 

[michaelgt]

I also notice if I put the AC on when it's cold it doesn't do it but when it's warm it does

 

[Mustang5L5]

When engine is cold? When outside temp is cold?

 

[michaelgt]

When the engine is cold once it hits operating temperature that's when it starts to want to die with the AC on if it's hot and cold outside doesn't matter it still would die with the AC on

 

[michaelgt]

I should let you all know that it only wants to die when I'm coming to a stop knot when I'm at a red light and then I put the AC on the car will still stay on if I do that but once I'm coming to a stop that's when it's kind of weird sometimes it catches it and sometimes it just wants to die

 

[Acj2001_Fordman]

No expert, but I would look into the Idle air control valve (IAC). It is suppose to bypass air around butterfly, during quick deceleration's, to keep the motor from snuffing out. Kind of a damping effect for quick throttle closes.

 

[Mustang5L5]

Agreed. Usually when you roll to a stop the VSS signals the ECU and the iac will open up s tad.

So either the signal isn't reaching the ECU or the iac isn't responding as it should .

However, doing it once engine warms up also is a hint it might be sensor related as well.

 

[General karthief]

My money is on the vss or the wiring in that circuit.
does the engine idle up when cold for a minute then drop to normal when you first start it?
as for the vacuum question, hook the gauge to the vacuum tree, low vacuum can cause stalling issues.