Max A/C Surging the ST

[Thomas Athanas]

Just noticed this yesterday morning on the way to work. I had the A/C set on MAX mode and while sitting at a red light my ST would surge or lunge forward hard enough to break the seal on the brakes (light pressure to keep from rolling while in drive) and I would roll forward a couple of inches until my brakes would stop me again. It would do it every other minute or so. Thought maybe a overnight rest would cure it, nope, still does it today.



As far as I can tell it doesn't happen while I'm in non-MAX A/C mode. It is a weird problem though, because it seems almost transmission related but it only does it with the A/C on, and in MAX.



Maybe related or unrelated, I just pulled the two CEL code cracked elbow problem and I replaced the part that was listed in the thread. Thank you guys for that help.



So anyone know what would be causing this? Thanks

 

[Todd Z]

When the ac kicks on the motor raises the idle to take the load off the engine and keep it from stalling...



IT actually is normal to feel that.....



Todd Z

 

[Richard L]

Your engine should not increase the idle beyond normal when the A/C kicks on. When the compressor clutch engages it increases the load on the engine and the idle will drop. The ECU sense this drop in the idle RPM and increases the idle speed to compensate by bring the idle back to the normal speed.



This may be irratic if you just started your engine and it has not come up to operating temperature yet, or if you may have a vacuum leak, or a sensor is out of tolerance.



If it continues after the engine is fully warmed up, you probably have a sensor or IAC motor that is going bad or dirty. You may also have a vacuum leak in one ot the vacuum operated AC doors under the dash that controls the air recirculation when in MAX AC mode.



...Rich

 

[Chad Sneary]

ok similar problem but mine surges with the heat on. I hear the compressor kick on, the idle drops to about 650rpm. When in gear the truck tries to break loose from holding the brakes. Any ideas? I need to solve this before snow falls.



Btw I replaced the common elbow leak beginning of spring.

 

[Chad Sneary]

so I have a similar problem. My ST surges with the heat on. It will drop to about 650rpm when the compressor kicks on. Then it tries to break loose while braking. Any ideas?