Very bad advice from a parts guy. There's no arguing with success but there's a lot of different things that cause the same symptoms.
Your automatic idle speed motor is bolted to the side of the throttle body. It only controls the amount of incoming air that flows through a passage around the throttle blade. At the same time the Engine Computer adjusts the position of that motor / valve, it also adjusts how long the injectors stay open during each pulse. That is done strictly to control idle speed. Above idle, that motor and valve have no function, and it definitely will not cause a rough idle. It can only cause an idle speed that's too low. Also, any problem the AIS motor or passage would cause will be there all the time, not just when the engine is warmed up.
You can also prove the AIS motor is working by observing its action at engine start-up. Without putting your foot on the gas pedal, the idle speed will "flare up" to 1500 rpm, then come right back down to around 800 rpm. If that occurs, the automatic idle speed motor is working and the air passage is not plugged. With the better detergents in the fuels today, I haven't needed to clean any of those passages in many years. That used to be a common repair.
I asked if the battery had been disconnected recently. If it has been, the Engine Computer has to relearn "minimum throttle" before it will know when it must be in control of idle speed. Until then, the engine will usually die at stop signs and fail to start unless you hold the gas pedal down about 1/8". You won't get that idle flare-up either.
For running rough, the first thing to check is if there's any stored diagnostic fault codes and whether the Check Engine light is on. Those codes will get you to the circuit or system that needs further diagnosis. Failing MAP sensors can cause a rough running engine but they don't fail very often like they did in the late '80s. '96 and newer vehicles have misfire detection built into their computers. When they're detected, the Check Engine light will turn on and there will be a code stored that will list which cylinder has the problem.
The next thing is to start is with the basics, spark plugs and wires, especially if it's an intermittent problem, and a compression test if the misfire is always there. Other less-common causes include a plugged fuel pickup screen in the gas tank. The clue is the engine will run fine at highway speed when less fuel is being pumped and it will stall at lower speeds, particularly coasting, when more fuel is being pumped. Chrysler has extremely little trouble with injectors so they should be one of the last things to suspect.
If you have a steady misfire, you can slide a grounded pick between the spark plug wire and boot to cancel one cylinder at a time to see which cylinder is causing the problem. Another approach is to connect a scanner that can display live sensor data during a test drive. You can see what the Engine Computer is seeing but it usually takes an experienced mechanic to interpret those readings.
Saturday, October 15th, 2011 AT 9:09 PM