Chrysler Repair: Idle Problem: 3.0L V-6, vacuum hoses, chrysler lebaron

Question
My son owns a 95 Chrysler Lebaron 3.0 V6. When at a stop in gear, the idle rapidly fluctuates back and forth between 700 rpm down to 250 rpm, sometimes going below 250 and stalling. It stalls more often when cold (and its cold in Minnesota alot).  Our mechanic changed plugs, wires, fuel filter, and checked timing, scan test, and engine balance test. All codes showed nothing. If you put the gear in neutral at the stop, the problem ceases and it idles at around 600. My mechanic cannot answer what is wrong. The vehicle is also very sluggish when cold pulling out in traffic.  Any other suggestions, it has cost alot trying to diagnose the problem and is getting frustrating.
Thanks very much  

Answer
Hi Dan,
It sounds like you have covered most of the possibilities except one:
My thought is to look over the exhaust gas recirculation (egr) valve and associated control and vacuum hoses. This system connects the rear (side of the engine next to the firewall) exhaust manifold to the intake manifold and the pipe that connects the two runs between the two manifolds. The valve is bolted into that pipe and controls the exhaust gas amounts that return to the intake.  The valve has to open and close firmly and quickly or the engine will not operate well due to a mixture of fuel and air that will not allow the engine to accelerate or idle well, if the valve is not where it is supposed to be. Often it will get sticky and fail to close all the way, for example which it has to do at idle and when stepping on the gas toward full open throttle. It is controlled by a vacuum operated and electrical control solenoid mounted near by, and those vacuum hoses should be examined to make sure there is no cracked, leaking, or disconnected hose fittings. There is a vacuum operated round fitting on its top, and also a vacuum line going to an electrically operated valve closer to the engine, nearer the throttle body. Look under the round fitting (which is mounted on a saddle with openings for access to view) on the top of the valve where you will see a rod with a slot in it which is the valve stem. Take a straight blade screwdriver, insert it in the slot, and then use the driver to lever the valve up (against spring pressure) and let it return to see if it is dropping back all the way down due to spring's action and closing positively to a firm closed position. Spray WD-40 or other penetrating lubricant on the valve stem to loosen it up in that in and out motion. Check to make sure the vacuum lines are all attached and not cracked. Then warm up the engine and observe the slot to see if it goes up when you rev the engine to 2500 rpm and then drops back down fully, closing the valve, when you let the engine go back to idle rpm. Make sure that the fully down position is really down because if it is not fully closing when you let go of the accelerator and coast that will cause the engine to stall. It may be that to get it to close you'll have to unbolt it and clean out the gas flow passage past the valve, due to buildup of exhaust fumes in the valve as sticky crud deposit around the internal valve.
Let me know if that doesn't help and we'll consider some other possibilities. But the egr function can be a big detriment if it isn't in the correct position for the conditions that are needed for a good mixture.
Roland