1999 Concorde 3.2L rough idle

I have 99 Concorde that has developed a rough idle. When stopping at a traffic light or in a parking lot the rpm's drop and the motor begins to idle roughly. It also will "diesel" after the ignition is turned off. Both problems began around the same time. I replaced the plugs hoping to cure the idle but it did not. Can someone give me som advice?

possible a leaking fuel injector...is there any check
engine light on....does your vehicle have a hard time
starting after it sits a while, is there also any black smoke when starting.. how did the plugs look when yoiu replaced them...

There is no check engine light. It always starts with no problem and I am not sure about the black smoke because my wife drives it. I have witnessed the rough idle and dieseling. The plugs looked pretty burned up. The gap was double what it should be. Most plugs had a rust/red color on them but no sign of black soot like when theres too much gas.

I'm surprised that no check engine light has come on. If you get a chance, take it to a dealer service department and have them pull any past codes in history of the PCM. It could be a "ghost" where it turns on when it wants to. What is causing all your problems sounds computer related, like a throttle position sensor, an idle air control valve, or a MAP sensor. If you had a leaky injector, it would still bleed off an amount of fuel and would make it hard to start b/c it would flood out the engine.

I had the pcm checked today and it turned up nothing. No codes whatsoever. Where should I look next?

You could also check for vacuum leaks. If the engine also makes a high whistling sound, then you have a confirmed vacuum leak. Or you fuel system could be clogged. If it is, the best thing you can do is just add some fuel system cleaner to the system, let it run itself out and go from there. Worse case scenario, you could have uneven cylinder compression in the engine. But I would first off check for a vacuum leak then check the fuel system.

Another thing that I thought about is check if your engine is running slightly hotter than normal. A hotter running engine is prone to dieseling.