My car has the same problem. The last time this happened to me it was a gunked up idle air control valve located on the throttle body. Going to try that first. If i remember, they are about $15-$20. It is what controls the amount of air is fed into the idle circuit. When they get cruddy they dont adjust quick enough and can cause that low idle on decel and it straightens out after a second.
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When I'm coming to a stop or slowing down to turn my rpm drops to +/-500. the car hasn't stalled on me yet but it seems like it's going to.
I know I have a small vacuum leak and I'm having an issue with the transmission also but i don't think it's related.
The car is an 86 regal with a mild cam 5.3 and a 4l60e.
It could be several things. It sounds like you need to check/fix all the little leaks you have going on. If the O2's are seeing more air than the engine is actually getting it will throw the IAC count off.
Also, are you running a stock PCM or has it been tuned by HP tuners or something similar? If it has been tuned that might be the problem or at least part of it.
There are several things in the tune that will control how fast/slow the idle drops off when you get off the gas.
The only way I could see it being a torque convertor issue is if its not unlocking and is pulling the engine speed down.
Don't rule out bad O2 sensors! Experienced this first hand while knowing for a fact that it was my O2 sensors being fowled. In addition to the RPM dipping when coming to stop, there was also rough idling when hot operating temp was reached (not sure why not when cold (random misfires P0300--not actually misfires, but rather imbalanced firing as confirmed by a Snap-On scanner)). But my sensors were severed fowled, so a mildly bad sensor might just give the low RPM you're experiencing.
Keep in mind also that, if it is/was your sensors, unplugging other sensors around the intake might make it appear as if it fixed the problem when, in reality, you've only bypassed it.