500 is what I use as well but as long as the WARM engine idle drops to under target idle RPMs when IAC Valve is unplugged its OK
There are a few reasons it might be higher than 500, one is that the anti-diesel screw needed to be adjusted to set TPS voltage a bit higher to get 4.5v at wide open throttle(WOT)
And you could be right, a small vacuum leak could cause the extra 100 rpm, but it wouldn't effect engine operation since IAC Valve can set minimum target idle(above 600) and its not enough extra air to even set Lean code
I'm gonna flush my coolent bc it looks really gross, maybe oil change, (I know this should have been first step before smog). I'm hoping/thinking that I haven't completed the drive cycle correctly, especially the coasting down from 55 to nearly a stop and returning to that speed - which is a bit scary in a busy city (SD), I'm gonna attempt it on a weekday mid-night. Also I talked to my local mechanic and he said, to put some 91+ fuel and injector cleaner in the tank and then do the drive cycle. So I'll try that. (fuel filter was changed about 15k miles ago)
Thanks for all the info. The positive is - I'm learning valuable car knowledge.