low idle issue..on stroker

1989 laredo renix stroker auto tranny. idles around 2 to 300rpm? i have tried swapping t-bodies,injectors,idle air valve,tps sensor etc. no catalitic converter, tried plugging off egr tube and even leaving the egr tube off and un-corked to manifold and idle the same. the odd thing thats getting me is unplugging all the vaccum lines on the tree, the idle increases close to what it should be but if i plug them all back in it drops the idle down too far and runs rough. i could sure use some help on finding my problem on what it could be..........thanks.

IAC stuck shut???

Since you say you replaced the IAC, that pretty much leaves the drive coming to the IAC. Follow the wires from the IAC all the way back to the ECU. If they are all fine, I would say you have a blown driver in the ECU and you need a new ECU.

If the power is way down, you might expect a plugged CAT, but you dont have one.

The computer knows what the engine rpms are and should sent the correct sequence of pulses on the IAC lines to force it to open..

IAC..that is the one next to the tps on the throttle body correct? if so it has been replaced with a new one with the same idle problem.

old man, can i back probe on both ends (computer and at iac) to trouble shoot it? if so what voltage would i be looking at?