I've installed the TPI from 85 Camaro onto a SB 400. All painless wiring products with 24lb injectors. The system is set up for 91. Computer from Howell engine development. Engine is all stock but rebuilt. Runs good and strong except for a decent idle. Idle vacuum is 8 inches to zero as the engine is searching. Up and down it searches. It has been driven long enough for IAC to reset. All parts are new!!! Base timing has been set at 0 with EST disconnected. And although I've been told it should be 6 and 8 neither one of those settings changes the vacuum. No codes. My best guess is there is a vacuum leak out the bottom side of the intake due to the surfaces not matching well enough. I've checked it numerous times for leaks above and nothing. When propane or starting fluid is injected directly into the intake the engine smooth's out but does little for the vacuum. I've got a Monitor 4000 connected to it and the BLM is 136 to 128 and seems pretty steady moving back and forth relative to the engine surging up and down. Any more ideas??

 

My suggestion is put ECM into diagnostic mode at ALDl connector,
this puts IAC extended into its port, then disconnect IAC, remove ALDL short, and set base idle RPM with its setscrew.
Once idle set, then eng off, remove short from ALDL, then turn ign on, eng off allowing ECM to set IAC. then ign off, then turn engine on.

 

I'll give that a try, Thanks

 

Say I just got around to trying this process you suggest. However theres a little confusion as I read it. You state to "remove ALDL short" twice. ? I'm not sure at what point did I put it back in"

 

Also, is the aldl shorted with the engine running? I forget, its been so long since I did this. Luckily I found my aldl short tool. In other words do I start it, then short it? Then proceed

 

Good read, My mistake.
Proper method is key off, eng off, install short.
Key on, eng off, this will stand IAC, unplug IAC.
Key pff, eng off, remove short.
Key on, engine on, set base idle.
Key off, eng off, plug in IAC.
Key on., eng off, sets IAC by ECM/PCM - 15 seconds IGN on
Key off, eng off.
Now drive vehicle for ECM self-learn