Sorry you’re have such trouble solving this problem.
I think the advice on the Holley forum about checking exhaust leaks and the secondary butterflies is a good starting point. I was actually having IAC troubles at one point and it turned out it was being caused by both of those. The secondary butterflies would stick open very slightly with a hot engine causing higher than set idle speed and 0 IAC. once that was identified and fixed, exhaust leaks (I went through a few sessions to find and fix them) would cause a false lean condition, wreaking havoc with my AFR and IAC. The last exhaust leak to fix was literally a pin sized hole in a weld joint just before the O2 sensor.
I know you’ve been investigating these based on the Holley forum advice.
here are some things I’ve learned from my personal experience.
- don’t trust exhaust band clamps to completely seal. I could never get them to seal and have since switched to either welded pipes or V clamps.
- header ball joint clamps leak, at least mine did. Switch to V clamps for a reliable seal.
- use a shop vac in blower mode and connect it to your tail pipe. Spray soapy water on all joints and exhaust manifold/header connection points. If you’re using headers, don’t forget to check the joint where the 4 pipes come together at the collector and check the O2 bung. You are using a weld in O2 bung, right. that O2 clamp that comes in the kit is not reliable.
- if/when you are experiencing an IAC of 0 at idle (assuming you’ve already accurately set the IAC to between 2%~10% at idle at running temp) it means the engine is taking in air from somewhere - that is why the IAC is at 0. Take notice, is your idle speed above what you’ve set? if you blip the throttle will it return to normal operation (IAC and idle speed)? Or if you pop the hood and manually work the secondary linkage closed, does normal operation resume? If yes to either of these, your secondaries are sticking open. Pull the Sniper off and go through the adjustment procedure. Secondaries sticking open with a hot engine is/was a common issue people were seeing.
sorry if this is obvious stuff for you, I’m not familiar with what you’ve actually done or your troubleshooting methods.