Trying to track down a rough/surging idle problem. I've already done a lot of troubleshooting:

- Verified vacuum hose routing.
- Checked for vacuum leaks. (I could have missed something.)
- Replaced a number of vacuum hoses. (Not all of them yet.)
- Replaced the throttle body gasket
- Cleaned/swapped the IACV.
- Checked/swapped the FITV. (Plugged the FITV port in the throttle body to check it.)
- Checked the Air Boost Valve.
- Checked/adjusted the throttle position sensor.
- Adjusted the idle adjustment screw
- Checked EGR valve
- Swapped MAP sensor
- Swapped ECU
- Bled cooling system
- Checked/adjusted timing

Now, here's the question on my mind: I removed the IACV and Air Boost valves, and blocked off their ports on the manifold. Still, the top throttle body port (the one used by them) had a lot of vacuum, and covering it caused the motor to stall out.

Where's that vacuum coming from?