Bad news. I went through the trouble of replacing the intake manifold gaskets and the surge is still present. If you didn't see before, I also replaced the throttle body gasket and cleaned the throttle body. O2 sensor also replaced.

Same issue. Surge from 1100-1500 when warm and perfect idle at 750 when IACV unplugged.

 

I see some people have issues with aftermarket IACV's, and I think I have one on there. Looks new though.

- Tried something else, though: blocked off boost valve with aluminum plate. No difference in idle behavior.

Debating buying a new ECU.

 

- Contacted old owner, said he replaced IACV and has the same problem.
- Replaced ECU. Idle hunted for a bit but resumed symptoms.

I noticed something though. On the low end of the surge (from ~1400 to 1100, so on the 1100 side) it feels like the car is misfiring/idling rough, and the car lets in more air to compensate. Could this be a fueling issue?

Should I check my fuel pressure?

This issue is a nightmare.

 

Fuel pressure test says about 20PSI engine off car off.
Replaced fuel pressure regulator, same issue.

Maybe stuck open injector???????? Will measure today and report back.

Edit: All injectors show 2.0 Ohms at rest