Who doesn't love a good GT40 idling issue?

Problem:
Starts fine, idles high(1200-1500) for a 15 seconds or so and eventually comes down to 800-900 rpms. After idle down it searches for a steady idle(rolls). This happens when it is cold, it happens when it starts to warm up and happens when warm. It does however after a few minutes it lowers in idle and then idles between 680 and 950.
Also, any time you go above idle it again takes time for it to idle down and then rolls.
While I know that a rolling idle is not an uncommon thing, it is so persistent that it makes it very difficult to drive because with all the starting and stopping you are constantly dealing with a 1100 rpm.

Attempts:
I have replaced almost everything at some point for other issues and trying to sort out the idling issues. I'll list all parts replaced even though I know some of them are not related.
Cleaned throttle body and checked for any hold up with valve and throttle arm
checked for vacuum leaks with starter fluid
Removed tank and cleared and cleaned all valves and hoses(besides supply line and return line)
Replaced:
Low pressure fuel pump
FCC fuel filter
Injectors
fuel line between LPFP and FCC
High pressure fuel pump
Fuel pressure regulator
Idle air control valve
Spark plugs
SP wires
TPS
MAP sensor
vacuum line from map sensor to throttle body
ACT sensor
ECT sensor
PCV valve
and finally the ECM(might still be the issue, bought used on Ebay)

I have read through almost all the threads on the GT40 but I cannot figure out what is wrong and what to try next.
B

If you go to the link below, Joe seems to have solved his idling issue with an adjustable fuel pressure regulator. You mention that yours has been replaced but was it with an adjustable one?

Compared to the cost of that laundry list of parts that you've replaced, it's a drop in the bucket cost wise (and it sure is pretty looking)

It may or may not solve your issues, but you won't know till you try


link

KENO wrote: It may or may not solve your issues, but you won't know till you try

Well at least you tried

DHMcFadin wrote: When you removed and cleaned the throttle body, did you replace the gaskets as well? If so, do you mind providing the source you ordered the gaskets from? Thank you!

I did not replace the gaskets because they seemed in good condition. However, you can find them at skidim.
Intake Gasket