if its not vacuum related then I would look to the opti or other electronic engine control unit.

 

Was thinking that measuring the intake vacuum is kind of the same thing as reading the MAP value (not saying I wont do it later anyways)
So I had a look at my live date recording and the MAP value, from the little glimpse I saw it fluctuated between 26 -43 kpa (3.7 - 6.2 psi) , and I’m thinking (might be all wrong here...) this is likely due to the IAC regulating, and if so its doing it as the ECU tells it to, and then next questions is what sensor/device tells the ECU to do so...

Regarding possible defect electrical components being the cause for the surging, two things points in that direction, its heat related and intermittent.
And I did place an Rockauto order the other day for a new ICM and Coil as the current ones are old, maybe even original, so good idea to replace them regardless as they are crucial parts for the engine.

 

Saw at least two youtubers that claimed that the engine should shut off if disconnecting the MAF, if it didn’t the MAF was bad, anyone who knows if this is applicable on our LT1’s?

Because mine doesn’t shut down, disconnected it a second time today while surging, and no response at all except the check engine light to come on.

Checking the live data for MAF it moves between 7-16 (g/s) while surging, and the numbers seems to go up when the rpm goes up, which gives a normal impression, was thinking about back-probing it but one of the youtubers I mentioned said chevys MAF needs to be measured in AC / hz, and I don’t have that on my multi-meter

Ran graphs on all 4 O2 sensors, both when driving and at idle (surge), looked similar for both banks, sensor 1’s a lot more active than the 2’s.

Also checked the short time fuel trim at surge, slightly below 0 but not much, -1 to -3 and +1 to -2, and on the road as mentioned earlier -5 to +5

 

just for heck of it unplug the ac pump or maybe pull ac fuse. see what happens.

 

Thanks for the suggestion
In the live data the OBD says AIR OFF plus my AC is not functional, but I will add it to the list on things to test anyway, makes sense that the IAC would be impacted if the AC was on or not

 

Below the latest updates in the surge hunt

Vacuum leak trace
Replaced the 90* hose under the elbow going to the PVC, a 10mm fuel hose that can be bent without block itself seems work fine there.
This is the very last unchecked vacuum leak place I can think of, the old hose had a small crack in the turn, but likely it was done when I pried it off with a screwdriver but can say 100% if there was a tiny crack before.
This hose is a bit difficult to reach with the carb cleaner spray as the elbow is in the way

Electrical stuff
Installed new Delphi coil and ICM, and while at it space them off good from the head to spare the components from heat (the coil bracket sat directly on the head before with just some washers in between that I had put there)

Also pulled the AC/Cruise fuse, just in case the AC should somehow affect the IAC, as you suggested Craby

And the result:
It actually got a lot better but its not gone, yesterday was the hottest day of the year around here so far (90*F), and I took it for a long drive with some WOT’s which under earlier circumstances would have made the surge monster furious.
Also have feeling the idling got just slightly rougher now, its acceptable but not so silk lean as before when not being in the surge mode.

One possibility I’m thinking of is if I have multiple vacuum leaks and the hose under the elbow being one of them, that would explain the improvement, but the question is where other one is at...

 

I was watching my live date recording when I spotted something interesting.
Just for a second the rpm jumped up to 5800 although very obviously the engine was still in the 800-1000 area, saw it happens twice and there could have been more as you have to scroll through the views on the OBD reader, so the recording don’t show all at one time.

 

how old is the optispark? dont recall if you replaced it or not.

 

The car came with a red MSD unit, definitely not new but does not look that super old either.
Its missing the vent hoses, which I after a lot of investigations managed to remanufacture myself with some fuel hoses, a brake booster check valve and a fuel filter.
When I was to install them I discovered that the Opti is very far from airtight as the big O-ring had crawled out at the bottom (see pict), likely due to oil leaks, so I decided to postpone installing the hoses.

My long-term plan is to replace the opti with a Petris (as this has the Mitshubishi sensor), and while at it replace all sealings in the area and of course the water pump.
Figure it would be a perfect winter job, doing it slow and right.

One thing I have seen in many threads is that problems turning out to be the Opti, typically showed driving issues too, miss fires, hesitation etc, my car drives good out on the road, pulls good at WOT, so its a pure idle surge problem in my case.

Regarding the RPM fluctuation, and I realize this might be a OBD glitch, but I’m thinking a closer look to the CPS wouldn’t hurt, that too is oily from leaks, to check if it positioned right and maybe back probe it to see if any similar jump shows in the voltage, at the moment I’m searching for info about how to this.

Other unturned stones are spark plugs (checked two of the easy one's that looked fine) and wire, the later looks old, also thinking on possible bad groundings to the PCM for ex

 

I finally found the source for the screaming lady I mentioned earlier in this thread , it turned out to be the electrical smog pump that for some reason occasionally gives this sound when turning ignition ON and OFF or early after the start up.
Will pull the pump off and see if it possibly can be repaired or find a working used one. Did check the check valves which both seems ok.
Tried to upload a recording with the scream, hope it works

As for the surge problem, its still there but lesser than before replacing the ICM and coil.

It is very definitely a heat related issue and I’m thinking a bad heat exposed ignition component or possibly a fuel injector that when reaching a certain temperature partly bails out and then the ECU tries to compensate for it, and hence the surge.

The fact that replacing ICM and coil (which could have slightly boosted up the spark) reduced the problem, might be due to they compensates a little for the really bad component in the chain.

So next step will be to replace spark plugs and wires and the Crank pos sensor that I ordered yesterday, there is the Nock sensor also but I think I’ll replace the Opti before that unless there is some cleaver way to rule one of them out.