Rough Idle - First start in 15 years

[verygq]

Posting to all the usual places so you regulars will see this a million times, my apologies. Not sure which forums are still active these days.

So, some background.

The chassis has 420K miles and then sat for around five years, the block had cracked due to overheating and so a donor engine was sourced. The engine came from a car that hadn't ran in 15 years but only had 85K miles. When I pulled the valve cover it looked almost brand new inside, that's as far as the inspection went but everything looked clean and clear. Valve clearances were also adjusted (minor adjustments needed).

In the last year or so I've begun the slow journey of putting everything back together so lots and lots of 'consumables' have been replaced.

Fuel pump, fuel filter, spark plugs, wires, dist cap and rotor, fuel injectors, fuel pressure regulator, IAC, oil/filters, etc. MAF has not been replaced but it was ran without it and it ran similarly. (Not to that say that any of these can't be the fault, but just running down a possible list of things which are 'presumed' good). Tank was also completely emptied and refilled before starting. Brand new cat but not a new O2 sensor.

It's been started a few times for 5-10 minutes at a time to bleed the coolant, etc., and has ran consistently the entire time.

Idle is rough and stumbles hard when given hard throttle input and then the idle is even worse afterward. Quick, small blips of the throttle are okay and long, slow throttle input is okay.

Reaching out to see if you guys have any input on where to start troubleshooting, it's been so long since I put my head under the hood of this one so I'm not as seasoned as I once was.

[shogun]

I would drive the car some hundred miles that all the drive components get to operating temperature and then see if the problems are gone.

[CamE32]

Something similar happened to me and it was coolant that had gotten into the distributor. For some reason it was fine until given more throttle and/or engine warmed up... then it behaved like the end of your video until stalling + impossible to restart until cold. It was infuriating to me as I had similarly replaced everything on your list (+ CPS, fuel pump, coolant temp sensor, etc.).

I replaced the water pump and then almost immediately experienced this problem on the test drive (which left me stranded!!)... I had to surrender it to a mechanic for them to discover that coolant somehow got into the (<10k new) distributor. I think you probably got some in there too, from bleeding - don't ask me how, but that's my theory for now.

My M30 is also low miles so I literally couldn't fathom what could be wrong - plus I have a parts car and swapped over the obvious stuff like MAF TPS ICV so I knew those were all good. My mechanic even checked the timing chain (which I can't fault him for because he knows of everything I've replaced/is new) before returning to basics and pulling the dist, cap & rotor.. lo and behold, soaked and ruined