VERY BAD IDLE QUESTION.

1991 Nissan D21 2.4L Manual. 97k miles. Extremely bad Idle. All original.
1st owner. Searching for known issues not obvious.

Searching for cause of extremely bad idle.  Started rough years ago and got
progressively worse as time went on. Runs almost fine past idle and high
rpms. Driving , it runs normal. Back to idle, its very bad.
Checks and parts replaced.

Vacuum leaks...check.
AAC, FICD....check Idle air control and assist...check
new plugs and wires...check
new O2 sensor...check
new cap and rotor...check
EGR removed and cleaned and holding...check...solenoid working...check
Fuel injectors working. ohm in spec....check
coil firing...check

Going to smoke engine to see if I'm missing a vacuum leak...nothing
obvious. all vacuum hoses...check
Going to do fuel pressure test next....runs fine past idle and drives normal.
Replaced fuel filter...check.
Leaky injector? Plugs all look normal. No soak. Check.
Anything im missing, out there , please respond.
I will check timing...no chain rattle. Positive the chain didnt jump. Excellent
mechanical condition. no valve clacking or engine noise.

Truck starts right up and idles like crap. Chasing a boogie man problem for
now. All sensors test fine and normal. Not obvious. Real Bad Idle like if you
unplugged the vacuum to the brake booster. It should be obvious , but so far,
cant find it.

I am a trained car guy, but just a DIY home mechanic. Any recheck or pro
advise, please.

Thanks for reading guys. Ive helped countless others, but I cant help myself,
lol.

camaro2778@aol.com
Thank you!