89 corvette idles rough when warmed up

You need to have the choke adjusted so it runs on a fast idle so the engine can warm up or you may need the carb to be rebuild and adjusted properly.

check the fuel injectors - seems the new fuel is messing these up...

mcdevito75 here Sounds like 1 of the fast or air idle sensors are bad, have your shop run a scan.

i want to give you the right answer but the only thing i think of is the mass air flow sensor.

Hello,
The car may be hard to start because it may need a normal tune-up. If the spark plugs are pretty used, they will slowly stop burning as much fuel because the spark slowly gets lesser and lesser. On a cold day the fuel is harder to ignite with a small amount of spark.
Normal idle when its cold means that your cars idle is probably too high. If you lower the idle, the choke on a cold day will keep the idle a little higher until the motor warms and then it will kick back off. If the idle is too high, it will run very rich, smothering itself with fuel, and causing the idle to stay low. When the car warms, its creating better spark when its warm, and with the idle being too high its running at a fast idle.
The car wont start sometimes can be the cause of a few things. If the ground is bad on the fuel pump, it wont make a good connection sometimes for when you turn the key on, the fuel pump dont pump the fuel to the engine. Another one can be that your accessory switch can be loose going to the fuel pump. I think its your ground though. Good luck! Hope this helps.
Have a nice day. -Steve

i would clean the injecters

on top or near the thermostat housing is a sensor it tells the ECM what to do corvettes run hotter than average cars 210 f i believe, i wish i could chek it hands on in '' REAL TIME" FIND A MECHANIC WITH A SNAP ON SOLIS, DONT SPEND ANY MORE $ ON IT UNTIL IT IS MONITORED IN REAL TIME

iwould say try some octane booster sounds like poor quality fuel p0300 is a multiple misfire so start with that and i believe vettes are supposed to run 89 or better octane

replace the coolant temp sensor, not the coolant temp sending unit

if u have a damaged plug it can be running on 1 less cylinder ,it will idle very rough,change it!