Hi,
The idea that it runs normally until it warms up points me in a couple of directions. First, a faulty coolant temperature sensor. Second, a fuel pressure issue. And third, a possible issue with the crankshaft position sensor (CKP).
When starting to fail, the CKP can cause the vehicle to stall, misfire, and even not restart until it cools off. Interestingly, the sensor may or may not set a trouble code/CEL.
Take a look through this link that explains symptoms related to a failing CKP. See if you feel it mirrors what you experience.
https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor
As far as the fuel trims, I would think that would change when warm. By chance, could you ask him again for the short-term fuel trims when warm?
Let me know.
Joe
Wednesday, January 26th, 2022 AT 4:12 PM