The first start of the day, my 1997 GMC Sierra starts like new. After driving it won’t start unless I hold the key ON for another 1-2 seconds. After driving the past 2 days there was a pre-ignition upon restart. I didn’t experience any problems while driving. It has 250,000 miles.
But it still starts? Just starts a bit worse when hot? But it still starts? Not sure you actually have a problem, just high miles.
I isn’t fuel since it starts right up cold. Possibly ignition module. Very possibly the engine doesn’t have quite the compression it once did and the cold thicker oil helps that compression on startup.
Go to the basics if you really want to find the issue… Run a compression test and see what condition the engine is in.