Alright, I'll try to keep it brief (been through tons of threads and I thought I had it). 4 mechanics have looked at it to no avail.
2005 Chevy Express 3500 4.8L V8 130k miles Code P0300 (not sure which cylinders)
Engine light flashes, but only on the highway after going above 52 or so for a couple minutes. If I slow down, it turns off. Sometimes, it stays on solid for about a day afterwards. Nothing at all seems off about the van other than the seatbelt light stays on always.
Fixes tried: New spark plugs/wires, Cleaned MAF/TB and ran some SeaFoam through gas tank, Crank Relearn
After tons and tons of research, I really thought the crank relearn was gonna do it, but I tested yesterday and...no good. Next up, I was planning on trying: Distributor cap? Whole distributor, haven't looked into it yet, O2 sensors (trying to start with cheap fixes first)
Thought of getting a diagnostic tool that I can use in motion, but I want to hear what you guys think. Been driving me crazy and I really want to get the stupid thing fixed. As a side question, I usually just drive under 50 so that it doesn't come on, but I have a feeling that it's probably still doing damage.
Thanks a bunch for any replies!