Scotty, HELP! I have a 1989 GMC SIERRA 2500. The truck has 192k original miles on it. Runs great except, after it's been running a while, I notice the oil pressure gauge will fluctuate from. 60 down to 30. Only when I am idling with my foot on the brake. Local dealership says it isn't throwing any codes, so tjey are at a loss. Any insight would be appreciated. Btw, love your videos and have seen more than I care to admit. Lol
A 1989 vehicle is not going to be "throwing codes" in the sense of a modern one with OBD2 diagnostics that can be read with a generic code reader. The limited OBD1 codes available via the "paperclip scan tool" do not include anything having to do with oil pressure. These would be the codes available:
https://mechanicbase.com/trouble-code/obd1-codes/
Use a mechanical oil pressure gauge to confirm what's happening. (The dealer should have done this.) For most engines 30 psi at idle would be perfectly acceptable, but you'd have to look up the specs for yours to check that.
