I have an 89 S10 blazer 2.8L, have had it for about 2 years. Since I got it, the oil pressure gauge stays at 100% until its been running for about 15 min, then starts to bounce around while idling. In gear and driving it will stay at 100% but chatter slightly at the top of the gauge. Ive noticed that while idling, the bounces of the gauge seem to correspond to a slight shudder the engine has always had, the gauge reading drops with every vibration I can feel. I have new plugs, wires, ignition coil, cap & rotor, cleaned the egr, and a new fuel pump and filter.

Any ideas? Fuel economy is at spec ~17mpg...

 

It sounds like the sending unit. Try taking the wire off and see if it drop to zero.

 

i might check the wiring to the switch and make sure no wires are broken, im thinkin a bad ground possibly. i would do an oil change and filter to to make sure your oil is clean, had a 91 jimmy with same problem your experiencing found wire was broken and who ever had it last really didn't take care of her. oil was sludge (penzoil=sludge)

did oil change every 1000 miles for a while and she ran top no more oil switch fluctuation.

 

Good idea on the ground wire... ill check it out. I do change the mile on a regular basis, about every 3 months.