1991 gmc sierra forum
low oil, loose connection to oil pressure switch or instrument cluster, bad oil pressure switch, too much clearance between oil pickup tube and oil pan, bad oil pump,
I went to synthetic oil in my 95 GMC Sierra and about the same time, my son went to synthetic oil in his 98 Chevy pickup. First my oil pressure gauge went nuts then all the way over. It drops back to "normal" sometimes now after several miles on the oil change. My son's gauge did the same thing. He changed back to regular oil and his gauge is working again. I'm going to change mine and see what happens.
I need to add that when mine "goes back to normal" I meant that it only does this sometimes when I let off the gas to turn off the road or at a red light. As soon as I take off, at around 1800 rpms it will go all the way over again. Not a tick, peck or knock anywhere. It sounds really strong.
These trucks are infamous for faulty or irregular sending units or "sensors". Both temp and oil pressure sending units are faulty on my 94. They are both really easy to fix considering whether or not you own tools and know what you are doing.Both are inexpensive to replace and each costs about 20 bucks at your local parts store. (For good ones). Oil pumps rarely fail and are replaced more often under actual suspicion than actual failure. Actual engine and bearing damage it rare and can be ruled out if there are no metal "shavings" in your oil or strange hollow knocking sounds from the engine
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