DISCLAIMER: I'm not an expert mechanic, and I've never experienced this kind of gauge problem first hand (so far...), so take any advice that follows with your desired quantity of appropriately-sized grains of salt.
From what I've read, it tends to be caused by bad grounds, somewhere near the gauge cluster or ECM, but potentially anywhere (grounds on the GMT400s – and, I suppose, any 20+ year old car or truck, especially in the "rust belt" states or other wet climates – seem to be prone to going bad or getting intermittent due to rust and/or corrosion), and the gauges (oil pressure, temp, fuel), as I understand, rely on a proper reference to ground to measure the sender's signal against, measuring the difference between them (I think?), and for some, if the signal goes beyond tolerance – say, due to a weak or broken ground somewhere – they will peg to either the left or right (depending on how they're configured; I think the fuel gauge is configured to read the difference and will peg to "F" if it reads an open circuit. Not sure about the oil and temp) like you describe because they're reading infinite resistance (basically, open circuit). Sometimes, the opposite can happen as well (a short can cause zero resistance, causing the gauge to peg to the left or right, again depending on how they're configured). Again, this is based on various threads I've read describing similar problems, and again, I'm not an expert mechanic, so I could be completely wrong.
That said, if you check the wiring between the sending units and the ECM or gauge cluster, it certainly can't hurt anything to also try checking and cleaning every ground you can find (I think I read that there's one in the engine compartment that can sometimes rot away completely and cause all kinds of problems; I suggest searching the forum about it for more information). Maybe that will fix your oil and temp gauges, or maybe it won't, but at the absolute minimum, your electrical system should perform better overall, because there will be less resistance at the ground points, so current can flow more efficiently (better grounds, logically, should help improve things such as headlight brightness, HVAC fan output, and windshield wiper performance).
c
From what I've read, it tends to be caused by bad grounds, somewhere near the gauge cluster or ECM, but potentially anywhere (grounds on the GMT400s – and, I suppose, any 20+ year old car or truck, especially in the "rust belt" states or other wet climates – seem to be prone to going bad or getting intermittent due to rust and/or corrosion), and the gauges (oil pressure, temp, fuel), as I understand, rely on a proper reference to ground to measure the sender's signal against, measuring the difference between them (I think?), and for some, if the signal goes beyond tolerance – say, due to a weak or broken ground somewhere – they will peg to either the left or right (depending on how they're configured; I think the fuel gauge is configured to read the difference and will peg to "F" if it reads an open circuit. Not sure about the oil and temp) like you describe because they're reading infinite resistance (basically, open circuit). Sometimes, the opposite can happen as well (a short can cause zero resistance, causing the gauge to peg to the left or right, again depending on how they're configured). Again, this is based on various threads I've read describing similar problems, and again, I'm not an expert mechanic, so I could be completely wrong.
That said, if you check the wiring between the sending units and the ECM or gauge cluster, it certainly can't hurt anything to also try checking and cleaning every ground you can find (I think I read that there's one in the engine compartment that can sometimes rot away completely and cause all kinds of problems; I suggest searching the forum about it for more information). Maybe that will fix your oil and temp gauges, or maybe it won't, but at the absolute minimum, your electrical system should perform better overall, because there will be less resistance at the ground points, so current can flow more efficiently (better grounds, logically, should help improve things such as headlight brightness, HVAC fan output, and windshield wiper performance).
c
