Bought truck in April. about one month later oil pressure would drop to zero and warning message would display Low Oil pressure and warning chimes would go off. Local GM dealer tested with mechanical gauge which fluctuated same as truck gauge, so they changed the oil pump at great expense. Oil pressure still fluctuates erratically but at a higher level and warnings have not come back on. Problem is still there because the pressure is erratic and weird the way it fluctuates. Cold start gauge goes halfway up, once warm drops below 1/4 on the gauge, driving on the highway at 60 mph when you let off the gas and decelerate for turning, gauge goes back up to about 1/2 way, every time you let off the gas oil pressure increases until the new rpm is reached and held, then it falls off again. 2 shops have tested the sender and gauge and say they are fine, it is an internal engine problem. Don't understand why pressure goes up when decelerating but only momentarily. Anybody had this happen?
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Does the angle of the vehicle affect the oil pressure?– HandyHowieCommented Oct 6, 2015 at 7:15
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It sounds to me like there could be an issue with the oil pickup tube, but they would have probably saw something here when they changed out the pump. Also, how many miles are on the truck?– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Oct 6, 2015 at 10:18
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Just under 300,000 kilometers on the truck, and the angle of the vehicle has no effect. One other answer on here about a related question proposes the theory that the thrust washer is worn allowing the crankshaft to slide forward and backward more than it should, in so doing the oil could leak out the sides of the main bearings. This would explain the "backwards" nature of the rise and fall in oil pressure. Sound plausible?– Rotten RonnieCommented Oct 6, 2015 at 13:59
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Finally bit the bullet, got a GM Reman engine, oil pressure is fine now. Still, wish I knew for academic interest.– Rotten RonnieCommented Nov 21, 2015 at 5:02
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2 Answers
Check the crank case ventilation. If it is blocked on the air intake side, a vacuum when under acceleration would suck against the oil in the sump. When you let off the gas, the vacuum would drop and allow the oil pressure to increase.
answered Oct 6, 2015 at 16:50
See this post, they cover several possibilities including the pickup tube, a damaged pickup tube seal, a defective pressure bypass valvue, or possibly a clogged filter screen located before the oil pressure sensor.
