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My 2005 Chevy Tahoe has an Oil Pressure gauge. The needle usually sits to the far right on a value of 80. Recently it's started randomly dropping to 0 and as expected, the car's oil pressure alarm triggers and chimes. A few minutes later it will pop back up to the max of 80. When it moves between these two points it does so quickly.

In researching what causes oil pressure to drop to zero I found that there were generally 3 items: (1) low oil (2) malfunctioning oil pump (3) malfunctioning gauge or gauge sensor. The car's oil was replaced within the last 2 weeks and the dipstick shows full so I rule out 1.

My question: Please can you explain how the Oil Pressure gauge works on this type/era of car. My objective is to be better armed at understanding the impact of this aspect of my car in the context of what's currently happening.

(I have every intention of taking this to the service station to be checked/repaired as soon as I'm able.)

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Since this is a Tahoe, I'm going to assume it has the 5.3L LM7 (LSx variant) in it. These have an oil pressure sensor at the back top of the engine under the cowl portion (near the windshield). It should be on the back left side of the engine. These are just pressure actuated and allow (IIRC) from 0 to 5vdc to flow through, which the ECU then takes and computes the pressure. Here is an image for location reference:

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My experience with GM vehicles tells me your pressure sensor is most likely bad or the wiring is faulty. A 16 year old LSx engine will not have that much oil pressure. The engines from your era of LM7 truck engines had issues with lifters and such, which caused a lot of noise when they are worn. Oil pressure at idle should most likely be ~20-30psi, while at higher RPMs would be between 30-50psi. This, of course, is all dependent upon whether any internal engine work has been done. If the oil pump and lifters have been changed, you'll see higher oil pressure, probably in the 60psi range at higher RPMs, but unless you've had a high volume/pressure pump put in, you'll almost never see it reach 80psi except on initial start up before break in of a brand new engine.

All that said, the pressure sensor can go bad in these. Usually when they go bad, it will show full pressure or no pressure, but usually won't fluctuate back and forth between the two. Due to this, the first area I'd search for is to see if there are any noticeable problems with the wiring. After that, you could pull the plug on the pressure sensor, then put the leads from a DMM set to Ohms to see if it fluctuates when the engine is being revved. If it doesn't move, the sensor is dead. If it does, you'd need to look elsewhere.

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  • The DMM is set to ohms? Are these sensors current based? I.e. they change resistance with changing pressure and the ECU is reading the current not the voltage from the sensor connection. That's really cool, I didn't know that. I thought the changes in pressure changed the voltage output from the sensor. Doing it this way is likely more accurate and requires one less wire to the sensor. Thanks!
    – cdunn
    Sep 24 at 23:00
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I would connect a "physical" pressure gauge (Bourdon gauge) with a pipe and check the actual pressure. I would do this as soon as possible to make sure that there is no problem with a pressure relief valve or similar causing a zero oil pressure.

Once you know that the engine oil pressure is physically sound, then you can check for the other causes in the other answers: sender, shorts in wiring, head unit etc.

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There is a oil pressure sensor that screws into the engine block or head. An electrical connector on the sensor connects the sensor to the vehicle's wiring harness.

The sensor electrical signal is likely read by one of the vehicle's computers which will sound the alarm if it sees a low oil pressure. The computer will also send the signal to the oil pressure gauge on the vehicles dashboard.

The fact that the alarm is triggering as well as the gauge dropping to zero indicates that the alarm and gauge are agreeing with each other, so the sensor and the wiring to the sensor are the most likely culprits.

First thing to check is that the wiring to the sensor looks OK.

If that looks okay, then it's probably best to replace the sensor.

Since you said that the oil pressure drops to zero for a few minutes and the engine hasn’t seized yet, I would guess that the oil pump is actually working and that this is a false alarm, but I wouldn’t rely on it. If the oil pressure did show zero, you really shouldn’t continue to run the engine, because it wouldn’t take long for the engine to seize without oil circulating.

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  • That being said, and I agree, there is another issue with these vehicles where the actual motors that drive the gauges fails. There are tiny stepper motors with plastic gears that are controlled by a computer to move the needle indicator. Over time the plastic teeth snap off or bend and then the needle can just flop around. So it's important to know if the pressure is actually low, the sensor is bad, or the indicator is faulty. It's not difficult to change the little motor if you are somewhat proficient with a soldering iron.
    – jwh20
    Sep 19 at 20:42
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    @jwh20 Surely if it was just the gauge indicator that was faulty, then the alarm would not be sounding at the same time, don’t you think?
    – HandyHowie
    Sep 19 at 21:28
  • Well you would think that but I had a 2000 Tahoe that sounded the oil pressure alarm when the needle was low. While I didn't confirm what caused it, when I replaced the motor on the oil pressure gauge, the alarms stopped. I had tested the actual oil pressure and found that it was within normal limits.
    – jwh20
    Sep 19 at 22:40
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    @jwh20 You should add that as an answer too, if you have had that experience. Thanks.
    – HandyHowie
    Sep 20 at 5:42
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    @Guy It doesn’t take long for serious damage. Someone I know once stopped driving because his oil light illuminated. He checked his oil level and found it OK, so he started driving again. I don’t think he managed another mile before his engine seized due to no oil circulation because his oil pump had failed.
    – HandyHowie
    Sep 20 at 14:22

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