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Oil pressure gauge needle is irratic

Tiny
PACKERS27
  • MEMBER
  • 2000 GMC SIERRA
  • 5.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 230,000 MILES
It seems like the needle is center in the middle at 40 but it bounces left and right from 20 to 60 really really fast. When I am accelerating, it seems to not do it. If I'm on the interstate and going a constant 70 mph or so, it hardly moves but still moves. There is no driveability problems and the oil levels are good. Maybe a sender?
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Wednesday, October 30th, 2019 AT 5:05 PM

24 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Hi,

It could be or it could just be a sensor issue. Oil pressure is created by the oil pump pressurizing the oil but it needs resistance to do this and that is created by the distance between the bearing. If this is excessive you normally have lower pressure all the time. Erratic pressure that goes with RPM is normally always the pump. Erratic pressure at various times is normally the sensor.

If you can get this to be erratic by doing the same thing all the time this is almost always a mechanical issue which would be the pump. I would suggest hooking a mechanical gauge and making sure it does the same thing your in vehicle gauge is doing. If so, it is the pump. If not, most likely the sensor.

Let me know if you have questions. Thanks
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Wednesday, October 30th, 2019 AT 5:51 PM
Tiny
PACKERS27
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I've had the truck for a few weeks since I bought it. So since it's cool in the mornings it reads about 60 when first cranked. Then it goes to 40 but its hard to tell because it starts bouncing. The gauge changes as the RPM's go up but the needle starts to settle when I'm at top speed. What kind of gauge do I use and where do I hook it up at? Also, what kind of headache will this be to me if it is the pump?
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Wednesday, October 30th, 2019 AT 6:41 PM
Tiny
PACKERS27
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  • 187 POSTS
Also, I thought this issue would throw a code from the sender.
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Wednesday, October 30th, 2019 AT 6:49 PM
Tiny
KASEKENNY
  • MECHANIC
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Okay. That sounds like the pump. A mechanical gauge will confirm it.

I attached the needed documents. As for replacing it, it is pretty involved. The oil pan and front cover have to come off but can be done.
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Wednesday, October 30th, 2019 AT 6:59 PM
Tiny
PACKERS27
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I apologize. Forget what I said about the oil levels. I checked it once and that's when the motor was warm upon looking to buy. I just checked it cold. It was a half quart low. I topped it off and cranked it. Here's what I noticed:

The oil pressure needle stopped acting crazy but there was a little quiver to it that may have fluctuated left to right by one little mark and that wasn't constant. I took these readings, as followed:

At idle: 58 oil temperature on cold start 160% at 800 RPM. Oil temperature at 50 180% at 500 RPM. I took the truck down the road, came back and at idle again 42 oil temperature at 190% norm 500 RPM. Then I revved up to 1000 rpm 58 oil temperature. 1800 RPM 65 oil temperature. 2200 RPM 65. 3000 RPM 74 oil temperature. After this, I notice a burning oil smell from under the hood as I've noticed since I've had the truck. I don't have any visible leaks from underneath and the motor looks immaculate with no oil to be found.
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Wednesday, October 30th, 2019 AT 8:11 PM
Tiny
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Can you get a video of the what the gauge is doing now? Sounds normal but it may not be once I actually see it.

The oil smell could just be due to the crank splashing it up on the cylinder walls and it burns it off. This happens when it is just a touch overfull. It should go away once it burns enough off to where the crank is not splashing in the oil any longer.
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Thursday, October 31st, 2019 AT 5:45 PM
Tiny
PACKERS27
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Okay, so I replaced the sending unit. I cranked it up from a cold start and the gauge is at 15 and hasn't moved a bit. It doesn't even move when I rev the motor. What's going on?
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Saturday, December 7th, 2019 AT 3:26 PM
Tiny
PACKERS27
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I just revved it up to 2,500 rpm's and the needle twitched between 15 and 20 and stopped back at 15. I tried to rev it up to do it again and it didn't flinch.
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Saturday, December 7th, 2019 AT 3:32 PM
Tiny
PACKERS27
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Now I'm getting a light that says change oil. I just had an oil change about 2,000 miles ago.
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Saturday, December 7th, 2019 AT 3:39 PM
Tiny
PACKERS27
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I was reading about the oil pressure staying low, but why was it flickering when I had the old sending unit in if the gauge is the culprit?
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Saturday, December 7th, 2019 AT 3:58 PM
Tiny
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Okay. We need to measure resistance of the sensor and see what it is reading. This will tell us if the gauge is accurate.

If you don't know how to do that, you need a multi meter and back probe the sensor on each wire and put the meter on ohms. Then see what it says. It should read about 24 ohms if the PSI is 15.

As for your attachment, what they mean by the gauge reads low and stays low is it stays at 0 when you start the engine. Not a low reading like you have. If the gauge is 0 when the engine starts and remains there it is the gauge. Then that is misleading as well because when they say the gauge, that could mean the sensor as well.

If it were accurate it should say "if the gauge does not give a reading after the engine starts and warms up then the gauge is not giving an accurate reading and need to be diagnosed. When an engine is running, it has some amount of oil pressure so if the gauge says 0 then it is not accurate."
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Saturday, December 7th, 2019 AT 6:31 PM
Tiny
PACKERS27
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So this needs to be checked for ohms while the truck is running?

Back probe? You mean just simply penetrate the harness wires?

Which lead goes on which wire?
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Saturday, December 7th, 2019 AT 7:28 PM
Tiny
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Correct, while the truck is running. You can penetrate the wiring if you are going to fix it. If not then you need to back probe the connector.

Here is a video that shows how to back probe. Around the 1:30 mark is the back probe portion.

https://youtu.be/g2ST8w_0w-Q

You put the red lead on the wiring leading to the cluster, and black on the ground. See the diagram.
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Saturday, December 7th, 2019 AT 7:56 PM
Tiny
PACKERS27
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Okay, will do tomorrow. Does this sounds like a possible oil pump issue? The truck is driving fine, no issues as far as I know of. It's funny how the new sender stalled the gauge and the old one made the gauge flicker.
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Saturday, December 7th, 2019 AT 8:26 PM
Tiny
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It could be. However, get a mechanical gauge and hook it to the engine and see what it says. If this is the same then you know the gauge is correct.
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Sunday, December 8th, 2019 AT 8:02 PM
Tiny
PACKERS27
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So I tried to back probe the connector. I got the lead in the positive wire but not sure if it was in there good enough. I shoved it in as far as it could possibly go. The only other wire I found next to it was not connected to the connector. It felt like it was connected to another connector on the bottom of the housing where the sensor screws into to. I tried to probe into the wire but don't think I got in there good enough because there is tape around the black wire. The gauge didn't read anything. Is there suppose to be 2 wires on top of the connector or just the tan one?
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Sunday, December 8th, 2019 AT 9:35 PM
Tiny
PACKERS27
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So the oil pressure gauge screws on the threads where the filter is?
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Sunday, December 8th, 2019 AT 9:50 PM
Tiny
PACKERS27
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I'm not understanding running the oil out of the truck while its running. How is this done safely? Please explain this to me.
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Sunday, December 8th, 2019 AT 9:53 PM
Tiny
KASEKENNY
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Yes. There are two components to this. The first is the oil filter replacement adapter and the second is the gauge with the pressure line that threads into the adapter. I put a square around each and an arrow to the pressure line.
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Monday, December 9th, 2019 AT 6:57 PM
Tiny
PACKERS27
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So before I put the components on the filter housing, I need to simply make sure the oil levels are good, that's all?
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Monday, December 9th, 2019 AT 7:12 PM

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