Diagnosing erratic/high oil pressure...

[The Raven]

Nice job on the sleuthing. I would also trace the wiring for the sensor carefully and look for chafed wires. The centripetal force of the turns is likely either moving exposed wires together, or messing with the ground point for the sensor or the indicator circuit.

Yeah I thought of that too, because the issue doesn't seem to happen when the truck is sitting, idling, on level ground. So i put it in the garage, let it run with the hood up, and made sure the gauge was stable, then had my son watch it while I poked and prodded the sensor harness and the bigger harness that those wires feed from. Got zero movement.

Having watched the weird behavior at length, i've found that it really is completely random except for two things - it never does it while sitting on level ground, and it ALWAYS does it during hard left turns. No matter what, I can always peg the gauge at 80 by turning hard left. It will not budge though during a hard right. Other than that, it's completely random...it can behave totally normally for an entire drive (without hard left turns obviously) and conversely it can go nuts while parked if i'm on a hill. One other thing I noticed last night, it SEEMS like I can increase the likelihood of the gauge going nuts by forcing a downshift. This lends some credibility to the idea that it really could be a bad sensor because the increase in engine revs would subject the sensor to higher oil pressure. But it's far from conclusive.

 

[Dustin Jackson]

@The Raven I know you checked it with a mechanical gauge already but do you have any OBD2 monitoring tools? I would compare what your dashboard oil pressure gauge to what the computer sees to give you an idea of if this is a sensor wiring issue or a cluster electrical issue. This won't tell you what the problem is only point you in A direction

 

[Geotrash]

@The Raven I know you checked it with a mechanical gauge already but do you have any OBD2 monitoring tools? I would compare what your dashboard oil pressure gauge to what the computer sees to give you an idea of if this is a sensor wiring issue or a cluster electrical issue. This won't tell you what the problem is only point you in A direction

That’s a good idea.

 

[The Raven]

@The Raven I know you checked it with a mechanical gauge already but do you have any OBD2 monitoring tools? I would compare what your dashboard oil pressure gauge to what the computer sees to give you an idea of if this is a sensor wiring issue or a cluster electrical issue. This won't tell you what the problem is only point you in A direction

Two issues with the idea of it being a gauge issue - first, the computer logged PO521 and PO523 which are both pressure sensor reading out of range codes (this is what spurred me to originally try replacing the pressure sensor) and second, both the dealer and the independent mechanic commented that despite servicing 8 model years of the GMT900 platform, neither had ever seen a bad oil pressure gauge. It was a common issue on the GMT800, but basically non-existent on the GMT900. The dealer also said that in all the GMT800 cases where the gauge motor went bad, the gauge would not reset properly. It would stick at like 40 when off, and often move beyond 80. Mine doesn't do that. It seems at this point that trying another sensor and then looking at harnesses would be the smartest path.

 

[Dustin Jackson]

@The Raven I agree with that statement. Here is a video I found on a GMT800 with a similar issue. The guy goes into probing the wiring harness to find where the problem is and ends up being the oil pressure sensor connector had a break in it. Might give you an idea of things to try.

 

[The Raven]

Well it appears that it was a bad replacement sensor after all.

After watching the above video I figured it might be possible that the +5v wire was shorting to the signal wire intermittently, cause that would cause the gauge to jump around and sometimes peg at 80...though it was tough to determine how that would cause it to drop to 0 sometimes too, unless there were multiple compromised wires. Nonetheless, I figured if this were the case then I should be able to make it act up by messing with the wires and the plug. So once again I had my son watch the gauge while I prodded around for quite awhile but the gauge would not move. I then completely unplugged it, figuring I might see the gauge move with the plug out (which would be pretty definitive proof) but no luck. Gauge went to 0 and stayed there. So at that point I figured it was worth trying another sensor (I picked one up from the dealer last week). Did the sensor swap, then proceeded to use a half tank of gas driving EVERYWHERE yesterday and the cluster gauge was completely back to normal. Pressure is stable at 35-40psi at idle, and goes up to just below 50 under throttle.

So I still want to watch it for a bit longer but it sure looks like this one is solved.

 

[Geotrash]

Well it appears that it was a bad replacement sensor after all.

After watching the above video I figured it might be possible that the +5v wire was shorting to the signal wire intermittently, cause that would cause the gauge to jump around and sometimes peg at 80...though it was tough to determine how that would cause it to drop to 0 sometimes too, unless there were multiple compromised wires. Nonetheless, I figured if this were the case then I should be able to make it act up by messing with the wires and the plug. So once again I had my son watch the gauge while I prodded around for quite awhile but the gauge would not move. I then completely unplugged it, figuring I might see the gauge move with the plug out (which would be pretty definitive proof) but no luck. Gauge went to 0 and stayed there. So at that point I figured it was worth trying another sensor (I picked one up from the dealer last week). Did the sensor swap, then proceeded to use a half tank of gas driving EVERYWHERE yesterday and the cluster gauge was completely back to normal. Pressure is stable at 35-40psi at idle, and goes up to just below 50 under throttle.

So I still want to watch it for a bit longer but it sure looks like this one is solved.

Awesome! Thanks for following up and fingers crossed!

 

[Dustin Jackson]

@The Raven What brand did you use for those replacements?

 

[The Raven]

@The Raven What brand did you use for those replacements?

GM OEM. Same exact thing that I took out. Got them from two different dealers.

 

[Dustin Jackson]

@The Raven Concerning. Thank you for documenting this.