Diagnosing erratic/high oil pressure...

[The Raven]

Working with my dealer and I think we are headed in the right direction here but before I spend a big chunk of money i'd like to engage some more experts. This problem is a little different than most that i've read about.

I've had a check engine light come on and then go off before I could get a reader on it three times throughout this past fall. Finally it came on and stayed on a few weeks ago and I simultaneously noticed that the oil pressure gauge was acting really weird. It would randomly spike to 70-80 psi and bounce around up there for a few seconds before dropping back down to 40ish where it normally hovers. With time it has gotten more frequent and sometimes just pegs at 80 for 20-30 seconds...I actually even had it drop to 0 for about 20 seconds one time. In reading of course I came across the bad pressure sensor problem and figured that must be it so I ordered a replacement sensor (OEM) and did the swap (that was fun as many of you know). Unfortunately that didn't fix the issue...just to be sure I went back in and removed that filter screen thingy too but that didn't help either.

So I took it to my dealer cause they have always treated me well and been very fair about prices. They spent three days with it and in that time they verified that the sensor is reading correctly, the oil pressure is indeed fluctuating just as the sensor indicates. They also changed the oil filter to verify that it's not a filter issue. It wasn't until the third day of messing with the thing that the tech caught onto one little detail that clued us in - the pressure always seemed to jump when slowing down or braking. He did a little further testing and found that he could make the pressure spike by braking hard or just getting the front of the truck way lower than the back (down a hill or whatever). So the natural conclusion seems to be the oil pump pressure relief, and based on my experience I tend to agree - I remember the first time I noticed the problem was while entering the roundabout closest to my house and that was the same spot that it happened again after I changed out the pressure sensor. You have to brake pretty hard coming into the roundabout from like 55mph and then do a pretty swift right-left turn motion going through.

What do you think - on the right track?

 

[OR VietVet]

Wonder what the mileage is on your rig? Could this be a sign of the oil pickup tube assembly o-ring failure? The hard braking and pressure climbing seems to be pretty close to the stories here where the pressure is low and fluctuating and it is recommended the problem vehicle be overfilled by two quarts and pointed downhill and pressure would climb, indicating the o-ring problem. Was this possibility brought up at the dealer? What type of oil do you use and filter brand?

 

[The Raven]

Wonder what the mileage is on your rig? Could this be a sign of the oil pickup tube assembly o-ring failure? The hard braking and pressure climbing seems to be pretty close to the stories here where the pressure is low and fluctuating and it is recommended the problem vehicle be overfilled by two quarts and pointed downhill and pressure would climb, indicating the o-ring problem. Was this possibility brought up at the dealer? What type of oil do you use and filter brand?

107k miles. Mobile 1 5w-30 and AC Delco pf48 filter religiously.

 

[OR VietVet]

107k miles. Mobile 1 5w-30 and AC Delco pf48 filter religiously.

Ok, was the o-ring brought up at the dealer?

More Information for GM GENUINE 12608835

Hope this fixes it though. But I cannot see how the hard braking and turning would make this the culprit. I have been wrong before though and hopefully the tech at the dealer is spot on.

 

[The Raven]

Ok, was the o-ring brought up at the dealer?

More Information for GM GENUINE 12608835

Hope this fixes it though. But I cannot see how the hard braking and turning would make this the culprit. I have been wrong before though and hopefully the tech at the dealer is spot on.

They didn't call out the o-ring individually, they said "there are several possibilities when it comes to the oil pump, but the pressure relief is the problem here". They also mentioned that there's a second pressure relief for the AFM system in there that they would also change out because it's cheap to do so, but they don't believe that one is the problem.

 

[Doubeleive]

interesting I have never heard of the oil pressure relief valve, I thought the pressure was controlled by the spring in the oil pump, hence different model pumps provide different oil pressure as needed/desired.

 

[Dustin Jackson]

Isn't the o-ring suspect when low oil pressure is the problem but I never heard of it being the cause of high oil pressure?

If they are opening the bottom and front of the motor might not be a bad idea to have them investigate a little and replace all suspected parts: oil pump, pick up o-ring, pick up tube maybe, oil relief valve

 

[Geotrash]

They didn't call out the o-ring individually, they said "there are several possibilities when it comes to the oil pump, but the pressure relief is the problem here". They also mentioned that there's a second pressure relief for the AFM system in there that they would also change out because it's cheap to do so, but they don't believe that one is the problem.

It makes sense to me that the oil pressure relief valve in the pan is a likely culprit here. It's a pop-off style valve that would cause this behavior if it's intermittently sticking closed. It's not common, but also no unheard of for these to fail in this way.

 

[donjetman]

interesting I have never heard of the oil pressure relief valve, I thought the pressure was controlled by the spring in the oil pump, hence different model pumps provide different oil pressure as needed/desired.

There's an AFM oil pressure relief valve in/on the oil pan:

 

[The Raven]

It makes sense to me that the oil pressure relief valve in the pan is a likely culprit here. It's a pop-off style valve that would cause this behavior if it's intermittently sticking closed. It's not common, but also no unheard of for these to fail in this way.

Does it only operate along with AFM or does it work on it's own? I had AFM tuned out a couple years ago so while all the components are still there, they aren't activating.

 

[Dustin Jackson]

Does it only operate along with AFM or does it work on it's own? I had AFM tuned out a couple years ago so while all the components are still there, they aren't activating.

I don't know how it works specifically but it is designed to help maintain a specification of oil pressure independent of the AFM system but if it is malfunctioning it could be restricting oil or allowing too much oil to pass and depending on where in the oil pressure system this valve is could cause the gauge to show a non-normal number.

I understand the one in the oil pan is a second oil pressure relief valve and the first one is built into the oil pump, things to keep in mind.

 

[The Raven]

I spoke to the tech and writer again today, in person, to get a complete rundown of exactly what they did so I have it documented. They were unable to verify the gauge reading while driving the vehicle because they couldn't get a tee on without removing the intake. They were able to verify pressure fluctuation while parked though. Also, on the drive home the gauge started out pegged at 80 for the first minute or so, then dropped back to 40 ish, then wavered in between for awhile, then shot back up to 80, then back down to 40, then to zero for a few seconds, then back up to 40. No strange noises from the motor - ran smooth and quiet the whole time. When I hit the brakes the pressure would go ballistic but it also did it a few times while driving on flat level road.

I think i'm going to get one of those oil-filter pressure gauge adapters and a long hose so I can get a mechanical gauge verification while driving. I really really hate to spend 2k on an oil pump replacement only to find out it was something really simple.

 

[Geotrash]

I spoke to the tech and writer again today, in person, to get a complete rundown of exactly what they did so I have it documented. They were unable to verify the gauge reading while driving the vehicle because they couldn't get a tee on without removing the intake. They were able to verify pressure fluctuation while parked though. Also, on the drive home the gauge started out pegged at 80 for the first minute or so, then dropped back to 40 ish, then wavered in between for awhile, then shot back up to 80, then back down to 40, then to zero for a few seconds, then back up to 40. No strange noises from the motor - ran smooth and quiet the whole time. When I hit the brakes the pressure would go ballistic but it also did it a few times while driving on flat level road.

I think i'm going to get one of those oil-filter pressure gauge adapters and a long hose so I can get a mechanical gauge verification while driving. I really really hate to spend 2k on an oil pump replacement only to find out it was something really simple.

Good call. It could be as simple as a wiring harness or connector shorting out.

 

[The Raven]

Ok got a mechanical gauge on it, and the mystery only deepens.

Background info - The mechanical gauge setup I used uses a pass-through block that screws on in place of the oil filter, and has a port for a mechanical gauge fitting. So during testing, the oil filter was not installed.

Before the install, I went out to the driveway, started the truck, gave it a few seconds to get the oil flowing and warm up a bit, and the erratic behavior began almost immediately. Cluster gauge all over the place. Pulled the truck in the garage, shut it down. Installed gauge contraption and routed gauge up to the side of the truck so I could hold it with my left hand and steer with my right. Started truck, backed out of garage and through driveway, all readings normal (PSI 40ish on both gauges). Tried moving up and down the driveway a few times (this was more than enough to get it to act up before) and readings stayed right around 40 the whole time. Backed out of driveway and did a loop around the neighborhood, again, no weird behavior. Pressure lightly fluctuated between 35-50 psi, both gauges in lock-step. In all I drove around for about 15 minutes and never saw a single hint of erratic behavior, and both gauges agreed the whole time.

So the oil filter would then seem to be the culprit. I re-installed it and the erratic behavior did indeed return (going by only the cluster gauge of course). Open and shut case right? Except this is the second oil filter i've tried.

Since it's cheap, i'm heading out to Autozone to get a Mobil 1 or Bosch filter. I don't have a lot of confidence in anything at this point but another $15 is well worth the try.

 

[The Raven]

No luck. Behavior continues, even with new oil filter. I have driven the same test loop now so many times that I have noticed a few things, which I confirmed on my last loop. The behavior is noticeably worse while going downhill or braking, and during left turns. I can peg the pressure gauge on command by executing a "spirited" left turn. Doesn't have to be at the limit of tire grip, just "swift" I would say. Same behavior does NOT occur on spirited right turns.

I'm going to put the mechanical gauge on one more time and try to figure out a way to route the gauge for safe driving.

 

[The Raven]

Ok finally some progress. I put the test gauge setup on it again, and this time I routed the gauge up through the fenderwell (around the frame rail) and out under the back edge of the hood so i could see the gauge while driving. Then I went out for a nice long cruise until it got warmed up and the cluster gauge started going all nutzy. The mechanical gauge read 35-50psi the entire time. The cluster gauge was pegged at 80 quite a few times yet the mechanical gauge stayed steady.

I think i'm going to see if I can find another mechanic who can verify these findings before I consider the truck safe to drive though. Then the next question is now what? Try another pressure sensor?

 

[OR VietVet]

Yes, do another sensor.

 

[Donal]

Ok finally some progress. I put the test gauge setup on it again, and this time I routed the gauge up through the fenderwell (around the frame rail) and out under the back edge of the hood so i could see the gauge while driving. Then I went out for a nice long cruise until it got warmed up and the cluster gauge started going all nutzy. The mechanical gauge read 35-50psi the entire time. The cluster gauge was pegged at 80 quite a few times yet the mechanical gauge stayed steady.

I think i'm going to see if I can find another mechanic who can verify these findings before I consider the truck safe to drive though. Then the next question is now what? Try another pressure sensor?

I would contact Eric at Pine Hollow auto Diagonstic, He is at State College, about a 3 hour drive from you. An ocillscope will reveal the problem source.

 

[The Raven]

I got a third (second professional) opinion yesterday and it matches mine. A professional was able to get his mechanical gauge on the truck and verified my results. Oil pressure is rock solid, so there's no REAL problem here. It's only the gauge. Next i'm going to try another sender, but it's probably going to be a few days before I have time.

 

[Geotrash]

I got a third (second professional) opinion yesterday and it matches mine. A professional was able to get his mechanical gauge on the truck and verified my results. Oil pressure is rock solid, so there's no REAL problem here. It's only the gauge. Next i'm going to try another sender, but it's probably going to be a few days before I have time.

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.