Lower oil pressure on Cold startup

[dnt1010]

2009 Tahoe 6.0 Hybrid (FLA) engine 256k miles. When I remote started the Tahoe yesterday morning to thaw it out it ran for about 3 minutes then shut itself off and had a check engine light on, Code PO521 "fault with the engine oil pressure sensor/switch range/performance". I checked the oil level and it was full so I started it back up with key and it was shoiwing below 40 on the oil pressure guage which is a lot lower than normal for it on a cold start but there was no valve chatter or other unusual noises. I let it run a few minutes then shut it off and restarted it, THEN the guage went up to 65 as it normally does???. It is typically at +60 oil pressure on a cold startup and stays around 40 when fully warmed up.
Current Oil 5w-30 full synthetic was at around 4000 miles so I decided to go and have that changed.
The check engine light was still on this morning so I could not use the remote start. I started it up this morning with the key and watched the guage and it did the same thing, it was well below 40 so I let it run a few minutes then shut it off and then restarted and it was once again at around 65. However now I am noticing a fluctuating guage it drops below 40 then zooms back up to +60 moving up and down every few seconds. To me it sounds like it just needs a new oil pressure sending unit and a new screen (if it actually has a screen?)
The check engine light went back off after I drove it around this morning (took kids to school) so it should start up remotely in the morning.
Trying to figure out my next step to properly diagnose without going to extreme measures of some kind. What do you guys think?
From what I am seeing on youtube the sending unit and screen are very hard to get at without removing the intake (do not want to remove intake)
Hoping that someone else has had this issue and resolved it and is willing to share?
I am rather fond of the old Tahoe and would rather not junk it out, at least for a few more years! Hahahaha

 

[wjburken]

A couple things come to mind on this.

1) the screen below the oil pressure sensor has a bad habit of getting clogged up and may be affecting what you are seeing for oil pressure. I know it it did on my 2007 Silverado.
2) the second is the o-ring on your oil pickup line. If it is starting to fail, it may have reduced sealing capability when cold and as it warms up it swells up a little and seals better.

If you are at 256K, I would bite the bullet and swap out the oil pressure sensor and clean out the screen below it and if that solves your problem, then great. Might consider getting an analog pressure gauge hooked up to see what it is reading as well. If the new pressure gauge and screen don't fix it, your not out much more than the time and cost of a new oil pressure sensor and now know to start looking else where. The o-ring on the oil pick up tube requires dropping the pan.

 

[RebelBelle]

Have done that on the Hy-Hoe a few times (3X) on the Tahoe during 316,000 miles. The oil pressure sender is a royal pain in the **** but can be done by anyone with patience. Get the GM switch instead of aftermarket in order to hopefully make it last longer…. Also, make sure and change the oil screen. It’s inexpensive and you don’t want to have to repeat the whole process. To remove the screen you can use a tool like a threading tool for chasing bolt threads, I actually welded a screw to the end of a broken screwdriver and that worked just fine.

A couple of tips get some plywood to span across the top of the engine and it makes access to it much easier and you don’t have to worry about breaking or bending various engine sensors or accessories. Pad with towels or shop blankets below the plywood. A light and mirror can help visualize the process and reduce time and frustration. I’ve done it in 20 minutes and in 3-5 hours. I always thought it might have been worth it to spray (only a little) some light solvent like WD-40 into the screen housing to flush away any debris down to the oil pan where the oil change and or filter could remove it.

My best recommendation is to search for You-Tube videos on the process and use their tips and tricks. Some of those mechanics are very good and can make it look very quick and simple and their tips are worth it. Myself, it seems to take longer ea h time instead of shorter but it’s certainly cheaper and you truly know what’s been down when you’re through and know that it’s a new filter and sensor.
I haven’t done it to the ESKY yet but it only has 60,000 miles so far.
Good luck!

 

[dnt1010]

I have spent hours online reading and researching. It was 33f this morning and I just started it and it exhibited below 40 so I let it run for a few minutes till I saw the temp gauge creep up just a hair to see if the pressure would come back up on its own as the engine warmed and it did not. I cut it off and let it set for 1 min then restarted and pressure was at 65 like normal. Not sure what to make of it but it does not appear to be low enough to cause any serious issues with the fresh oil and filter. I am not sure how low it had gotten to set off the PO521 code! Maybe the fresh oil change will keep it from dropping that low again until I figure it out. To be safe I am not going to use the remote start for a while and I will set in the ICE COLD drivers seat and watch the oil pressure for a few days to try to get more info.
It seems to be important that when started and pressure is low that a restart after a few minutes brings the pressure back up.
I need to start it in the morning and allow it to warm completely up and see if the oil pressure comes up on its own without restarting I guess.
A few thoughts after reading a bunch of info online.
1. I may have some serious sludge in the oil pan that partially blocks the oil pickup screen. Theory is when I cut if off after startup that th oil backflows enough to partially clean the screen?
a. I know this engine has some sludge in it from changing out the drivers side valve cover when I first bought it a few years back at 195k. Did not look terrible but it was pretty obvious the oil had not been changed as needed to keep it clean.
2. Possibly a blocked screen underneath the sending unit. When I cut it off after startup the oil drops back down and partially unblocks the screen?
a. Could be partialy blocked screen since I know the engine contains some sludge? But why every time it is started in cold weather but maybe I just have not tested it enough?
3. A bad oil pickup tube ORing that when the oil is cold and thick it allows air into the oil pump reducing flow and when it warms up it seals back? I saw a GM TSB that stated to change the Oring at 150,000 miles
a. I can test this theory by overfilling by 2 quarts and being on a incline apparently. Might do that soon if nothing else pans out.
4. Bad oil sending unit that is affected by the cold temp that corrects itself when it warms a bit?
a. I am ordering a new ACDelco sending unit and screen from rock auto this morning but they have 2 different plug styles (oval and round) so I am trying to figure out which one to order. Will have to crawl into engine bay and see if I can get a pic of the connection I guess. Looks like a PITA to change out but much less effort than the ORing changeout.
5. Bad oil filter since I have been getting (full synthetic) oil changes at Walmart and they are using a low quality filter?
a. Hmm wally world is probbaly not a good place to use for oil changes I might go get a high quality wix filter and trade that filter out today before my test in the morning. Could it be that simple????? That is super easy to do so I think this will be the 1st step Haha.

 

[Fless]

Have you compared the oil pressure value PID using an OBD reader, looking at live data, to see if that is comparable to what the gauge is reading?

 

[dnt1010]

I cannot find oil pressure in the INNOVA OBD reader sub menu?
After some thought, back a few months ago when I was changing my own oil I always used the ACDelco PF48E so I decided to go with that to test my bad oil filter theory. Somewhere in the past I had heard that it has a special backflow valve or something that is needed for these engines and this is a known correct filter that i had used for many past oil changes. I figured that would keep my multiple factors down on testing since that is a known good filter for my truck.
Anyway I just picked up an ACDelco PF48E oil filter and removed the WalMart Fram10060acc oil filter. Lost probably a pint of oil so no big deal to spin the Fram off and spin the ACDelco on. Going to go run some errands and pickup kids at school later today so I will see if I can determine any difference. And the BIG test will be in the morning it is supposed to be down around 15f here tonight. Fingers crossed/knock on wood/throw salt over shoulder/hang horseshoe over the door etc. Confidence is High! Amazing that at 60 yrs old I can still have this much optimism ROFL

 

[dnt1010]

Update: no noticeable improvement after changing from the Fram to an ACDelco oil filter. The fresh synthetic oil seems to have helped the cold start oil pressure a little bit, at least it does not seem to drop below 35 so that should keep the low oil pressure code PO521 from coming back. I guess the old dirty oil and the extreme cold caused a real low pressure on that one day that I remote started it and the PO521 code was set. I have been watching it closely now and it takes it about 5 to 8 minutes idle time to warm up enough to raise the oil pressure up to the normal cold start pressure of +60 then it goes back down to around 40 once fully warmed up. It is to cold to mess with changing out the sending unit/screen right now. That will be the next step though unless it suddenly gets a lot worse. Hopefully I will not have to do another update until around April or May when the weather warms up Hahahaha
Found another pic of the sludge on the valvetrain, I have no idea what it looks like now as that pic was 60k miles ago and about 3 years hopefully it is a lot better now after regular oil changes. I am super leery of doing a engine flush it would probably plug up everything inside my engine knowing my luck!

 

[dnt1010]

Update 6-12-2022 at 261k miles I am now doing 3k oil changes and keeping fresh synthetic oil in the engine seems to hold the pressures at this New Normal. I can see that the oil pressures overal drop if I run the oil over 3k between changes. On hold now for doing any repairs as I am not sure that there really is a problem except it is a +261K GM gas engine LOL

 

[j91z28d1]

Update 6-12-2022 at 261k miles I am now doing 3k oil changes and keeping fresh synthetic oil in the engine seems to hold the pressures at this New Normal. I can see that the oil pressures overal drop if I run the oil over 3k between changes. On hold now for doing any repairs as I am not sure that there really is a problem except it is a +261K GM gas engine LOL

old post I know, but have thought of just running a thicker oil in the old girl? go 10w40 to even a 15w40 diesel oil?

I'm not sure I'd waste the money on synthetic if you're going to change it often. we have a ton of work horse gas engine in equipment at work. things have a million hours, but just get fresh non synthetic 15w40 bulk oil changes every 300h and the look perfectly clean inside.

as I understand oil, unless you're running into high oil Temps that need synthetic to keep from braking down, or the little extra sheer strength in say a racing engine. it's over kill, my truck never sees over 3k rpm and I doubt oil Temps ever get over 200. I did put synthetic in it last change cause eh, it will be 5k plus miles before I change it again. but if I was doing it often, I'd have no worries throwing cheap Rotella in it.


now my c6. that thing I fight oil Temps all the time. last track day I had to shut it down at 300deg and do a cool down lap. very annoying. gm and the boards seem to say 300 is OK for quality oil, but I don't like it(I think 330deg triggers limp mode but I don't wanna find out haha) . that's with oil cooler and a fan on the cooler. but it's an issue everyone runs into with these cars, so eh. more cooling mods before the next one.

 

[dnt1010]

Good suggestions thanks. Engine was not very well taken care of the 190k miles before I bought this rig. Tons of blow by in the engine and some great blue gnat Smoke out the tailpipe at times. A major failure is imminent and if I lose a transmission or engine will probably just call the local scrap yard to come and get it...... just not worth investing significant dollars into this vehicle.