stuck oil relief valve?

Chris Bassett · February 24, 2018

So fairly recently I noticed that my oil pressure is reading a bit low. I have topped off the oil, replaced the oil pressure switch and filter (located beneath the switch) to no success. It still reads around 20 psi idle (warm) and, after a bit of driving (ie. warmed up) it only reads about 25, maybe 30 psi tops, at around 1700-2000 RPM. Someone had mentioned it could be one of two things, since I dealt with the sensor already: oil relief valve could be stuck open, and/or the siphon tube on the intake side could have a broken/craked o-ring.

Since I've done the sensor and the filter below it, are these the next recommended things to check? There are no physical leaks. It does not appear to be burning oil (although it might, but is quite slow as I don't see any evidence from the tailpipe, even of cold starts in cold weather--at least none of the exhaust fumes look to be like smoke from oil burning). Even so, when the oil is full, it still reads low pressure even with the new sensor installed (so the old sensor was actually fine. and yes it was tested with a mechanical gauge before the sensor was done ).

The engine runs fine, no ticks or noises, just low pressure. Some people on other forums say that my oil pressures are normal, while I see the opposite, and some people are reporting 40 psi idle, and 50-60 psi while cruising, which is why I'm wondering about this. I have not gotten a warning light, and the gauge has never dropped to 0 or been pegged at 80 psi. When it starts cold, it is around 35-40 psi and then drops off as the truck warms up. I did take it to the dealer, they said it needs a new pump (now, it occured to me later that perhaps what they meant to tell me is that the relief valve might be stuck open but just replace the whole pump). All they said was "you need a new oil pump" which I'm sure that they are glad to say because it's an 8 hour job, and at dealer shop rates, that adds up quick (I got an estimate from them and the local independent shop which I've used before for stuff. Dealer said $1500 installed plus tax (so maybe $1700 in the end?) Independent shop said $1000 plus tax, so if it does need replacing, I'll probably just go to the local shop as I trust them (as they fixed trans problems on some of our other company vehicles to our satisfaction).

Am i correct in looking at the o-ring and relief valve. I know that if the relief valve is bad, I need to just replace the pump, but I'm also tempted to say if they drop the pan and check the o-ring, they might as well just replace the pump, as the pump itself isn't expensive. Doing the labor is. Like I said, the truck runs fine, doesn't make any unusual noises (I do get piston slap in the morning which I mistakenly thought maybe was a stuck lifter, but that quickly goes away after it warms up, and is apparently a "problem" with these engines). I should also mention I checked the trans fluid and the coolant to see if possibly oil was getting mixed in, but it isn't. Those fluids are clean and look normal. No overheating either, or strange noises from the engine.

Thoughts?

Edited February 24, 2018 by Chris Bassett

Chris Bassett · February 24, 2018

I should quickly note some pressures that I was having before this came up: up until about 3 weeks ago, cold start pressure was between 40-50 psi, and idle was maybe 25-30psi, cruising at 1700-2000 RPMs it would be between 35-40 psi, which I thought these pressures were normal, and I think they are (it would drop slightly as the truck burned oil probably over the 4 month period between oil changes) but I would always top it off. But what I'm getting now doesn't seem normal to me, even though they might be within GM's "specs." or minimums. (Also what I am referring to as not burning oil above, is I mean it's not burning a significant amount, to a point where you would get a huge cloud of smoke on startup. I imagine it is slowly burning oil, hence the slight drop I talked about here, when at cruising speed--it might drop 5 psi to 35 psi when it needs to be topped off, but like I said, that was before this issue surfaced.

Other potentially helpful info: 5.3L Vortec, 6 speed automatic, 4WD, LC9 engine, 3.42 gear ratio. Engine is all stock, no modifications. Oil has been changed every 5,000 miles, using Mobil 1 synthetic. This problem started appearing (slightly lower oil pressure, but no where near where I'm at now) say, about 3-4 weeks ago (but didn't really get noticeable until about 2-2.5 weeks ago). Initially needed 1 quart of oil, but that isn't entirely alarming, given these engines reputation for burning oil after a while.

Edited February 24, 2018 by Chris Bassett

muddkatt · February 24, 2018

I just read through the essay Chris and didn't see where you stated the year model.

I would suggest that before you get all set on changing the oil pump that you do an oil and filter change (you failed to mention what weight oil you are using and what filter). If that does not change your numbers, then buy a mechanical oil gauge and see what the real pressures are. It could be a bad stepper motor, bad sending unit, etc.

Chris Bassett · February 24, 2018

Sorry, it's a 2013 1500 LT. Is the sending unit the same thing as the oil pressure switch? I replaced the pressure switch and filter already and it hasn't changed. The shop I took it to did hook up a mechanical gauge, and based on what they told me, it looks like the dash gauge might be off by 2 or 3 psi, but not much. and if that's the case, it appears that at high RPMs, say like 3000 RPM, it still maxes out at only 30 psi (this takes into account the gauge reporting a little short and only showing about 25 psi when at 3000 RPM). (I used the manual speed control to limit the gears to get higher RPMs to see where the pressure would land without having to actually drive faster, not sure if this is an accurate methodology, but short of trying to drive fast, this was all I could think of to simulate higher RPMs while driving. Never did go past 4000 RPM though and each one was for a short time, maybe 3-5 seconds at most).

I did also take it to the dealer, and they are saying a new pump is needed (which I'm wondering, like I had mentioned above, if they are referring to a stuck relief valve which requires the pump to be removed--and you might as well replace it then if you have it out I guess. They also did a pressure test and they said that it was reading low (the other shop said 20-22psi is normal, and 30 while driving is normal, but I'm finding online that people with similar years 2011-2013, are reporting anywhere from 35-40 psi idle warm, and 40-50 psi while driving. Some are reporting the same as me, but they may be exhibiting a similar problem as I am.

I was also told by the dealer that the warning light will come on around 15-18 psi. So far the lowest it's gone is 20 psi (adjusted) at idle at a stop light, warmed up.

As for the oil, I'm using the recommended 5w-30 spec (Mobil 1 though, not Dexos 1). I am going to get the oil changed today and see if that changes anything. I was thinking it is slightly possible that the dealer accidently put in 5w-20, for example, and then low pressure might make sense if that happened, but there's not really a way of telling (the bill I got says 5w-30 but there could have been an error made when filling it).

So, sorry for another "essay." But it's been pressure tested with a mechanical gauge, oil pressure switch and filter have been replaced. If the oil change doesn't solve it, then I need to move onto the next thing (which is the oil pick up too o-ring, and then possibly the pressure relief valve in the pump).

swathdiver · February 24, 2018

Your truck has the Melling M365 oil pump. Maximum oil pressure is 33 psi with the yellow spring in this pump. The red spring had the oil pressure relief set at 43 psi in the earlier trucks. The 365 pumps 1/3 more oil per rpm compared with the 295 pumps used in the iron block motors, LS2, LH8 and Gen III motors.

Mine idles around 25-30, less as the oil gets dirtier/older once warmed up.

The oil pickup tube o-ring can also cause low oil pressure as it begins to deteriorate and fail.

Google and download this pdf file, "Rebuilding GMs Gen IV Engine".

muddkatt · February 24, 2018

What oil filter are you suing?

Dnt · February 24, 2018

Did you clean the screen filter behind the oil pressure sending unit before you put the new one on? If not that is most likely your issue. There is a pretty good chance here that they would replace the pump and you will still have the exact same issue........ check out this link.

https://noln.net/2017/05/31/low-oil-pressure-message-locating-mystery-filter/

Grumpy Bear · February 24, 2018

1 hour ago, Dnt said:

Did you clean the screen filter behind the oil pressure sending unit before you put the new one on? If not that is most likely your issue. There is a pretty good chance here that they would replace the pump and you will still have the exact same issue........ check out this link.

https://noln.net/2017/05/31/low-oil-pressure-message-locating-mystery-filter/

That is a great resource. Thanks!!

txab · February 24, 2018

5 hours ago, Dnt said:

Did you clean the screen filter behind the oil pressure sending unit before you put the new one on? If not that is most likely your issue. There is a pretty good chance here that they would replace the pump and you will still have the exact same issue........ check out this link.

https://noln.net/2017/05/31/low-oil-pressure-message-locating-mystery-filter/

He replaced the screen when he replaced the OPS. Post #1

Dnt · February 25, 2018

2 hours ago, txab said:

He replaced the screen when he replaced the OPS. Post #1

Woops I see that now. Oh well lets continue. Some people have mentioned that the screen stops right back up because there is debris in the tube below the screen. If it had anything much on the screen when you pulled it out you may wish to pull the oil filter and clean/flush that tube while the screen is out (it goes straight down to the oil filter) with some cleaner and compressed air?? I just hate to see anyone tear into some major work if it is something super simple...... Good Luck OP

CKNSLS · February 25, 2018

This guy has been all over the Internet looking for a solution to a problem that doesn't exist. His oil pressure and consumption are both within GM specs.

He's a little OCD.

Chris Bassett · February 25, 2018

Well, check engine light is on now (no oil light though, no messages on the dash display) so it's going into the shop to be looked at. One thing I can think it might be is a recently clogged PCV valve (it may have been restricted before, as I have been seeing oil loss--must be through burning as there is no oil leaking out anywhere and really no visible smoke from the back (tailpipe). So it may very slowly be burning the oil then (or possibly burning it under the hood somewhere while I'm driving but in a way that I can't see it while driving maybe, not sure, but I'm thinking the PCV needs to be replaced. performance hasn't changed since the light came on (and it is only the check engine light that came on). Engine runs normally still, but will still be going in to be checked.

I'm about at the point where it's got to be one to three possible things (oil is probably being burnt somewhere) but it doesn't explain what I think is the oil pressure loss:

Probably Cause #1: PCV valve / system, (probably the cause of the check engine light)

Probably Cause #2: o-ring on the inlet pick-up tube in the pan (possible cause for low pressure)

Probable Cause #3: oil pressure relief valve is stuck open (in the pump, thus contributing to low oil pressure)

I guess a fourth could be oil sludge, but I don't see any evidence of this, and the oil does look to be fine for 4,000 miles (it's not black and doesn't have any contaminates in it that I can see)

So I guess I'm going to figure out what the check engine light is on for with a reader. If it's the PCV system, I'll get that replaced first. If that doesn't help the oil pressure issue, then I'll move and do items #2 and #3 as I guess you have to remove most of the stuff to get the pan off, and I think the guy at the shop said if you're going to do that, you might as well just replace the pump since you're basically there. They quoted me around $1000 to replace the pump. I think the dealer said to have them replace the PCV valve was around $200 (dealer quoted me $1500 for the pump replacement so I think if that gets done, I'll have the local shop do it as their hourly labor rate is a bit lower and they do decent work based on previous work I've had them do for our company vehicles, including trans works).

So that's where I'm at right now with this. If it does end up being the o-ring in the pan, that's annoying, because it's like a $10 part that will cost me $1000 to replace (assuming I get the pump done at the same time, according to the shop). Even if the o-ring isn't the culprit (ie. not cracked) I'll still have them replace it anyway if they go in there to do the pump.

(Also for those asking about the oil filter and oil, I am using Mobil 1 full synthetic with the AC Delco oil filter -- it's been done by the dealer all this time, and never had an issue up until now, so I doubt it's related to the oil filter or oil being used, UNLESS they accidently used 5w-20 instead of 5w-30). I've always heard that synthetics will cause the pressure gauge to go a little funny and maybe read a few psi of pressure lower, but even then so, it doesn't explain some of the "behavior" I've noticed with the oil pressure.

Edited February 25, 2018 by Chris Bassett

Chris Bassett · February 25, 2018

Here is one quick question: if the AFM malfunctions, does it just stay in 8 cylinder mode? I didn't see it switch into 4 cylinder mode after the light came on, but I also don't hear anything odd (no noises, tapping, etc) and don't smell anything odd, like anything burnt).

Chris Bassett · February 25, 2018

3 hours ago, CKNSLS said:

This guy has been all over the Internet looking for a solution to a problem that doesn't exist. His oil pressure and consumption are both within GM specs.

He's a little OCD.

Yes probably. But now that this has happened, I'm starting to wonder. Read my latest post. So I guess "losing" 15-20 psi of oil pressure over a, 2 week? period is normal I guess. And apparently to you all over the net means this site and another site...

Edited February 25, 2018 by Chris Bassett

Chris Bassett · February 25, 2018

13 hours ago, swathdiver said:

Your truck has the Melling M365 oil pump. Maximum oil pressure is 33 psi with the yellow spring in this pump. The red spring had the oil pressure relief set at 43 psi in the earlier trucks. The 365 pumps 1/3 more oil per rpm compared with the 295 pumps used in the iron block motors, LS2, LH8 and Gen III motors.

Mine idles around 25-30, less as the oil gets dirtier/older once warmed up.

The oil pickup tube o-ring can also cause low oil pressure as it begins to deteriorate and fail.

Google and download this pdf file, "Rebuilding GMs Gen IV Engine".

Now the only time it gets up to 25 psi is at around 2000 RPM. Below that it's around 18-20 psi at idle. In fact, after warm up, it really doesn't go above 25 psi, even forcing the engine to 3000 RPM (suing the manual transmission control mode).