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Extremely Low oil pressure at startup while idling

acedown13

Full Member
Joined
May 27, 2013
Messages
54
So randomly the other day my wife called me and said the oil pressure on her lunch break tanked to 0 . When i got home i started it up and everything was fine, oil pressure was between 40-60 ( engine was warm still).
The next morning i cranked it up and the oil pressure gauge was jumping  all over the place between  20 psi and 40  just kind of twitching back and forth , i could hear a slight tapping of a lifter and the engine sounds rough , i pressed the gas and the gauge jumped up a bit to more normal psi, i took my foot off of the gas and it kind of dwindled down to maybe 20 , pressed the gas back up to normal, the engine no longer sounded rough, took it off and kept it off and it went down to 10 then 5 then 0 and my low oil pressure warning light came on, i turned it off and left it off for a min , turned it back on and drove it down the road, since i kept my foot on the gas the pressure was around 40 the tapping of the lifter went away, it got up to normal operating temp and the oil pressure was just fine even at idle. It seems to be only when it hasn't warmed up it's got the low oil issue. I don't believe it's just the gauge because of the tapping and also when it dips down from like 20 and below i can hear what sounds like a power decrease in the engine if that makes sense?
Should i just go ahead and replace my oil pump first, try a new sending unit, or drop the pan and check the pickup screen ? I kind of want to get this figured out today but have an 8 month old with me so it should be interesting.
oh my truck has like 215k miles and the oil level was normal. Thanks Everyone!
 
When was the last time the filter was changed?  Sounds to me like a viscosity issue and something plugging at higher viscosity (colder temps).  Could be the filter, pump or pickup screen.
 
i just replaced the oil and filter about a month ago. i change it every 3k miles
 
acedown13 said:
i just replaced the oil and filter about a month ago. i change it every 3k miles

Well, I've seen filters fall apart and plug up, so cheap check would just be to replace it again.  Otherwise, might sound like time to drop the pan.
 
completely dumb question, what exactly would i be doing while dropping the pan? changing the screen or the oil pump? I know on the 02 i can pull the oil pump from the top without having to drop the pan. I'm kinda just at a loss of what to do first since it all seems to be kinda involved whichever route i go.
i'm just confused about why this is just happening at a cold start and then is fine after it warms up.
 
Oil pumps aren't the easiest to change on these, especially 4x4s because the manual call for dropping the front diff in order to remove the oil pan.  A mechanic once told me these engines were susceptible to having oil return passages from the tops of the heads get blocked on engines that were a bit sludgy due to having gone long on oil changes at some point - that may be your issue.
 
bad gage?
bad sending unit?

Check the cheap things before you start replacing the motor
 
the sending unit just controls what the gauge reads out and if the warning light comes on correct? could there be a correlation to the rough idle and noises and the sending unit?
 
You wouldn't have the rough idle and noises if it was just a sending unit / display issue.
 
i just read on multiple sites that the oil pump doesn't create pressure , that the bearing clearance creates the pressure and that i most likely need an engine rebuild? does this sound correct?
 
Change the oil and filter first.  $20.  If that doesn't change anything then start worrying about the other stuff.  You could have a trashed filter, could be a little low, could be a bad batch of oil...do the cheap stuff first.
 
acedown13 said:
i just read on multiple sites that the oil pump doesn't create pressure , that the bearing clearance creates the pressure and that i most likely need an engine rebuild? does this sound correct?

That doesn't sound right....the vortec has a relief valve on the pump for reducing pressure...don't think that would work if the pump itself isn't producing pressure.

Read this...sounds like your symptoms

https://www.motor.com/magazine-summary/trouble-shooter-jan-2017-low-oil-pressure-vortec/
 
just took a sample of my oil, has metallic shine to it and seems pretty dirty, just changed it about 1200 miles ago
 

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WHat year AVY do you have?
 
it's a 2002 5.3 z 71
i went to check the oil pressure today with a mechanical gauge, ended up breaking my old oil pressure sensor while taking it off, when i go to put the threaded end of the mechanical gauge in where the old sensor is, it doesn't really want to spin more than like 1/16 of a turn and i can't get it to spin more with my fingers, i'm kind of afraid to put a socket wrench on it in fear that it might cross thread, then i'd be up a river more than i already am. What a horrible place to put such a thing.  Went to just put the new sensor on since i had to buy a new one and the deep socket oreillys sold me isn't deep enough. Kind of stranded at home until the wife gets home.
Bought a new filter and oil today, going to change those in a bit.
Since i bought a new oil pressure sensor, do i even need to check the pressure with a mechanical gauge since it's replacing the old one? I realize that the gauges on the dash aren't always correct but i imagine if a bad sensor was my issue a new one would fix it without having to test it. Also, started it up yesterday and today from cold starts, the pressure was dropping but no tapping or loss of power sound. This is my life.
 
acedown13 said:
it's a 2002 5.3 z 71
i went to check the oil pressure today with a mechanical gauge, ended up breaking my old oil pressure sensor while taking it off, when i go to put the threaded end of the mechanical gauge in where the old sensor is, it doesn't really want to spin more than like 1/16 of a turn and i can't get it to spin more with my fingers,

Sounds like 1/8" BSP (28 threads per inch) vs 1/8" NPT (27 tpi). Don't force it. Very carefully compare the threads before going further.

Since i bought a new oil pressure sensor, do i even need to check the pressure with a mechanical gauge since it's replacing the old one? I realize that the gauges on the dash aren't always correct but i imagine if a bad sensor was my issue a new one would fix it without having to test it. Also, started it up yesterday and today from cold starts, the pressure was dropping but no tapping or loss of power sound.

If the problem is the sensor itself, then changing the sensor will correct the problem and you won't need a real, physical, measurement. If the problem is in the wiring or the gauge, then you won't know if it's electrical or oil pressure. So personally, I would take the measurement while you are in there PROVIDED you can address the thread issue.

The way you  describe the pressure "twitching back and forth" makes me believe this is an electrical issue. Oil pressure tends to rise and fall smoothly. Even a relief valve kicking in and out is not likely to produce fluctuations that I would describe as "twitching". But I'm not there and can't see it for myself, so that's just my 2?, take it for what it's worth.

With regards to the question about whether or not the oil pump produces pressure: Positive displacement pumps produce pressure, which can be converted to flow by discharging into a larger volume passage. Centrifugal pumps produce flow, which can be converted to pressure by discharging into a reducing passage. The difference is really academic, but your oil pump is a positive pressure design. If your bearing clearances increase, then you are converting more of the pressure into flow (ie: the pressure drops). Doubling the bearing clearance will square the volume lost.
 
buickwagon said:
With regards to the question about whether or not the oil pump produces pressure: Positive displacement pumps produce pressure, which can be converted to flow by discharging into a larger volume passage. Centrifugal pumps produce flow, which can be converted to pressure by discharging into a reducing passage. The difference is really academic, but your oil pump is a positive pressure design. If your bearing clearances increase, then you are converting more of the pressure into flow (ie: the pressure drops). Doubling the bearing clearance will square the volume lost.

That's some good info, explained very concisely.  I'm a control's engineer and deal with PD pumps daily, and it would have took me 10x as long to describe what you did very simply. :)
 
ON start up of cold 2002 engine you get PISTON SLAP which makes the tap tap noise most think is lifter tapping but is not.

Search PISTON SLAP here and on the web.

Nothing to do with oil pressure but could everything to do with the noise you hear on cold startup
 
All of the advice from everyone is making me feel like i'm understanding this all more than just trying to fix the issue, i thank you all for all the great advice.
Update: yesterday after fighting to get the sending unit issue fixed, i got the new sending unit on, i wasn't able to use the manual pressure gauge due to the threads being larger than what was on the sending unit. Life would be much worse with a stripped out hole . I also changed the oil as well as put a new filter on. Results, it seems worse now. There hasn't been any noise the past 3 days but now when i cold start it, it just sits at 0 psi with the light on then bumps up to around 20 then dwindles down and  twitches around 10 and 15 before it drops down to 5 and then possibly 0. Before it would start around 20 at start up. I also noticed today driving it around, before after it warmed up it would go between 40 and 60 and sit at 40 while idling. Now that i changed everything, it barely gets to 40 when accelerating and twitches around 20-25 when idling. Also, today from a stop at a traffic light with a fast acceleration up to about 60 mph it just sat at 30 and didn't move. and last but not least , i realized yesterday when i shut the truck off, the oil pressure gauge sits at it's last pressure reading when i turn the truck off until i turn the key , then it resets as it should. So, when i turn it off all my other gauges on my cluster turn off but the oil pressure needle sits at the last reading as if it were stuck. Could this be an electrical issue other than the sending unit sin ce that was replaced? Bad stepper motor maybe? i had one of those on my fuel gauge a few yrs back that was bad and just went to no fuel out of nowhere, had to replace the instrument panel. Overall there seems to be no change in sound or anything now when the pressure drops vs, idling, vs, accelerating. I'm sure tomorrow it will be different because my life. Thanks again for putting the time into all of these answers ya'll, i really appreciate it.
 
If it was me, I'd take the broken sender in to a fluid handling or auto parts shop and find a manual gauge with the same thread. Get someone to rev the engine while you watch the gauge.

But to answer your question: yes, it is entirely possible to be an electrical problem with the gauge itself or a wiring connection between the gauge and sender. FWIW, the oil pressure gauge on my 04 drops to 0 when the key is shut off.
 
If the oil light is coming on its not the servo.  The oil light I'm pretty sure is driven by the PCM based on the pressure sensor...not the servo gauge position.

Oil is the life blood of an engine.  I wouldn't be farting around checking ghosts until I ruled out the actual pressure with a mech gauge.
 
robertmee said:
If the oil light is coming on its not the servo.  The oil light I'm pretty sure is driven by the PCM based on the pressure sensor...not the servo gauge position.

Oil is the life blood of an engine.  I wouldn't be farting around checking ghosts until I ruled out the actual pressure with a mech gauge.

I agree wholeheartedly
 
acedown13 said:
it's a 2002 5.3 z 71

acedown13 said:
it just sits at 0 psi with the light on then bumps up to around 20 then dwindles down and  twitches around 10 and 15 before it drops down to 5 and then possibly 0.

What light? My 2004 doesn't have an oil light. The owner's manual does not show an oil light. The wiring diagram in the service manual does not show an oil light. So I downloaded an owner's manual for a 2002, and it doesn't show an oil light either.

All 3 sources indicate that their should be a message in the DIC in the event of low oil pressure (there's also a separate DIC warning message for low oil level. The sensor seems to be a 3 wire type, and wiring diagram suggests there are 2 functions in the one sending unit -- One conductor sends a signal to the PCM and the second conductor sends a signal to the instrument cluster logic board directly. The PCM and the instrument panel can also communicate with each other via the serial data bus.

So, some questions: Is there actually a low oil pressure light? If so, does IT flicker on and off when the oil pressure gauge drops to 0? Is there ever a low oil pressure warning in the DIC?

But like everyone has said, an actual mechanical pressure reading would remove all doubt. The sooner, the better.
 
sorry for the confusion everyone, by oil light i meant the warning light that comes on that says " low oil pressure"
 
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