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Discussion Starter · #1 ·
Hello All,

I have a 2009 GMC Sierra 1500 SLT 4x4 with the 5.3L LC9 with 145,000 miles.

Truck has been more or less trouble free thus far.

I dropped my truck off for some body work 3 long weeks ago, and shortly after getting it back, it gave me a "low oil pressure - shut engine down immediately" warning. The oil pressure went from ~35psi down to ~20psi and then slowly tailed off to 0psi. With the body guys taking their sweet time fixing my rear bumper and quarter panel, my truck did quite a bit of sitting, compared to a 30+ mile drive each way to work every day. I'm thinking this may have allowed some sort of sludge settling or something of the like. When the pressure drops, the truck does not seem to shudder or make lifter noise, so I have a feeling that the gauge is lying about being low and the engine is still receiving oil.

Checked the oil - I have plenty but it's a little on the old side, ~22% life remaining according to the monitor. I have had AFM disabled since 90,000 miles.

Anyway, first thought was replace the oil pressure sending unit, so I did that. No Dice. Today, I'll be changing out the pressure sending unit filter can underneath the sending unit (Wish I knew that was under there that yesterday when I swapped the sensor) and changing the oil & filter.

If that doesn't fix it, it looks like the Oil pump and Oil pump siphon tube O-ring have to be replaced? Any input, thoughts or suggestions would be super helpful as to what to do next.

Could my oil pump pressure relief valve be stuck open even though I've had AFM disabled for the last 55,000 miles? Could a damaged siphon tube O-ring be causing this great of a problem? Finally, if it worked at the beginning of the month when I dropped it off, why the hell did it pick now to malfunction if it's a mechanical problelm?

Thanks in Advance.

-DS

PS: I did thumb through a few threads about this but it seems like the answers were a bit jumbled so I wanted to ask specifically about my case.
 

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Discussion Starter · #4 ·
This oil probably has ~5-6000 miles on it, I changed it in the late spring. I drive 60 highway miles round trip for work so the Oil Life Monitor (which I started listening to after I got tired of changing perfect oil every 3,000 miles) lets me go for ~7,000 in between changes. I always use Mobil 1 5W30 full synthetic and a purolator filter.

Unfortunately I do not have a mechanical gauge, but I have the new sending unit in there with no change in behavior so I think I've ruled that out as an option.

-DS
 

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Discussion Starter · #5 ·
UPDATE: I removed the new Oil Pressure Sending Unit (OPSU) and used a pick to remove the old OPSU Filter. Installed a new filter and re-installed the OPSU. I then added 8oz of Seafoam Engine Treatment to the crankcase via the oil fill port.

Started it up and drove around and the pressure started at ~40psi, spiking to ~45psi under acceleration. Then the pressure dwindled down to ~18psi and STAYED THERE. I thought for sure I was going to hear the "low oil pressure" warning again, but it didn't go down any further. It sat at 18 until I got back home. I made sure the engine was at operating temperature for 10-15 minutes and then drained the oil, and put in 6 quarts of fresh Mobil-1 Synthetic and changed the filter out for a new purolator. Started it up again, this time, the oil went up to ~35psi and stayed there at idle, spiking to ~45psi under acceleration. I drove around a mix of highway, stop and go, and back road to try to shake it down and get it to fall again. Stayed in spec and fully oiled the entire time... Looks like it's fixed for now, but tomorrow's drive to work will tell.

TL:DR:
Installed new OPSU Filter
Seafoamed Crank Case
Consistent but Low oil pressure
Changed oil
Good Oil Pressure.

I Will update if anything goes bad in the near future.

-DS
 

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Discussion Starter · #7 ·
It was pretty darn dark for synthetic.... it almost looked like regular oil to me. Definitely darker than usual but otherwise I couldn't tell much from it. The oil filter was heavy as hell though... thoroughly loaded up with junk for sure.

-DS
 

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It could have been as simple as your old oil filter or maybe even the oil itself.
 

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Discussion Starter · #9 ·
Concur. What had me worried was that all was well before the truck sat for the 3 weeks, so I started with the sensor. Should have changed the oil first!

-DS
 

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Yep, that is what I would have done. Especially since you were already so close to the OLM change time. Glad you got it solved though.
 
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