2003 LQ4 (6.0L) Low Oil Pressure

[DAVe3283]

tl;dr I was drag racing my 2003 Denali with an old Maggie and on the 3rd pass of the day, it dropped oil pressure during 2nd gear. Normally holds 70 PSI above 4000 RPM, but it dropped to 50 PSI for the rest of the run. Normal hot idle is 30 PSI, now is 6-7 PSI.

So, taking all bets: what happened?

The long version is I picked up this 2003 Yukon XL Denali a year ago with a very early (casting date of 2002) Magnuson supercharger installed on it. Previous owner clearly cared for it at one point, but abused it the last couple years. Radiator was cracked (and full of stop leak), everything except the rear main seal was leaking BADLY, and the engine had clearly been run out of oil more than once. Cam was trashed, lifters were trashed, cam bearings are... not great. Exhaust valves were pitted, spark plugs had no electrodes left (not sure how this even ran TBH).

I decided to do a sloppy-mechanics style "repair" on it and see how it went. I pulled the heads off, lapped the valves, new valve stem seals. New Mahle MS16124 headgasket and GS33380 headbolts, back on they go. Swapped to a LS9 cam, new LS7 lifters, PAC-1218 valve springs. All new seals everywhere to stop the ridiculous oil leak rate.

Popped off a main cap, and the main bearings are freaking mint. Looks brand new. Crank thrust is well in spec. No play in rods either, so I buttoned the bottom end back up. Yay main-priority oiling, I guess.

Existing oil pressure senor was bad (as is common on these Gen3 LS motors) so I put in a brand new GM 12677836.

Oh right, the cam bearings are still trash, and changing them without pulling the engine sucks. ? I know, install a Melling 10296 high-flow oil pump! Surprisingly, this seemed to work, as oil pressure was awesome, until it wasn't halfway down the track today while dialing in the tune.

This is a little over 2,000 miles after the "rebuild". Blower is making 7.0 PSI, AFR according to a wideband was 11.0:1 for every pass, so not leaning out. I would see 1-2° KR right at the start of each run but decay to 0 halfway through 1st gear, probably false/burst knock, as I was only running 8-10° total advance. I am spinning it a bit faster than stock. The last pass was shifting at 6,600 RPM, but the first two shifted at 6,800 RPM. It starts to choke on the stock exhaust up there (stock redline was 5,900 RPM) but the cam should otherwise flow north of 7,000 RPM.

Once I get some time, I will verify the oil pressure is actually low (not just another sensor failure), and if so, start tearing into it and see what happened. But I am curious on your guys bets and thoughts.

Edit: best pass was a blistering 14.991 @ 90.89 MPH ?

 

[Chevy1925]

Does that year have the pil pressure bypass in the pan? That be my bet, shes stuck open

 

[DAVe3283]

Hmm, it does have that plastic "barbell" at the back of the block, which is original. My understanding is if that failed it would just bypass the oil filter, but I will read up on that. I don't believe it has anything in the oil pan itself.

Forgot to mention, the engine sounds perfectly normal, no ticks/knocks/nothing. At 6 PSI, I would expect the lifters to start collapsing, but no? Though I obviously didn't run it very long to be safe.

 

[malibu795]

Did the cam bearings finally walk into the oil pan? Kinda reads like you left the cam bearings in?

Oil/Oil filter look like?

Definitely double check oil pressure with mechanical gauge

 

[ikeG]

Spun cam bearing i bet. I chased this similar issue one a few years ago.

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[gassux]

how is the 4l60 holding up?

 

[NC-smokinlmm]

If it’s actually low and not a bad sensor I vote spun cam bearing as well.

 

[DAVe3283]

I did leave the original, worn cam bearings in, so it is possible they had enough. I'm going to pick it up from the track today, but I probably won't have time to drain the oil or cut open the filter.

Oil level on the dipstick is normal, and no shiny or metallic color, looks like nice clean oil.

As for the 4L60, since the trans had to come out to replace the rear cover seal, I put a built 4L60 back in, and it's doing great.

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[2004LB7]

What about sending in an oil sample? That might give you a heads up or what to look for before you tear into it.

I voted overheated oil before reading the post. But I'm on the spun bearing wagon now. But James's guess at the relief might be a good one

 

[1FastBrick]

No it doesn't have the oil pan relieve. That was a Gen 4 thing in the 07 up trucks.

If its not nocking and the PSI is actually low, I vote cam bearings. Common to spin when they have been run low on oil. How ever, Usually when they spin in the block and seize to the cam, they chirp. Unless it walked out completely.

 

[DAVe3283]

Just picked it up, now back to the farm. An oil sample isn't a bad idea, but I don't have a baseline for this engine.

Driving it on the trailer and it only made 20 PSI on a cold start and quickly fell off after driving it the 20' onto the trailer. Still sounds perfect, which gives me a glimmer of hope it's just a bad sensor. But I usually see them fail to max PSI or 0 PSI. I'll keep posting back as I make progress, but I've got some work travel coming up so it might be a while before I can actually tear into it.

 

[NC-smokinlmm]

I’ll bet that thing is fun.

 

[1FastBrick]

Which Maggie kit do you have? Is it an old Side mount set up that flows through a stock intake or is it a MP112 that bolts on top in place of the intake?

 

[DAVe3283]

I've been having fun with it so far. A bit cheaper than modding the blue truck, and the goal is I race something cheaper to break, so I suppose I'm achieving that goal LOL

Now that you mention it, I do believe the supercharger is the MP112. I ended up replacing the intercooler pump with a modern Pierburg as the original had failed, but it flows way too much for the tiny-ass reservoir the kit came with, so it mostly pumps high speed foam instead of water LOL. I have some plans to address that once the engine is fixed.

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[1FastBrick]

I was curious because they also had a side mount set up back then as well.
It's hard to find a 90mm inlet J tube for those blowers I can't remeber if they were just ported or if they were a special casting. If they were ported castings you might be able to find some one to modify your existing one. I am going to have see if I can dig up the info on that.

 

[Mikey52]

You might fix it by just unplugging, the plugging back in the connector to sensor. I've fixed min 2 or 3 times doing that. Now when the alarm goes off, I just get out and wiggle the connector and that fixes it for a while. I replaced the sensor years ago. Make sure you get the special socket for it. It's much easier that way.

 

[NC-smokinlmm]

If it’s the sensor you can drill a hole in the block drivers side near the front oil galley and put in a new sensor there.

 

[DAVe3283]

I did unplug and re-connect the sensor with no change. If the sensor does turn out to be bad, I will stick a new GM one in (because that worked so well the first time) and also add an aftermarket pressure+temperature in the side of the block like NC-smokinlmm mentioned.

I got a mechanical gauge in today, but probably won't have time to hook it up this week. Given the engine sounds perfect, I am still holding out some hope it is just a dead sensor or some debris got in it or something.

 

[PureHybrid]

Did you remove the oil pump during all this work? Properly line it back up?

 

[DAVe3283]

Yes, I replaced the original oil pump with a Melling 10296 high-flow oil pump. I followed Melling's recommended alignment procedure (bolts in finger tight, rotate engine 360°).