2014 Tahoe (LMG motor with VVT and AFM). 150k mi. Got it about 6 months ago. Started having low oil pressure (to my eye) and eventually went low enough that the warning came on. Ruled out a bad sensor (saw pressure drastically drop on a mechanical gauge when hot). OK, time to pull the motor and see what's up. Motor came apart and looked PERFECT inside... no scoring on the bearing surfaces, lifters all looked in good shape, no sludge, etc. OK, must be a bad pump or failing relief valve in the AFM system. So, I did the following:

First pass:
- Crank journals measured and all within spec
- New cam bearings (confirmed centered in the bore and oil ports aligned)
- New AFM delete kit (Melling MDK300 - included cam (mc1390, lifters, buckets, oil control valve, cam plate)
- New timing chain and sprocket (including VVT sprocket and control valve)
- AFM VLOM manifold oil supply port blocked off in the valley cover, valley feed holes also plugged
- New GM standard volume pump, new o-ring matching the original (red)
- In-pan relief valve port plugged
- Engine completely disassembled and all oil galleys cleaned, new front plug and rear Improved Racing barbell installed
- AC Delco PF63 oil filter
- 5W-30 Full Synthetic
- All new gaskets and seals
- ECU retune for AFM delete

Put the motor back in, and still poor oil pressure. Motor also lot compression during test drive... turns out the new camshaft sprocket didn't engage fully on the pin and it slid out of time by a few degrees... OK motor out again with the assumption that the sprocket not sitting properly provided a "bleed point" contributing to my oil pressure issue.

Pass 2:
- Inspect all valves, none bent, but replace pushrods since they were ever so slightly bent
- New rod, and main bearings (standard) - just did this to see if it would help bring up the pressure as well.
- Cam bearings confirmed centered in the bore and oil ports aligned
- New VVT oil control valve and new Melling MC1390 cam (since the alignment pin was damaged on the 1st one)
- As before, oil pan relief, VLOM oil galley are both still blocked.
- New GM standard volume pump reinstalled and added a Melling pickup tube (I used the oring that came with the tube)
- While engine back apart, confirm that oil galleys still clear
- AC Delco PF63 oil filter
- 5W-30 Full Synthetic

Engine back in and fired up - STILL only seeing ~7-10psi at idle when hot (confirmed on cluster and with mechanical gauge), and I can't really get it above 30 even when revving. Motor has been out and apart 2x now for this. I didn't put a HV pump in because I know the AFM engine pumps are supposed to have decent pressure out of the box. I did test (to the best of my ability) the relief valve on the pump and it started opening at 60-70psi air pressure, so I don't think that is faulty. Truck is running normally around town now, so I don't think sprocket/cam alignment issue is present and I did take extra care to make sure it seated properly this time.

I'm at my wits end on this thing. I've had it out 2x now and cannot get it to produce decent oil pressure. The engine is actually in excellent shape internally, everything looks great when checked with a micrometer. I thought for sure blocking the AFM/VLOM oil galley would vastly increase pressure (since a lot of pressure can get lost there), and the oil pan relief. I inspected everything I can on this when I had it down to a bare block but just cannot see where I would be losing that much pressure. The only thing I can think is that the new pump itself isn't good, but I did disassemble it and inspect and it looked fine on the second go around. There is also an oil cooler on this truck but I'm not losing oil and it doesn't appear to be mixing. I did notice oil coming out the front of the camshaft oil valve, which I believe is part of how it works, right?

Any input appreciated... losing sleep over this thing at this point!!! I just want it to run with decent oil pressure. Any thoughts would be appreciated!

 

Where are you measuring oil pressure at?

Is there an engine oil cooler in the circuit and have you tried bypassing it to see if pressure improves?

There’s only a few things that can cause low pressure -

​​​​​​1) Faulty pump and/or pressure relief valve.
2) Pickup tube not properly sealed at pump.
3) Restriction in one or more galleys.
4) Excessive bearing clearances.

 

Thanks. Galleys are clear, pickup tube and oring are new (used oring that came with pickup tube. Bearings are new and matched to what I mic'd on the crank (still within standard spec). I plastigauged the bearings too before install and they were good. At this point I'm leaning towards potentially some kind of restriction in the ACDELCO PF63 which is a larger size and apparently the ACD filters have a known issue causing low pressure. I have a WIX on order to try today (smaller size closer to original).

Measuring OP at the top of the motor this round, have had measurements at top and bottom when I was diagnosing, but likely need to do another test on the bottom side this go around.

 

Just tried a smaller WIX filter, no noticeable change in oil pressure...

 

Did you remove or delete/block off part #455 (lower right above oil filter)?

 

Yes it was blocked off with a plug (https://www.improvedracing.com/ls-oi...alve-plug.html)

 

I’d recommend getting an adapter to check pressure at the oil filter so you can determine if it’s a problem at the top or bottom end.

 

Yep that's what I'm tracing down now. I am seeing much better on the oil pump outlet plug (20psi hot idle, jumping quickly to 35-40PSI at 1kRPM). Tomorrow I'm going to test at the sender port at the top. Using an adaptor like this, what does it tell me in addition to those two points? Just making sure I'm understanding the testing procedure...

Thanks!
-Phil

 

Reference the flow diagram in my previous post.

Oil is pumped through the lower galley straight to the oil filter, giving the filter ports direct and maximum pressure from the pump.

Good oil pressure at the filter indicates the pump, pump relief valve, and pickup tube seal are all intact and functioning as expected.

With good pressure at the filter and low pressure at the sensor bore, the problem is isolated to the hydraulic circuit(s) between the sensor and filter.

Note that if it’s equipped with an engine oil cooler, post-filter oil will first flow to the cooler, then it will return to the block and flow into the upper galleys.

In this case, at the sensor port, Test pressure with the cooler attached and with it bypassed. are cheap and worth the investment if the cooler proves at fault - if you intend to repair the cooler, the bypass allows short term use until it can be addressed.

(P.S. It doesn’t have to be that fancy of a plate to get the job done. )

Good pressure with the cooler bypassed = plugged or leaking oil cooler and/or lines. Replace both if the circuit is indicated as the source.

Good luck.

 

Thank you so much - I was definitely considering perhaps something going on with the cooler, although it all looks in good shape and the oil that came out was clean. But at this point I need to put this level of troubleshooting in so I appreciate your input tremendously!!!

-Phil