Thought I would share an issue I had with the board for all Chevy owners with AFF or Active Fuel Management.
My wife 2007 avalanche has 211,000 miles on it. So one little Engine issue no big deal.
The in January she said the Engine had a miss in it and the Stabilitrak light came on. So she drove it home and had me look at it. I started it up and a definite tick could be heard from the Engine. This sound very familiar to me. It was either a lifter stuck lifter, bent push rod, bad rocker or stuck valve. No big deal other than she drove it ten miles homes so I ruled out stuck valve or the Engine most likely would have shelled. Pulled the valve cover on the even side and found a loose push rod and thought ok lifter or push rod. Pulled rocker arm and the push rod was good. so I went after the lifter. The thing with an LS Engine the head has to come off to replace and the lifter was an AFF type which is 40.00 and a is of a roller configuration which is good for me in this case.

After pulling head checking cam lift I dropped a new one in and reinstalled the head and rocker assembly. I did go though the rocker assembly making sure all was good as well checked the other lifter on that bank. Ran like a charm so changed the oil and told to give it hell. After all 200,000 + miles one stuck lifter not bad. Went and gave it a complete tune up was I was at. Now this brings us to last week when she brought it home with a slight miss in it, this time the check Engine came on. Had a slight roughness to it however the Engine sounded solid. I thought maybe I had a bad coil which LS are not known for going bad like the ford ones which a lot of ford guy change over to LS coils. So I plugged in a tester and the computer said #7 miss fire. Cool for me bad coil or cracked a plug when I installed it or a new plug wire went bad.
I swapped 5 and seven reset computer and started it up miss still there so plugged in tester and #7 miss fire detected. Damit I thought nothing simply. Before digging into the issue I drove it checking all gauges etc. as no other code flashed. Thought had to be a mechanical issue.

This takes me to observation and recollection. Keep in mind as a rule I change the oil filter twice for every oil fluid change unless I pull with the truck over 250 miles, then its a one to one. For the past 100,000 mile I did notice the oil pressure dropping between oil changes and when it goes into AFF usually as it reaching time to change oil and filter. once the oil or filter was changes the pressure returned to normal cold and dropped a little warm. this is kind of typical for a high millage Engine and wear on the crank and rods, oil pressure drops when warm. I thought the pressure fluctuation at AFF was normal for a high mileage Engine. After all we were talking 200,000 miles at 50 psi cold and 40 psi warm at idol and running operation as steady speed with a 5 pound drop at gauge when AFF engaged. I thought noting of it. As the miles increased on an oil change the pressure would drop a little and to me that's ok for this Engine. Change the oil and filter and its back to normal on the dash gauge. No Engine noise at all with the exception of cold start.

As I test drove the truck it ran like poop and like a Engine that lost a cam or crank and the oil pressure was low like 20 pounds and surged up ad down. So I thought # 7 miss with no electrical issue kiss the Engine good by. The wired thing was the Engine at idol sounded solid with erratic oil pressure. I knew if the crank let go with that type of oil pressure it would have a knock at idol and easily could be heard. With a cylinder miss fire detected I new it had to be in the valve train, dang it lost a cam. The thing that puzzled me was the oil pressure and rough running at a higher rpm. I thought what the chances of a bad oil sending unit or pump timed with # 7 misfire. Made no sense to me as that is just not the way engines work. My only thought was lost a cam and plugged the filter and the sending unit was not getting enough oil except during filter by pass. Here is where it gets weird if your following my way of thinking. I could see the cam on the odd side when I removed the even side lifters and check cam lift and rollers on lifter for wear. All looked good, so did I screw up or do I have another lifter failure. I bet on lifter failure again. So I pulled odd side valve cover and all push rod were tight no play to speak of like one would have if the lifter was stuck down in the bore. As I cranked the Engine over by hand (after removing plugs) there was no lift on #7 intake. So the thought of cam being flat and wasted was becoming an reality. I just could not get over the way it sounded, ran and oil pressure acted.

On the AFF system the #7 cylinder was an AFF lifter so I opted to pull the head and take a look as it would need to be removed to replace the cam anyway. Surprise, Surprise the lifter suffered an internal failure and it was acting like it was in AFF mode keeping the push rod tight and not allowing the valve to open. Cool another lifter but why?????????????????
I check the lift on the cam before any parts were bought and replaced. The cam was good so I had to figure out would cause this other than time. I knew the AFF system ran off of oil pressure in the valley manifold which had had electrical components that controlled the oil pressure to the AFF style lifters to basically unlock them to float and drive on 4 cylinders. So did I suffer a failure due to oil pressure or suffer an internal failure or both. Thought process was one lifter stuck on even side and one lifter failure on odd side, different oil control valves. This lead to oil pressure to the valves as it did not through a code for the AFF.

So why did I have an oil pressure issue going to the Valley Oil Manifold. I had to have one or the oil sending unit would not have been telling the gauge to have erratic movement. The oil passage in the block going to the sending unit mounted on the VOM (valley Oil Manifold) came from the filtered side of the oil filter. So do I have a blockage or obstruction. I would have to and answer why the pressure reading were being so erratic. I pulled the VOM and gave it a once over cleaned it and blew of the passages as well as the AFF passages leading from the block to the lifer bores all seamed ok. still did not make since until I removed the oil sending unit to clean this passage. The mother bleeping GM engineers or darn maintenance manual makers. Who new they put a stupid screen type filter in this passage which could become partial blocked from the bottom and only allow a slight by pass under high pressure. This was a bad design or failure to put in a maintenance literature.

All I had to do was clean tis filter and oil pressure at the gauge was restored to 60 pound with only a slight fluctuation when the AFF was engaged allowing the oil manifold valves to allow proper oil flow to the AFF lifters.

Moral to this story you Chevy with AFF has low oil pressure or fluctuates more than 5 pound when in AFF mode clean or replace this filter before lifter failure. Any AFF Chevy over 50,000 miles should have this done or do it yourself. It will take one day of your own time and no parts required if carful. Pull intake remove oil sending unit pull out filter and clean than reinstall. If you tare it get a new one from Chevy there not expensive.

See pic of filter.
I spent $620.00 and four days of my time to tract down this issue. I would have only had to spend worse case 65.00 for filter and intake gasket when I did my tune ups.

Just out of curiosity, do you use Dino oil or synthetic?

I have heard of Gen3 and Gen4 5.3L's generating a ton of carbon and sludge in the oil. Have you peeled open any of your oil filters? May be filling up your filter with carbon/sludge, and then running the filter in bypass mode could cause the erratic oil pressure and maybe even starve it. Gen3 & Gen4 are also known to burn through 1-2qts oil in a couple k miles. Some people (in warranty) will get on an "oil consumption program" where the dealer will montior oil level. If in warranty and drinking oil, they may opt to replace rings, hone, etc. But obviously you're out of warranty, lol.

I had a similar issue on my 08 Avalanche. I didnt notice oil pressure fluctuation, but had misfires on #8 (non AFM cylinder). I changed plugs, swapped wires and coils and still had the issue on #8. The Engine always ran and idled smooth. I could not tell it was missing and there was no unusual ticking or knocking. My issue began at about 95K miles, but outside of the 5 year Engine warranty period by 3 months. I had the local Chevy dealer run some diagnostics and they found a slightly low compression reading on #8. Without going into the Engine ($2500 for the dealer to tear it down) the speculation was that either a lifter had partially collapsed or had a bad seal (GM used AFM lifters on all cylinders) or the cam/push rod had some wear. I drove it for almost 2 years (30K miles) after that and nothing got worse or completely failed.

The issue seemed to occur at start up, if I started the Engine and immediately began driving I would not get a check Engine light. I could drive for hours with no issue, but if I started it and let it idle for a few minutes before driving I would get the check Engine light with in 2 or 3 minutes. GM was no help to me on this, but I speculate the computer was looking at the number of allowable misfires at startup differently when compared to driving speed.

Because I was outside of the 5 year/100K power train warranty by a few months GM would not do anything for me. I was told if I had been the original owner of the vehicle (I bought it with 9K miles on it) they would pay 50% of the repair bill. My options were $4500 for a top end rebuild/ $7500 for a new crate Engine.

After 2 years of carrying my scan tool around resetting the light hundreds of times (the check Engine light drove me crazy), I solved the issue once and for good with a 2013 Avalanche. Since that little change I have not had a single check Engine light.

LOL...awesome solution.

The sad part is I saw my old truck listed for sale at another dealer... "like new". I wonder if they actually fixed it or just cut the wire for the check Engine light.