My 2013 Yukon XL 1500

Let’s see how far this goes….


I picked up a high mileage 2013 GMC Yukon XL SLT 1500 4WD the other day with a misfire and lifter knock on cyl 6.
Odometer shows 186k miles, but GM dealer replaced the engine at 88k miles due to a failed afm lifter. So currently motor has just under 100k and surprise surprise another afm lifter failure. Despite the high mileage this thing is actually really really clean. CA truck its whole life, started as an Enterprise Rental for the first 11 months and 24k miles. Then to a private owner down the street from me for the last 8 years as a family hauler. I got it for a steal so couldn’t pass it up.

My plan was to jump head first into a full afm delete. However after reading a lot more and watching some videos I’m tempted to try a trick to release a stuck lifter and see if it comes back to life. If I’m successful then I’ll plan to get it smogged (lovely CA) and just drive it around for a few months to see if anything else major is needed. If it seems pretty solid after that time, then I’ll likely plan for a full afm delete so it can be a solid reliable runner.

I’m usually biased towards 3/4 or 1 ton rigs but after the experience we’ve had with my wife’s current daily (04 Yukon 4WD) I’m curious if the gmt900 is a decent contender once you get rid of that crappy afm system. Time will tell.

Feel free to follow along as I document what I learn and accomplish some things along the way. This is my first foray into the gmt900 platform so it might get interesting…

Some more interior shots (sorry for the poor quality photos). I’ll try to get some better ones soon.

Good looking truck.

So tomorrow the plan is the drain the oil and cut open the oil filter to see if there's any carnage caught in there.
Apparently the lifter started knocking and misfiring about 9 miles from the house where they parked it. I did toss in a bottle of MMO in the crankcase and drove it about 6 miles to get it home. Noise seemed to come and go during that drive but still misfiring.
So I'll drain and cut open the filter tomorrow and report back.

If the filter is clean then my plan is to try to uncollapse the lifter and then run a motor flush and drive it for a while. If there is anything in the filter thats suspect then I'll plan to pull the heads and lifters and cam and check the cam for damage and go from there (likely full afm delete).

Drained the oil and went through it with a fine screen and magnet, cut open the filter and went through all the filter media/pleats. Found no metal anywhere but definitely noticed some grit in the bottom of the oil filter and in the pleats.
I can't say it was like sand but definitely gritty but smaller particles than sand. not much but it was definitely there. Pictures didn't work to pick it up. so I know the oil that was in there has about 4500 miles on it and they added BG MOA Engine Oil additive at the time the oil was changed (January of 2024). I also added a bottle of MMO before driving home ~ 6 miles. Glad there's no metal but not sure what this gritty stuff was. It was a pretty cheap no brand filter at a quick lube oil place. I filled it with fresh full synthetic 5w-30 and put a new Purolator One filter on and planning to see if I can get the lifter unstuck tomorrow.

We'll see how that goes.

So got the intake manifold, passenger valve cover and valley cover off last night. After confirming the intake lifter on #6 was collapsed as expected (checking the rocker arm was flopping back and forth), we proceeded with a trick to shock the lifter and cause it to release. I used a tool from a YT channel (Crazed Performance Repair) I'm sure many of you have come across. Lo and behold it worked. The lifter released and the pushrod popped right out. Checked the push rod to confirm it wasn't bent, cleaned and lubed it and returned it to its home. Then reset the locks on the lifter as he explains in his video. Got the rocker arm and manually rotated the engine to confirm the valve was now opening/closing as it should. Put the valve cover back on and torqued last night. Planning to button up the VLOM cover and intake manifold today and hope it runs without collapsing right away so I can run some AMSOIL motor flush in it. This thing is pretty dirty. Not so much sludge but definitely caked on oil residue.

For a fun comparison.....

The Left image is the passenger valve cover from this 2013 5.3L AFM w/ 94k miles. Appears to be ~10k mi OCI (following the message on the cluster) with cheap synthetic oil and no name filters used.

The Right image is the driver valve cover from our 2004 5.3L NON-AFM w/ 198k miles. 3.5k mi OCI with conventional oil but purolator or mobil 1 filters used.

Clearly these AFM engines need to keep the oil change interval below 5k miles regardless if its synthetic or not.

For fun you can see where the rocker arm was smacking the underside of the valve cover in the left image .

ohh yeah and when we pulled the air filter, found a squirrels jackpot. I bet he's pissed.

Great pics, good luck with her. Thanks for sharing everything!

Alright so we got it all back together tonight and...
Got it all buttoned up and ready to fire with fresh oil, filter and Amsoil motor flush.


After Two rounds of Amsoil flush (with fresh oil and filter each time) cut open both filters, no debris or metal found in either. Put in a final filter and some Amsoil Signature 5w-30 and let it run for about 30 minutes at idle before driving. Found cyl on #3 spark plug was loose (ie I could unthread it by hand). so Pulled it out, inspected and reinstalled.
It still had a significant misfire/stumble. Drove it to the gas station got some Chevron and Techron additive and dumped that in. Drove it for another 10 minutes or so and the misfire was significantly better. I'm betting that the spark plug/injector is just fouled up from the misfiring prior. I'll likely end up changing out all the spark plugs. But there's no active misfire code (flashing CEL) and no lifter noise anymore. So thats a win so far.

Plan is to drive it some more and get some miles on it so that it can complete the emissions monitoring tests we can get it smogged. I'll probably change spark plugs before the end of the week.
I've been driving it in Manual mode to keep it from going to V4 mode right now until It gets a chance to run well enough on 8 cylinders without and rough idle or stumbling. Then I may let it run in full AFM mode and see how it does. Hopefully the lifter doesn't get stuck again right away.

Here's a clip from driving on the highway tonight and giving it a little bit of a run...
So far im thrilled it's running without the lifter knock and there was no metal in it. Oil still looks super clean right now. After about 500-1000 miles I'll probably send a sample to Blackstone labs for an oil analysis to see if there's anything to be concerned about.