2015 Tahoe. Oil in cylinder 6 after lifter replacement

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TYF Newbie
Sep 29, 2023
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Good morning everyone. Thanks for letting me post. A little background on me, I've been a technician in light duty vehicles for 20 or so years. Been a diesel tech for the last 14. I moonlight out of my shop at home to subsidize my income in this economy. Usually I'm pretty knowledgeable and can fix just about anything that comes my way. However, I've got one that has me scratching my head. I had a 2015 Tahoe with AFM come through my shop for a knock when entering 4 cylinder mode. I diagnosed it as a bad afm lifter on the driver's side. Vehicle has 145,000 miles on it. Customer did not want to delete the afm so I replaced the 8 afm lifters and 8 standard lifters. Cam was in great shape. New head gaskets lifter baskets, intake seals and everything in between. Even had the heads gone through at the machine shop. They checked out great and everything was a straight forward repair as I've done in the past. However, once back together, it ran for 12 or so miles and started missing on cylinder 6. Pulled it back in the shop and started diagnosing. Swapped coils and wires with no change. While trying this, I noticed a little smoke out the exhaust. Went to swap plugs and 6 was wet with oil. And I mean wet. I pulled the exhaust manifold away from the head and #6 port was saturated. Pulled the valve cover and all valves are moving as they should. Put a compression tester on it and sometimes cranking it shows 25 lbs and then it will show 200. So I pulled the head back off (was pissed and forgot to do a leak down test before I pulled the head). I was really careful pulling it to not get coolant in that cylinder so as to not contaminated my findings. Once head was off there was probably 2 or so ounces of oil in the cylinder on top of the piston. The intake manifold and intake runners in the head just had a slight film of oil but nothing alarming. But the #6 exhaust port of the head is soaked. Had the head checked again for a bad valve seal but head checked out fine. I thought about piston rings, but it was dry when I originally pulled the head and the plug was dry. And the cylinder is not scored and still has factory hone marks. I'd think to put that much oil in that fast I'd see some cylinder issues but I could be wrong. Just wondering if any of you had any input. Thank you in advance.

Joseph Garcia

Supporting Member
Aug 2, 2018
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Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

That is indeed a strange issue.

I cannot personally help you with your question; however, other members on this Forum that are much more knowledgeable than me in this area will chime in.


Oct 13, 2022
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Centennial, Co
Welcome to the forum.

Seems like the only thing it could be is a cracked ring? If the oil isn't coming from up top the only place it could come from is the bottom? No? You went through the heads.. Push rods good? No noise when running? The odd 25 then 200 psi compression could only be the rings if the valves aren't hanging open. Right?



TYF Newbie
Mar 23, 2015
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Customer did not want to delete the afm so I replaced the 8 afm lifters and 8 standard lifters. Cam was in great shape.
I was under the impression you had to do a cam change if you did not put the AFM lifters back in.

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