leaking piston rings: seeking advice on process

[persistent]

i've been on here for the past few months posting about engine trouble with my wife's 2020 tahoe l86 that has 90k miles, and i believe i've narrowed it down to piston rings or similar on cylinder 8. i'm seeking advice on what process i should take when attempting to repair it.

i had the engine down to the block a couple weeks ago, but i made the (painful) mistake of not doing compression and leakdown tests _before_ taking it apart. after rebuilding the top half of the engine and doing a dod delete, i started getting misfires on cylinder 8 again after startup. i did compression and leakdown tests on all 8 cylinders. for compression i got 225-240 psi everywhere except cylinder 8, which had 95. i added a couple squirts of engine oil to the cylinder and redid the compression test, which gave 125. for leakdown, i got 12-18% loss for all cylinders except 8, which gave 85%+. when i covered the pcv tubes on the valve covers during leakdown on cylinder 8, it would spray oil from the dipstick tube. additionally, when idling with the oil cap off, air surges out of the oil cap pretty forcefully. both the service manual and what i've read elsewhere suggest this indicates a piston ring failure or similar.

now that i've diagnosed the issue properly, i'm wondering what the best approach is to do the repair. i'm already familiar with the process of removing the heads and oil pan, but it isn't clear to me whether doing this as an engine-in procedure is better than doing it engine-out.

when i disassembled the engine previously, i did not see any visible scoring or damage to the cylinder bore or any cracks. if the bore is substantially scored or the block damaged, i assume i would have to remove the engine anyways. thoughts on whether i should pull the engine or not for this repair? i've not pulled an engine before, so i'd need to read up and get equipment.

assuming there is no other substantive damage, does it make sense to replace just the piston rings on the damaged cylinder or should i replace the rings on all the cylinders?

in the meantime, i will be reading up on how to remove the engine to assess that route versus attempting the repair engine-in.

 

[Bill K]

It almost sounds like you might have a cracked or broken piston ? Its unusual for rings to only fail like that on one cylinder. At this point I think you are definitely looking at pulling the engine and going through it. I just finished fixing one for a good friend and it had 4 pistons that were cracked between the ring lands GM has been having a lot of issues with those engines. One problem is that they are a lot higher compression than the 5.3 and 6.0 and really require 93 octane fuel. My friend was running mostly 87 in his and was not aware that it needed the higher octane. The Direct Injection engines also have a problem with what is called Low Speed Pre-ingnition. Here is a good article on it.

Low-Speed Pre-Ignition: What Is It And How To Prevent It - Valvoline™ Global KSA - EN

www.valvolineglobal.com

Here is a picture of one of my friends pistons. If you get to that point Silvolite is the one aftermarket piston available at this time that I know of.

 

[persistent]

i pulled the right side cylinder head and confirmed what i saw on a bore scope - very light longitudinal scoring mark on the top of the bore. despite the visible scoring mark, i cannot feel the scoring and it does not register when i gently scrape the surface with my nail.

i'm thinking about honing this bore a bit once i get the piston removed. note that i would be doing this engine-in. thoughts on whether this is a bad idea or not?

something notable about the history of this vehicle is that from roughly 50k mi, it had slowly developed a strong vibration that did not appear to be an engine mount going bad, going off the usual tests with the hood open watching for engine movement while shifting and pressing the gas while braking. given no CEL appeared until roughly 90k mi, i'm guessing cylinder 8 was having problems the whole time and slowly failing, just not to the point of misfiring and triggering CEL.

since i was trying to be maximally time efficient, i pulled the head after draining the coolant from the radiator petcock but without removing the water pump. i ended up with quite a lot of coolant coming out onto the floor as a result. is there a trick to drain the coolant to avoid removing the water pump and coolant hoses when pulling heads?

i see that there are coolant drain plugs on the engine that require some mega hex socket, and they don't look very accessible with engine-in. i have a vacuum refill kit, so i wonder if i can use it to pump out the coolant somehow.

 

[Bill K]

i pulled the right side cylinder head and confirmed what i saw on a bore scope - very light longitudinal scoring mark on the top of the bore. despite the visible scoring mark, i cannot feel the scoring and it does not register when i gently scrape the surface with my nail.

i'm thinking about honing this bore a bit once i get the piston removed. note that i would be doing this engine-in. thoughts on whether this is a bad idea or not?

Depends on what you find when you pull the piston out. Something caused the scoring. You need to find out what.

If you do hone it DO NOT use one of those parts store three stone hones. Use a "flex hone" type hone or nothing. And see if your local machine shop will sell you a bottle of honing oil.

I think you are going to find something else wrong when you pull the piston that will almost certainly require more than just new rings. I do hope I am wrong but I do it all day long so .......

 

[persistent]

i dropped the oil pan previously to install a new oil pump, and i didn't see any obvious debris or damage when looking up at the crank and pistons from underneath or in the oil pan.

i should have the pan dropped tomorrow, so i'll have a closer look for damage before pulling the piston.