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.

 

[persistent]

i've removed the piston and found what i more or less expected:

- a lot of carbon buildup on the outside surface of the piston
- top compression ring was so stuck inside its groove that it was hard to find the gap, and it took several minutes to carefully pry it out
- bottom compression ring cracked in 2 as i was removing it
- connecting rod bearings are visibly worn to the point they need replacement, and the bearing surface is notably rough to the touch where worn
- crankshaft surface where connecting rod bearing seats has some mild wear, but nothing in comparison to the rod bearing

i have a set of replacement piston rings here, part 12685185, but i noticed there are only 4 rings in the kit instead the 5 that were on the piston. the 3rd ring from this kit appears to be approximately twice the thickness of bottom rings i removed. the piston ring kit with 5 rings, part 12687779, appears to have a substantial lead time (3+ weeks). any thoughts on using the 4 ring kit instead of the 5 ring one?

i see that the relevant oem connecting rod bearings, part 12735504, are hard to come by (even gmpartsdirect lists them as "backordered"), and i am unsure of aftermarket compatibility. not sure what to do here.

 

[Geotrash]

i've removed the piston and found what i more or less expected:

- a lot of carbon buildup on the outside surface of the piston
- top compression ring was so stuck inside its groove that it was hard to find the gap, and it took several minutes to carefully pry it out
- bottom compression ring cracked in 2 as i was removing it
- connecting rod bearings are visibly worn to the point they need replacement, and the bearing surface is notably rough to the touch where worn
- crankshaft surface where connecting rod bearing seats has some mild wear, but nothing in comparison to the rod bearing

i have a set of replacement piston rings here, part 12685185, but i noticed there are only 4 rings in the kit instead the 5 that were on the piston. the 3rd ring from this kit appears to be approximately twice the thickness of bottom rings i removed. the piston ring kit with 5 rings, part 12687779, appears to have a substantial lead time (3+ weeks). any thoughts on using the 4 ring kit instead of the 5 ring one?

i see that the relevant oem connecting rod bearings, part 12735504, are hard to come by (even gmpartsdirect lists them as "backordered"), and i am unsure of aftermarket compatibility. not sure what to do here.

Yikes. It's a safe bet that your other rod bearings are in similar shape. And given the issues cropping up with the stock pistons, I would be inclined to strip the whole engine block and build it back up with aftermarket parts from reliable suppliers. As part of that, I'd have a machine shop look at the crank and clean it up if needed. Call and talk to a pro at either Summit Racing or Brian Tooley Racing and see what they can offer you in terms of aftermarket parts that will fit.

To answer your question directly though, I wouldn't use a 4-ring kit on a 5-ring piston.

Update: Summit lists several aftermarket pistons, rings and rod bearings for the L86.

CHEVROLET 6.2L/376 Chevy small block Gen V Pistons Pistons & Piston Rings Engines & Components - Free Shipping on Orders Over $109 at Summit Racing

Find CHEVROLET 6.2L/376 Chevy small block Gen V Pistons and get Free Shipping on Orders Over $109 at Summit Racing!

www.summitracing.com

 

[Pressureangle]

Tough one. Consider that you have a bad injector on that cylinder, which would explain the rough running, excessive carbon buildup on the ring lands (detonation blowing the oil off the cylinders and overheating the rings, causing oil from below to coke up) and possibly collapsing the piston itself seizing the rings. The bearings may or may not be limited to this cylinder only. As long as it's out, I'd have the injectors tested and cleaned ( injectorrx.com )

 

[persistent]

Yikes. It's a safe bet that your other rod bearings are in similar shape. And given the issues cropping up with the stock pistons, I would be inclined to strip the whole engine block and build it back up with aftermarket parts from reliable suppliers. As part of that, I'd have a machine shop look at the crank and clean it up if needed. Call and talk to a pro at either Summit Racing or Brian Tooley Racing and see what they can offer you in terms of aftermarket parts that will fit.

To answer your question directly though, I wouldn't use a 4-ring kit on a 5-ring piston.

Update: Summit lists several aftermarket pistons, rings and rod bearings for the L86.

CHEVROLET 6.2L/376 Chevy small block Gen V Pistons Pistons & Piston Rings Engines & Components - Free Shipping on Orders Over $109 at Summit Racing

Find CHEVROLET 6.2L/376 Chevy small block Gen V Pistons and get Free Shipping on Orders Over $109 at Summit Racing!

www.summitracing.com

after reading your suggestion re full rebuild, i called a friend of mine who was a chinook mechanic in the army for several years to get his take re the connecting rod bearings.

his take is that the bearing wear is not atypical for a car with 90k mi from a visual inspection, and when i demonstrated the limited longitudinal (along crank axis) play in the connecting rod bearings for the installed pistons, he thought there was no need to change out the bearings imminently.

re the 4 vs 5 ring piston ring set, it ends up i was being a doofus and there are indeed 5 rings in the kit. the 2 oil rings that go above and below the spring ring were stuck together and looked like a single ring on initial inspection.

since this is my wife's car, i'm hopeful that it will make it another 50k mi until i'm ready to get her a new car (tahoe), so i don't have to pull the engine out and do a full rebuild.

 

[persistent]

Tough one. Consider that you have a bad injector on that cylinder, which would explain the rough running, excessive carbon buildup on the ring lands (detonation blowing the oil off the cylinders and overheating the rings, causing oil from below to coke up) and possibly collapsing the piston itself seizing the rings. The bearings may or may not be limited to this cylinder only. As long as it's out, I'd have the injectors tested and cleaned ( injectorrx.com )

that is a decent hypothesis.

i've pulled the right side head with the fuel rail and injectors still installed, so i'd have to remove the rail and injectors to test them. i haven't done that previously, and my recollection is that this is typically done with the head installed using a prybar and some puller tool.

i will probably remove the injectors once i get the rest of it back together and see about testing them.