Stock 2008 c6 with 115k miles. Seen 6 track days and many autocrosses, a couple of drift days. Daily driver.

I've been hearing a bit of a tapping/growling noise under part throttle beginning about two-three months ago. No noise during cruise or idle. No constant knocking, like rod bearings.

Oil test showed a small spike of lead at 96k and nothing else. No noise then. By 115k there was everything. Same lead, but iron, aluminum, tin, nickel, chrome also up. Indicating bearings, pistons and rings.

Here are the problem wear metals. The order of the numbers is: this last oil test at 115k, second last oil test at 96k, average from previous tests.

Al 47, 7, 7
Pb 23, 29, 9
Sn 6, 0, 0
Mg 840, 620, 15
Fe 55, 35, 35
Cr 5, 2, 2
Ni 5, 3, 3
Cu 61, 65, 82 - lower

Measured low compression (130 vs 180) and high leak down by the rings (40% vs 10%) in #4 and #5.

I've always heard a bit of a ticking/tapping at high throttle that I think might've been piston slap, since new. Some slight piston slap at startup since new. Could this have exacerbated into the current condition, or unrelated?

I also ran the engine with no oil for a few minutes by accident at around 70k. Surprised if that took this long to manifest.

I'm here because from what I've taken apart so far with the engine in the car - #2,3,4 main caps and #4 piston/rod/rod bearing - I see no obvious wear. Can you guys see anything from the pics (below)?

I see a bit of a rub mark on the back/passemnger side of the #4 cylinder wall, but I can't feel it much. Didn't have a bore gauge yet to measure. Is this enough to cause leakage by the rings? Can I just hone it out?

All the bearings look nice to me (pics are probably worse than real life), and I don't see bad wear on the piston crown or skirt. Rings also feel smooth. But please let me know if you see anything from the pics.

http://s1094.beta.photobucket.com/us...vt=email_share

Trying to figure out where all the metal is coming from. It's indicating bearings, pistons, and rings. Not seeing that much metal loss. Is lead and tin 100% indicating bad bearings?

At this point I might have to pull the engine to get a look at the #4 and #5 main bearings and the #5 piston/cylinder. I guess the cam bearings might be the bad ones, as well.

Any similar experiences? Any tips?

Thanks a lot!

 

Do ya think?


I make the decisions after events like this quickly. Option 1. Turn up the radio when driving and get a mechanical oil pressure gauge piped into the oil pump feed and stick it right in front of your face on the dash and run the hell out of it until you lose oil pressure then stop the car and rebuild it. or option 2. Run it til she blows.

Of course theres always everyones favorite option 3a. Pull it out and freshen everything and start over right now

 

3a is what I'm doing. I'm just not seeing the damage I'm supposed to fix. I also don't wanna fully machine the block and replace everything, because I can't be without a car for that long. I was hoping I'd get in there, see the bad bearings and pistons as the oil test suggested, replace them, hone the bad cylinders, and hope she runs for another year or two. But I'm not seeing the damage.

 

Geeeeeeeeez

Did you check the valvetrain(Lifters,etc..)

After all of the events that have occurred, you are lucky the **** still runs.

I have seen less **** happen and cause a rebuild.


Either refreshen it or just run that bitch till she blows.

 

I'll put money on the bearings being washed. Sometimes, they can wear and it will be hard to see with the naked eye.

Here's what I see happened..

You ran it without oil at 70k. Which took any coating left off the bearing. Shortly after, you started to lose the ring seal for whatever reason, causing excessive blow by. This caused fuel loaded oil, washing thru the already worn bearings, causing the spike in abnormal metals in the analysis.

 

X2

Heck, I am surprised it lasted this long!

 

If the damage is undetectable to the naked eye just throw a new set of bearings, rings and a dingle ball hone in it. Have a valve job done on the heads and you will be okay for a stock daily.

 

This is what I would do as well.

Pull the engine:
-Polish the crank
-Tank the block, and hone just enough to break any glaze. There are some nice DIY hones avalible.
-New rod bolts
-New bearings
-Re ring kit
-New head bolts
-Quality gasket kit.
-Get the heads tanked and freshend up with new valves seal, and springs

With a crank polish you might need +.001 oversized bearings, and you might want to order some hand gapped rings. So, double, tripple check all clearences, and use a top notch torqure wrench. You will come in under a grand, and be down for a few weeks.


Good luck!

 

Note that my oil tests, even the last one, showed very good oil quality, not diluted by fuel. Was surprising to me, too...

Checking the lifters is a good idea. Though I don't see them causing the various metal spikes, like lead. The rockers and pushrods look good... The one valve I removed showed no wear.

Why do a couple of you insist on a valve job? I'm not losing much if any compression by the valves on any cylinders.

Not keen on dropping anything off at a shop. Bad experiences with downtime.

Any tips on popping the retainers loose with the head off the engine? They're very stuck, and I can't figure out a good way to push up on the valve and not have the retainers fly randomly around the garage, or possibly even bend a valve...

 

What was the ring end gap installed in the cylinder? This will be a better indicator rather then just looking at the ring

-I prefer not to have my oil analyzed as it will most likely freak me out and make me over think the issue...I keep my oil clean and changed and make sure it has oil pressure...thats it.

 

I wouldn't be so afraid of the oil test. I did them every couple of oil changes and the numbers were consistent until the real problems appeared.

I thought about checking the end gaps on the current rings, then forgot to do it. Will do tomorrow.

I did hit the cylinders with a bore gauge tonight. I don't see anything different in the bad cylinder. Measurements vary by up to 0.002in per bore depending on the position in the bore (0.001 or less at nearby points is more common). Saw both taper and out of round. The numbers are quite wild, and consistent in the same spot no matter how many times I measure, but I dismiss it since the head's not torqued down, it's the same in all cylinders, and only two don't seal.

The "rub" spot visible in the pics doesn't really measure different than the pristine cylinder wall around it, so I doubt it would cause sealing issues.

I assume piston to wall clearance is measured near the bottom of the skirt. Going by that, I have around 0.003 - 0.0015 piston to wall clearance. In some spots it tightens up though, and at the bottom of bore #4, perpendicular to the crank, I measure slight negative clearance (the bore really tightens up there). At least compared to the brand new piston. The old piston is a tiny bit smaller, by one thou or less. I'll try sliding the new piston down and make sure it doesn't bind... Maybe the bore shape will change enough when the head's torqued down, and open up...

 

The reason for I reccomend a valve job is because they are a cheap thing to do that will get you back a near perfect head and pick up some power. The factory heads don't have that great of quality control, the cuts on the valve seats aren't that precise and sometimes valve guides get off center as well as the surface not being completely smooth and square. For around $250-300 a good machine shop will do a three angle cut on the valve seats, check all the valve guides and replace ones that are out of spec, clean up the surface of the head, install and check the springs for proper height and pressure. They will clean and magnaflux the heads to check for cracks too. They can also mill the head down at your request if you want them to to bump compression. You don't have to get the heads done when do a stock overhaul like that but you might miss problems that way and you would definaty be leaving some power on the table.

 

I'd get a close look up in the block at each of the 5 cam bearings. If one of them is going you'll see lead in the oil. In my case I had a cam bearing moving out of it's bore and rubbing a lifter, putting lead in the oil. Found it from oil analysis just as you have done.

I'd pull all the mains/rods at this time...would only take one bearing to be putting the metal in the oil. The ones you have pulled so far certainly look just fine.

with 115K I'd agree with Gunslinger on this ....have the heads freshened up and guides checked. Break glaze on the cylinders to restore cross hatch

 

Ended up pulling the engine cause I couldn't see anything wrong.

Turned out that it was the cam bearings. Upper halves are rough, lower halves have a copper spot. I'm assuming if I hadn't run it with no oil two years ago, they'd still be fine.

Cam journals look perfect, and rod and main bearings look perfect. Even pistons and rings look perfect.

I checked the ring gap on #4 that had a bad compression and leakdown, and the gap was .016 top and .025 bottom. That's pretty much spec. I really can't figure out how air was getting by those rings, given that the bore looks great as well. Any ideas?

Either way, doing a full rebuild. Valve job, heads checked, decked. Block hot tanked and honed just a touch to break in new rings. All new bearings of course. Crank polished (though it looks perfect).

That should do er...

Hope to have it running by the end of this long weekend.

 

Are you upgrading the cam or staying with the stocker?

 

Ended up sticking with stock. Ran out of time. Need to throw it together asap. Interestingly, the cam journals are pristine.

 

I was hoping you were going with the LS9 cam. I wanted to see exactly what it would make over the stock LS3 cam.

 

If everything was in good shape then air likely wasnt getting past the rings. Easy way to tell is pour a bit of oil in the cyls and do another compression test. If it makes big gains on compression its your rings/cyl/piston if it doesnt make any difference its your valves not sealing up right.