don22u

2008 after sitting a couple of days has a loud clapping knock. GM replaced the lifters, that didn't do it. GM replaced the camshaft assembly and new lifters again.

The knock is still there. The factory rep got frustrated and said knocking is natural, "its under warranty"????? So I asked him, why did they authorize repairs multiple times. You could hear a pin drop by the silence.

The knock lasts up to 1 1/2 minutes with engine rough and burping. You rev engine up/down there a slapping knock or clacking then it starts to subside
Second time they removed the heads and had to scrape carbon off a fully loaded piston where the valve contacted the carbon. Never seen buildup like that on these engines.

??? Anybody have knocking at startup and runup as normal. Ima thunkin someone is trying to pull something over me.
Any suggestion:

VETTEPLAY

I know this Don: I wouldn't settle for bs from the dealer until the warranty ran out. Them authorizing the repairs is their acknowledgment there is a problem. Sounds like "maybe" its not lubricated/oiled sufficiently until you start getting oil circulation going again. My chevrolet truck always makes a tapping on startup until oil circulation gets up nd running then it disappears.

don22u

Yes but a corvette is not a truck and you didn't specifically address my questions. See I'm trying to identify solutions as this clacking is deeper than a lifter; likely wrist pin or piston, not likely rod/rod bearings. That is why I am seeking advice from others that might have encountered the same or similar situation, but thanks for responding.

Don

JCtx

You need to record it to more or less know what you're talking about man. It's hard enough with a recording, but don't expect much input with only a verbal explanation; it can be many things. In addition, you need to add more information, like mileage, if you just bought the car or if it started doing that all of a sudden, mods on the engine, etc. Good luck.

SLAZ06

GM had piston slap issues a few years back on the LS1 motors. That problem was "corrected" on the LS2/3s, but it could be your problem as the noise is similiar. If you floor the gas before hitting state button the engine will turn over but not start. I do this if the vet sat up for several days to prevent the lifter knock I get. Trying that may help narrow the trouble down, if the the knock is reduced or disappears you then know it's most likely lubrication issue.

Larry/car

What oil are you using and the visicity ( 5W50)? What does you're oil pressure gage indicate at start up? Since the dealer replaced lifters we can discount them. Interesting that the engine is rough and burping? A piston slap or excessive connecting rod/crankshaft clearances wouldn't do that. With out hearing your engine, maybe it is not the engine but one of the accessories? Carbon build up, has to be running excessively rich; that would explain rough and burping. Fuel injection system component? When the engine settles/quiets down, does it perform normally or is it running rich?

JCtx

I can tell you from personal experience piston-slap was NOT corrected with the LS2 . I had to sell both my LS1 and LS2s because of that (and extremely noisy drivetrain). Haven't heard of that on the LS3 because it has deep-skirt pistons. And yes, my LS3 Vette finally has a healthy sounding engine, and no piston-slap .

I didn't say anything about piston-slap to the OP because we need to hear what the heck he's talking about. But he has another problem, since true piston-slap only lasts a second or two, and it's sickening to hear. He might have an oil-pump issue as well. Oh, and that reminds me to ask the OP to state what oil and viscosity he used.

Cory1745

A friend of mine has a 11' SS Camaro (LS3) with 17k miles & was having the same problem. They said it was the oil pump and replaced it. Got it back a couple days ago and its back to normal

don22u

Hey thanks. No its not an oil pump issue. Thats the first thing I looked for and watch each time at startup 40 psi and steady
thanks for the feedback

don22u

Mobil One. Fuel system and pressure is fine. Carbon buildup a result of the malfunctioning camshaft assembly, due to not replacing the camshaft with the lifters the first time. I even questioned them on how lifters can set to the camshaft and if there's a problem it transfers to the next component not replaced. Their answer: Hey that's what the factory authorized. I'm looking for another dealership in the So Cal area who takes a more proactive stance in figuring out the problem

Yes in part to the engine settling down. After minute or two.
thanks

MR_JLR

Sorry you are having this trouble. Last year after not driving my 08 for a week, temps under 50 outside, I went to start the vette and it made a very loud CLANK CLANk CLANK, that went away after a few minutes. However it was very disheartening to hear and only happened when outside temps were under 50 degrees. I ended up having camshaft and lifters replaced for another reason, so I can't say if that was the fix, but the only other thing I had changed was the manifolds, cats, x pipe to Z06 components. I also stopped using mobil 1. I have purposely started the car in temps below 50 and haven't heard the noise again(knock on wood).

There was another thread on here years ago of a guy with the same problem, I believe he had camshaft and lifter replaced as well as 1 of the catalytic converters. So with me having my camshaft and lifters replaced, as well as upgrading to LS7 exhaust components and not hearing the noise again, it very well may be something with the exhaust involved. I think he had a check engine light though, I never did. I will try to locate the thread.

Al Gumby

My `08 LS3 has piston slap every morning until the oil temp comes up. Sounds like a diesel, but I drive it anyway.

don22u

Thanks, Don't see a problem with Mobil One. I'll look into the cat and manifolds
As to Al Gumby on the ls3 with piston slap, there will come a day when,........ don't let that happen, get it fixed
thanks all, keep em coming. We'll fix it

Mike's LS3

My 2008 coupe developed a piston slap knock at cold start-up a couple years back. I performed two treatments of BGK 44 power enhancer in the gas tank. After the first treatment the noise reduced 80%. After the second treatment, 100% gone. This "piston slap" I experience was carbon deposits on the ring-lands, piston tops, valves, etc. Not "piston slap" from tolerance issues. I regularly use BGK 44 power enhancer a couple times a year and installed a catch can to reduce carbon buildup.

MR_JLR

thread i referenced earlier
http://forums.corvetteforum.com/c6-t...3-problem.html

similar
http://forums.corvetteforum.com/c6-c...-06-vette.html

http://forums.corvetteforum.com/c6-c...when-cold.html

http://forums.corvetteforum.com/c6-t...-start-up.html

hope this helps. I know too well how unnerving it is to hear your car making an abnormal noise.

Kadorja

Mine sounds like I have some nasty cams if its a decently cold day (for Florida) but it usually goes away after a few seconds.