RyaneC

OK.


2005 C6, automatic, 87k miles, purchased with 75k back in May. Daily driver for the most part. No issues with the car so far, other than the known bad solenoid in the shifter (replaced shifter) and airbag light (bypassed the connector and spliced the wires as recommended).


Now that it's started getting cool outside in the mornings (45 or below, 30 this morning), when I first crank it up, I have a knocking noise. Once the car gets to operating temps the noise is gone. It sounds like what I would expect a leaked-down lifter to sound like, but can happen 12 hours after it's parked. And I don't exactly know what a lifter would sound like either, just what I *think* it would sound like. I've had 2 LS1's and neither had a sound like this that I recall... similar but not as loud. It would also go away if I heard it.


It's getting close to oil change time, maybe 30% left on the indicator. I did put about 3/4qt of oil in it a couple of weeks ago; it was still in range on the dipstick but on the low side. Got it back up to above midrange.


So is this the piston slap noise everyone talks about? It's loud enough to hear about 30-50ft away but is usually 90% quieter in 1-2 minutes and is down to the typical "sewing machine" noise within 5, sounding like a typical LSx. Kind of an embarrassing sound if I'm leaving work at 7am but if it's normal than so be it.


Thanks guys!

Mike's LS3

It could be excessive carbon build up on the piston, heads or ring lands. Excessive carbon build up causing the piston to contact the carbon on the head will cause the piston in the bore to rock or slap. The knocking noise is more audible when cold, until the piston heats up and expands.

You can try a few treatments of BG 44K # 208 ( check their website for availability) in the gas tank and some good WOT runs might do the job. However, the most effective way is to remove the spark plugs and inject a carbon dissolving chemical into the spark plug holes. Let car sit, then start it up and see all the smoke (carbon) blow out of the exhaust. GM has a specific chemical to decarb the engine. Specific instructions must be followed so not to hydrolock the engine. Also, once the spark plugs are removed, you can exam the inside with a bore scope for carbon before and after the treatment.

I had the same issue as you are describing and used BG 44K. That helped, but later came back. Then, I had GM perform a couple of decarb treatments which improved considerably to the point that I sometimes I only here a faint knock at cold start up.

EVRose

My 05 has made that same knock noise every winter for the 6 years I've had it. I just live with it.

Gearhead Jim

I can't diagnose the problem, but...
LS engines are not quiet. Even in summer, our LS1 would sound like a Peterbilt for a couple of minutes when first started.

Our current LS3 has been better but far from silent.

butch353

Have had good results with SEA FOAM. in both the mark viii s that i have run 6 an the GMC's an Vette's 4 pop a bottle in with a fill up an take for a couple of runs. it could be piston slap as the piston rocks in the cylinder until it warms up from wear on the skirt or excessive cylinder clearance. JD

butch353

Just a add on. while installing a cam in the 06 we cleaned the piston tops with sea foam an a razor blade you can see how it lossens the carbon as it works. JD

Spasetrucker

This may not be helpful, but I used to have a 1984 Z/28. It came with forged pistons. It made a noise similar to what you are describing in the winter when it was below freezing. It would get quiet after the engine was warmed up. I was told that the pistons get a little smaller when it is cold, but expand to their normal size once the engine warms up. You might want to take it easy until your engine is properly warmed.

WW7

From what I recall, piston slap is more on forged pistons then on cast or hypereutectic pistons..What you may be hearing is a lifter that that has bled down and takes a while to pump up with the cold oil...I have a lifter in my car that bleeds down every once in awhile after the car sits a few days, it makes a little noise for about a minite until it fully pumps up...WW

Piston Slap
Piston rattle or slap when an engine is cold indicates too much clearance between pistons and cylinders. This may be due to excessive assembly tolerances or worn cylinders. Some forged pistons have a higher rate of thermal expansion than cast pistons, and may require slightly greater cold clearances to compensate. This, in turn, may produce some unwanted piston noise when a cold engine is first started, but it has no effect on piston performance, durability or longevity.
.

Dub Mc

I've been there and done that. The LS2 can be a noisy engine even though all are not. Mine is a bit noisy when cold. Let it warm up in cold weather and quieten down before taking off. My mechanic and a LS2 guy finally told me to find something real to worry about because with this I have nothing to be fretful of.

Mike's LS3

The OP developed this knocking noise after he purchased the car. The car had 75K miles when purchased and now has 87k miles. It is difficult to diagnosis a cold start up knocking noise over the internet, however we can only speculate. I would take the car in to a reputable mechanic, let the car sit overnight and perform a cold start up. Pull a few plugs out and bore scope random cylinders to exam carbon buildup. Perform the least evasive repairs, i.e. de-carb the engine with one or two treatments. If no improvement, take the next step, possibly a bad lifter?

cclive

To answer the OPs' question...piston slap sounds more like the noise of a diesel engine idling rather than the tap tap tap noise of a lifter. Both can go away when the engine gets warm. Which is it?

ROJ#79

I had it in my C-5, but not in my C-6
Just a small knock on start up and then it goes away.

09_BSM_Z51

My LS1 car had piston slap on startup. What did help was the famous LS1 snake oil called casterol edge made in belgium. There used to be german casterol that everyone was calling the miracle LS oil but the production moved to belgium. I don't know if it's exactly the same, but it did quiet down the valve train and piston slap. I think it was 0w30 or 0w40. SS seemed to run better on it than M1.

Might be worth a try for you, I don't think I can post other forum links on here but just google 'LS1 Castrol Edge Belgium' and look at the first results from ls1forum. I'm happy with M1 on my LS3, plenty of valve train tick. If noise gets worse I'll give the castrol a chance.

RyaneC

She. When she purchased the car. ☺️

Its hard to say if the noise was there when I purchased the car as I got it out of Florida in 90 degree temps and live in South Carolina. It's only been in the 30's this week, so it could have been there the whole time, just not cold enough outside for a really cold start.

We bought some Seafoam this evening to try out. My fiancé has experience with the procedure so we plan to try it out to see if it helps. He hasn't heard it yet, but based on my description, he's thinking a lifter that's leaking down. I'm still thinking 12-24 hours is awful quick for a lifter leak down. We also plan to change the oil soon too, as it's been 12k miles since purchase but the oil life indicator is still showing over 25% life left. Will be starting with the Seafoam in the gas tank the next time I fill up.

RyaneC

i wouldn't describe it as a tick, like the typical LS valvetrain, but not as loud as my stock Duramax either. Maybe... similar to rapping on an empty solo cup with your knuckles? I guess if the only two noises I had to pick from are the tap tap tap and a diesel idling, I'd lean more towards the diesel sound. It's heavier-sounding than a tap.

RyaneC

I found this in a other (old) thread. This sounds very close to what I hear.

https://m.youtube.com/watch?feature=youtu.be&v=xdcbZSi04Dg

Mike's LS3

Sound is very similar as the link. As it warms up it quiets down to the point it's gone at normal temps. We had my car up on a lift it was definitely coming from bottom end of the engine.

As far as Seaform, running it through a vacuum source is another effective way, but at a slow rate so you do not hydrolock the engine.

The most effective treatment is injecting a de-carb chemical directly into the spark plug holes, letting it sit to dissolve, then starting it up about an hour later. Smoke City!!

redrckt97

the noise in the video above sounds just like my C5 did-
(several years ago) turned out to be a bad roller on one of the lifters (ruined the cam too)

my c6 has some noise on cold starts but is softer and deeper

Rick369

Do you know what type of oil and brand is in your car? Even if it is standard Mobil 1 5W30, I wouldn't be running 12,000 miles between oil changes. 10,000 miles max. You should get your oil changed immediately and send a sample of your 12,000 mile oil to Blackstone Labs for analysis.

Mike's LS3

Did your noise go away once the engine reached normal temps or was it the same?