"Piston Slap"?

[skyguy]

I have a 2007, 1LT, 6 spd. manual with 37K miles. There is a noticeable engine knock around 2500-3000 rpm when motor is cold. When oil temp is above 200° it fades away, but is still slightly audible. Dealer mechanic says it's "piston slap", is common in most LS motors, and will do no harm. Is he right and will it get worse with time? Should I be concerned? It's got a GM Certified warranty. The car runs smoothly with no other issues. Thanks in advance for your replies.

[ChuckG]

I've been told this is a carbon build-up issue.

Of course, wait for verification from one of the master techs, but I understand it can often be solved by a tank or two of BG 44K and an oil change.

[skyguy]

Thanks, if you're right that's great news. I'll wait to hear from the techs.

[NORTY]

CEL?
Piston "slap" shouldn't be common in anything. Unless the engine has been built "loose" on purpose for racing applications.

Are you using oil?
What oil are you using?
Any change in oil use?
What's your OCI?

[LLC5]

A small amount of piston slap is normal and doesn't hurt anything, and carbon build up can aggravate the noise. As ChuckG stated using BG 44K can remove the carbon deposits and limit the piston slap noise. Using Top Tier fuels after decarbonizing will limit the amount of carbon build up on the pistons and intake valves. I doubt if you will get any warranty coverage with this problem as decarbonizing is considered a maintenance issue, more so with premium fueled vehicles than with vehicles that require lower octane. Good luck with it.

[JBsZ06]

I successfully used BG44K in my past vette. I haven't thought to use it on my relatively low mileage c6 though...(28K Miles)

Always use tier one fuel when possible..

Shell, Chevron and others on the list.

[NORTY]

Carbon build up is generally caused by the use of an upper cylinder lubricant. These products are designed to not "burn off," so they leave a glaze that builds to a varnish.
Low quality gasoline (as well as an inferior tune) can also contribute to carbon build up.

I'm still puzzled about the "knock" the OP is experiencing between 2500-3000rpm. If an engine is going to "knock," it would be when the operating temp is high...not "cold." (Unless of course, "knock" does not = pre ignition.)

Piston slap will not get "better over time."

Sure would like to see your timing/fuel curve tables...

[c4c5specialist]

Hi there,

"Piston slap' as the term is used is very broad based. This is essentially when the clearance of the piston skirt to cylinder bore is slightly loose. Slightly being .0001-.002 between the two. Carbon always buildsup in any engine and then when cold, the piston rocks slightly creating the nois

Now, that being said, many factors contribute to this. Low combustion chamber temperatures in conjunction with higher octane fuels create carbon buildup on the top piston ring land. This creates a rocking type noise, ONLY WHEN COLD and may be audible at certain engie speeds.

I will leave tuning out, because you have not stated you have one.

And this condition will NOT EVER create a relibility concern.

Use one can of BG 44k to a full tank of fuel and see if the noise is reduced. If it is, then you know that it was carbon buildup that created your condition.

Allthebest, c4c5

[skyguy]

Thanks so much for all your replies, especially c4c5's detailed analysis... I trust and respect your professional opinion. I'm greatly relieved to know that it will not cause any reliability problems. I will try the BG 44k and see if the noise is reduced. Just bought the car a week ago and the motor was warm when I test drove it, so I didn't notice the knock at that time. It's bone-stock and accelerates smoothly with plenty of power.

[NORTY]

Excessive conrod angle also causes cylinder "elongation."(at the bottom of the cylinder.) Using superior lubricants can stave this off, but eventually, it'll occur.
This is caused by the rods being "too short."

The Honda Superhawk motorcycle is a perfect example of this pnenom.