7) There are many questions and advice on the internet related to the clattering rattling engine noise that occurs from cold startup until it reaches normal operating temperature. Some web forum authors claim that this is "normal", no damage is occurring, and the life of the engine is not being shortened. Some claim that General Motors is aware of this but never offered a solution.

Several suggestions reside on the internet, two of which I have tried without success. First, I poured a can of Sea Foam into a near-empty gas tank then filled it up with premium gasoline. No change in the clattering. Next, I tried ethanol-free gasoline, three tanks-full. Again, no change in the clattering.

I have not yet tried adding Sea Foam to the engine oil. It's not due for an oil change for awhile. I also have not tried adding Sea Foam to the vacuum system. I'm scared about this procedure, and have nobody to help me anyway.

My mechanic offered to do a "decarbonization" procedure, or something to that effect. I don't recall exactly what he called his process. It entailed circulating gasoline, or something like that, which would necessitate removing the fuel hose to the injection system. Something like that. I didn't really understand. He didn't make any promises, of course, except to bill me regardless of whether it resolves the problem or not!

So, my good Corvette friends, what is causing my engine to sound like a diesel truck? Is this rather unsettling noise a signal that damage has, or is, being done? What "solutions" have worked, what has failed, and what is recommended?

 

The C5 has been known to have what is called piston slap, where it makes a knocking noise on startup until the car's engine warms up. As long as it is not excessively loud, it is normal and not much you can do to prevent it. Just the way it is.

 

If I were you I would not add sea foam to the crankcase under any circumstance. Like jack said the c5 has a known problem of piston slap on cold startup. Search you tube for videos about piston slap on LSx engines and you can hear and decide if it is the noise you have. I know of ls engines with over 200,000 miles that have had piston slap since new and still going strong. Good luck

 

Yep, it's PISTON SLAP:



These sound just like my engine. So I'll just have to live with it like everybody else.

Thank you for replying with good information,

Dave

 

Yep that's normal for GM small blocks.

 

Nothing wrong with adding some seafoam to the oil just can't keep it in there long 100 miles is what is recommended. I put a 1/3 a can in mine a week before doing an oil change. Then 1/3 sucked through the vacuum line and 1/3 in the gas tank.

 

My explorer was making a horrible racket when I started it when cold. My neighbor said to try some STP. I did that three oil changes ago and the noise is gone. I was going to get rid of it so its worth a try.

 

Good grief. And I thought my soft ticking noise on start up was bad.

 

Another vote for piston slap. My 2000 has 109,000 on it and on cold startup you would think a gremlin with a small ball peen hammer was sitting in the lifter galley tapping away at one side of the cylinder block. It has been doing this since I bought it with 89,000 miles on it. As soon as the engine starts warming up, about five minutes running time, the noise goes away and it doesn't come back until next cold startup. Don't run the dog crap out of it when cold and all should be fine.

 

From what I have learned, at one time when building an engine every piston diameter was measured and matched to a cylinder in the block, as it was time consuming GM decided that the tolerances were close enough not to have to do this procedure (probably save 30 min.) So now when building engines the engine builder simply grabs a piston and slaps it in a cylinder hole. The result is some of the pistons are loose and clatter until they expand to the cylinder diameter after start up and the rattle stops until they cool and start rattling all over again. So you have to live with it for the life of your engine because GM wanted to cut some labour expense!! Thankfully mine doesn't piston slap but I can sympathise with you, it would be very annoying. Rob

 

It isn't really a big deal if you aren't into ******* a cold engine; which you shouldn't do anyway. By the time I drive a half mile from my driveway it is gone. Car runs great. At 89,000 it ran a standing quarter mile at 13.1 and 106 mph. And it is a convertible, A4 and 315 gear.

 

I hate to live with GM's imperfections like electrochromatic rear view mirrors that don't darken unless another car is Right Behind Me, moldy A/C ducts, and piston slap. All these nuisance conditions could have been prevented through better engineering.

But I always wanted a C5 and overall I am thrilled with mine. I will ask my mechanic to work on the A/C ducts, I will consider a manual aftermarket rear view mirror, and I will blast the stereo to hide piston slap, as soon as I get one of those double-din mods and a sound system that I actually like.

 

We're not alone. Mighty Lexus has the same problem on some of their newer 3500 engines. It just takes a little longer to show up.
I guess I shouldn't complain because my C5 only exhibits the noise under a load, i.e. drive it before it's warmed up. Also, for what its worth, the noise decreased just a little bit when I went from Penzoil to Mobil 1.