Knocking noise on intial movement for the day

[bthomas02tahoe]

I have noticed once my Tahoe sits overnight in the cold upon intial start up, and drive I hear a loud knocking noise from the engine.

The noice only lasts about 20 seconds or so then goes away, but I am wondering what this noise is.

Anybody know?

 

[HitandRunDriver]

It's piston slap and completely normal in tahoes, nothing you can really do about it

 

[2000Yukon]

nothing you can really do about it

Ya there is, spend alot of money and get it bored out. lol

 

[bthomas02tahoe]

It's piston slap and completely normal in tahoes, nothing you can really do about it

Why does this happen though?

It only happens on the first movement of the day. Should I let it warm up longer before driving?

 

[TnTahoe]

yeah a couple minutes should do the trick

 

[terrible_buddhist]

VERY common in those engines, and usually nothing to worry about...

Piston slap is caused when there's too much space between the piston and the cylinder wall. The piston moves up and down in the cylinder and the extra clearance results in a greater amount of rocking in the cylinder, producing a loud knocking noise. Vehicles with the engine knock problem include 1999 - 2002 GMC and Chevy pickups and sports-utility models with 3.1, 3.4, 4.3, 4.6 (Northstar), 4.8, 5.3, 5.7(LS1), 6.0 or 8.1 liter engines. Specific models include (but may not be limited to): Chevrolet Silverado, Tahoe and Suburban, GMC Sierra, Yukon and Yukon XL. The 2002 Cadillac Escalade SUV also reportedly suffers from the problem.

 

[Blkout02]

mine does the same thing good to know the hoe is okk

 

[badtothe bone]

Actually GM diagnosed this problem a long time ago to be a build up of carbon above the top ring of the piston.
At one time, while they were under warranty, the resolved the problem by taking out the pistons, cleaning them and reinstalling them with new rings.
In 40,000 miles, the symptoms reappeared.
If you look at the pistons you will see that the skirts are too short.
The way to increase power and mileage is to lighten the reciprocating mass.
By making the pistons, rods and crank lighter, you get more power and better gas mileage at the expense of a little noise during start up on cold mornings, due to the pistons not riding firmly in the bore.
So if you want to say that the pistons are too loose, your assumption might be right. But pistons are not round like the cylinders. They are actually oval and they grow as they heat up - so as to fill the cylinder as tightly as possible, yet be able to go up and down without galling and getting stuck when they grow.

 

[T3rry]

basically it just needs time to cycle the oil n stuff.

 

[TheFuzz]

Actually GM diagnosed this problem a long time ago to be a build up of carbon above the top ring of the piston.
At one time, while they were under warranty, the resolved the problem by taking out the pistons, cleaning them and reinstalling them with new rings.
In 40,000 miles, the symptoms reappeared.
If you look at the pistons you will see that the skirts are too short.
The way to increase power and mileage is to lighten the reciprocating mass.
By making the pistons, rods and crank lighter, you get more power and better gas mileage at the expense of a little noise during start up on cold mornings, due to the pistons not riding firmly in the bore.
So if you want to say that the pistons are too loose, your assumption might be right. But pistons are not round like the cylinders. They are actually oval and they grow as they heat up - so as to fill the cylinder as tightly as possible, yet be able to go up and down without galling and getting stuck when they grow.

Where did you get your information from? I've heard the whole carbon buildup diagnosis before, but it isn't accurate IMO. Think about it for a second...you're blaming a temporary ticking/tapping noise on a permanent condition (carbon buildup). If the noise goes away when the engine warms up a little, how could it be carbon buildup? A physical buildup around the piston rings would lead to a constant noise, don't you think? I've spoken to several GM master techs who dispute the carbon thing fervantly.

Here's my take on it, courtesy of one of those techs:

The reason you get the noise only when it's cold, is because the pistons are aluminum. Aluminum expands as it warms, so when the pistons are cold, there is a little excess room around the skirt (bottom) of the piston. After the pistons warm a little, they expand and the noise goes away. It's a simple and sensible explaination for a common problem. GM even introduced teflon coated pistons to help alleviate this problem, although the pistons didn't solve it completely.

In your example, the reason the noise went away upon cleaning the carbon out is because of the new rings they installed...not the cleaning. Once the new rings wear in a little, the noise comes back. The last part of your post was accurate though. It all has to do with the pistons expanding as they warm up.