Knock when cold

[Denalibro76]

Good morning all you fine folks. I have a 2009 Yukon with the L9H and correct me if I’m wrong but it doesn’t have AFM right? Reason for my post is I have a knock when the engine is cold. Not really noticeable at idle (except initial startup) more noticeable while driving until the engine gets to about half normal operating temp. Not sure if it’s piston slap or rod knock? Since I haven’t experienced either issue before I thought I’d ask you what is more likely?

 

[SpareParts]

Sounds like piston slap.

 

[strutaeng]

Piston slap, but usually that goes away in 30-60 seconds (when cold months around here, but really depends on ambient temperature).

Definitely is not rod knock if it goes away.

Could also be broken exhaust manifold bolt(s) which is super common.

 

[Joseph Garcia]

I've had a similar knock since I purchased my new-to-me 2007 Yukon XL Denali, and it goes away as the motor warms up. I attribute it to piston slap, since I've replaced all of the lifters and installed new exhaust manifolds with ARP bolts.

I just baby the motor, until it is fully warmed up.

 

[rdezs]

If you upload a video with audio..... Piston slap should be 100% confirmed by many of us hearing it. Interpretation of sounds without actually hearing it is very objective. Like the difference between a slap and a knock.

As for timing of the sound, a rod knocking tends to get louder as the engine warms up and the oil gets thinner. Also louder with RPM increases and usually a pretty good oil pressure drop.

While the "official" position on piston slap is that it's harmless..... And this is just my personal opinion that doesn't coincide with the official position from GM..... Is that as a former machinist in years gone by..... Two pieces of metal smacking each other audibly is not good. I view it as a defect in the Piston skirt design, or the lack thereof. Common Sense tells me that if you slap an aluminum piston hundreds of thousands of times against something to where you can hear it, you will begin losing material. Now who knows, this might take 200,000 miles. And that is probably why GM says not to worry about it.

Sorry, a bit off topic, and in the form it's probably been beat to death like a piston slapping a cylinder

 

[Denalibro76]

Piston slap, but usually that goes away in 30-60 seconds (when cold months around here, but really depends on ambient temperature).

Definitely is not rod knock if it goes away.

Could also be broken exhaust manifold bolt(s) which is super common.

Yeah. On cold startups it definitely goes away after a minute or so. I’m going to have to take the fender liners out and have a real good look at the exhaust manifolds to check for issues as I have heard what sounds like an exhaust leak when I’ve started it.