07 Silverado Z71 cold knock

[Vol4Ever]

Found this in several spots, but still no definitive answer. 135k on my trusty single owner Z71 but over the last couple of weeks it has started a knock. Only does it when engine is "multiple hours" cold, usually has to be full overnight. Upon starting there will be a loud knock maybe 2-3 times at most that just goes away for rest of ride. I cranked with hood up today and listened from under the hood. Almost sounded like a hammer blow inside the engine (not a light ping in other words), but same as always it stopped after 2.

 

Ran some errands and parked at home. About 6 hours later I cranked again and no knock. I am close to trading in this truck, and the issue is scaring me that I better do it fast. 2 quick questions:

 

1 - Possible that it's something easy to fix since it only hits twice and then doesn't do it again until really cold? Tips to check?

2 - Fully plan to disclose issue to dealer,especially since I won't be able to duplicate on the fly. Would be easy to hide! Should I expect a huge hit on vehicle value? Even though 10 years old, I have all service records and it's in exceptional condition.

 

Left at Chevy overnight last week when there for regular oil change. Had to do that so they could replicate, which they did. Now they tell me they want $1,200 to tear down engine for further diagnosis, and who knows how much more to fix whatever they may find. OBD device not throwing any codes to help.

 

Sorry for a long first post. I am an enthusiast but not even close to a mechanic. Thoughts or advice from new bow tie friends? Thanks in advance.

 

~Vol

[Chebbyfan]

Piston slap? My 99 k1500 has better than 200k on the original 5.7 and does that when I first start it cold in the morning. It lasts a little longer than just a couple knocks tho. I understand yours is a 5.3, but I was under the assumption that the 5.3 did the same thing after a few miles.

[O_J_Simpson]

You can't fix it. GM says it is normal.

[Vol4Ever]

So it's nothing that's damaging the engine? If GM has acknowledged issue and not recommending a fix, maybe the truck has some life left and won't take a hit on resale value? Runs perfect other than the startup hammer hits.