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Cold start Engine knock

04-11-2004, 10:55 PM
Just purchased a 2001 Sierra
The dealer says it is normal for my 4.8L EFI Vortec V8 to make a knock
or valve tap sound when it is first started. The sound goes away after the engine warms up. It does not hurt the performance cold of the engine. Is this normal? I paid good money for the vehicle and I love my first truck. It was a good investment. Can someone give me a right answer as to what is going on here?
I still have 1500 mi. on the original warranty.


04-12-2004, 09:32 AM
My silverado does the same thing, over in the Silverado Forum there has been a ton of threads on "Piston Slap" as it seems to be called I change my oil every 2500 miles and baby the crap out of my truck and it does it when its cold, once warm the noise goes away.I have an K&N intake with dual exhaust and it runs the balls,the knocking noise when cold doesnt seem to affect my performance either, Look through the silverado forum and you will see a ton of replys on the matter.

04-13-2004, 07:29 PM
Thanks Jimmy....I read the posts over at Chevy.
same problem. Guess I have to live with it.


04-23-2004, 12:43 AM
I just bought a 2001 Sierra with 106K on it. Has the 5.3 and sounds like a diesel when I start it. Thought I had big prob till I found this forum. Thanks guys!


06-14-2004, 03:14 AM
My 96 gmc did the same thing found out that that bearings turned. It knocks for about 45sec's than it is good till the next cold start. GM has a bullitin about it it seems the crank was torqued up uneven.

06-14-2004, 08:34 AM
I was on the phone fianlly with a young lady from GM. She said that their engineers have disected the problem and that there is no need to be upset. My vehicle is 470 miles out of warranty. If it was under warranty they would have offered me a 100K engine warrnaty on the vehicle. I really think this is all :screwy: but who am I to judge?

I spoke to a mechanic that works for Chevy and he said that the piston skirts have been shortened to compensate for the emissions controls they have put on the engine. Thus the slapping is heard by the piston hitting the cylinder walls. Told me that it will not hurt anything.

Thanks for your input
John Thoden

06-15-2004, 04:31 AM
I guess it could be piston slap. On my 5.7L mine had a cold start up knock so I went every where to find out what was going on and one engine rebuilder went one to his info disk about the Vortec's and printed it so I could see it for my self. So I told him to rebiuld the engine it had to be bored 10 or 20 thou because of the piston slap. They started to make a oval cylider. I also had to get the crank turned because of the bearings. I also talked to other rebuilders seems I caught mine in time,the crank has gone so far that they had to buy a new crank. Anyways I got it fix and I happy I am a GM fan and always will be.I have to find out how to put pic on here.

06-15-2004, 04:38 AM
I also put longer piston skirts in

06-18-2004, 10:57 AM
SIERRAK1500...if you have proper documentation from dealer, were you reported noise while truck was still under wty..should still be able to get ext wty ???

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