red 95 z28 lt1

on a completley cold start my engine knocks, but it goes away after 30 seconds or so, i got a vid of it, in the first half it does it and you can hear it over the valvetrain if you turn up the volume and listen closeley

motor is almost 3 years old and has aprox 10k miles, eagle rotating assembly, clevite bearings. dyno tune was done in the spring time.

it started doing this just about a month ago when it started getting cold out, car is not daily drivin, but when i noticed the problem went away when it warmed up i drove it arround with no problems (dident get on it at all) oil pressure was good (over 40psi) i even changed oil to a lighter weight oil and still have same problem, this vid is the first start in about 3 weeks.


http://s175.photobucket.com/albums/w...urrent=010.flv


what do you guys think, should i have the bottom end rebuilt again, what parts can i keep, i know its hard with out pics but is it possible that i can keep my rods crank and pistons

Green96Z

Sort of sounds like piston slap.

ThisImminentDay

Hi, I'm new to the boards, but I was reading some posts and came across yours and thought I might add my thoughts.

There are a number of ways to diagnose an engine noise. It's difficult to tell from the video, but I will agree with Green96Z in that it kind of sounds like piston slap. This is caused by either too large of a cylinder bore, or too small of a piston diameter, i.e. collapsed piston skirt. An easy way to tell if it is piston slap is listen how the noise reacts to different circumstances. Does the noise seem to come from the lower section of the block? Does the noise increase as the engine load increases, such as on acceleration? Is it loudest when the engine is cold, and then as it begins to warm up closer to operating temperature, begin to fade? All of these indicate a piston slap. If you have a stethoscope, you can even listen to each cylinder and figure out which one is producing the noise the loudest.

Hope that helps!

marshall93z

Just let it warm up a bit.

red 95 z28 lt1

well, it has no rythem to it, its deep in the motor i know its not valvetrain because i checked that out to be good. when i first start it i can hear it and it has no pattern like an exhaust leak would or a collapsed lifter or loose rocker arm,

if its piston slap would it sound like the notorious ls1 piston slap? because i have heard that on a friends car and it dosent sound like that at all,

it literally goes away in 30 seconds of the engine running and i can probably count how many times it does it because it has no pattern

if i start the motor up cold like in an hour of sitting or 5 hours of sitting it wont do it but if i let it sit for a full day it will do it but for only 30 seconds.

thanks for all the input and help

Kevin97ss

Did you change oil filter brands just before you noticed the noise by any chance?

red 95 z28 lt1

no i use ac delco filters never had any problems, and oil pressure is up instantly upon startup. only diffrence is that i used the regular one insted of the extra capacity one this time.

Kevin97ss

Only other suggestion I could make is to see if it is rod/piston noise. Isolate to a particular cylinder by pulling either the injector or plug wire and start the engine. If you get to a cylinder that makes no noise on start up then you know you have a bearing/pin/piston issue in that cylinder. I realize this will take forever since it needs to sit 12-24 hours but it will at least answer some questions.

red 95 z28 lt1

thanks ill start to give that a try lol 1 day at a time