Hey guys, I have a 91 Trans Am with the LB9 (305 TPI) engine that makes a very brief knocking noise on start up. It does this only when the engine is completely cold. The noise lasts for about a second tops and the engine runs fine with no smoke. It has 102,000 miles. I'm thinking it's piston slap but I thought I'd ask for piece of mind. Anything to worry about? Thanks!

 

my lil 305 does the same thing. not sure if it piston slap on mine or something else.

 

so the sound goes away when it warms up, sounds to me its a lifter tap. pull valve cover and adjust

 

mines a deeper noise than the lifter tap.

 

my 350 does this as well, interested to see what some people say. My dad says its normal...

 

Well the thing is it only makes that noise for a half second tops following startup, and that's after the car has been sitting at least overnight. I don't think the engine has much time to warm up in 1/2 a second. And it sounds a little deeper than valve noise, it sounds almost like a light rod knock. But again, it disappears as quickly as it comes.

 

Sounds like an oil pressure issue. Rod bearings may be worn enough to cause the brief knock until your oil pressure builds up. You could try a heavier oil. But it's probably nothing to freak out about just keep an eye on it as well as your oil level.

 

i actually tried a thicker oil and it was worse. actually you want an oil that gets to bearings quicker.