Bone stock 1997 Z M6 with 90k. For the last couple of years when the outside air temprature gets cold this car has developed a knock when under power before the engine warms to normal operating temp. The frequency or repitition increases with rpm and totally disappears when the temp gauge indicates warming up. I cannot hear any knocking with no load on the drivetrain, it only occurs during acceleration, from about 1900 until about 3100 rpm. No noticable loss of power. Single cylnder noise. My two theories are a main bearing slowly failing or (I hope) a lifter gone south. Anyone have thoughts or ideas? Thanks, J.B.

 

What you hear is the piston skirts slapping against the cylinder walls. My (brand new at the time) SS started doing this before I had 3,000 miles on it. GM replaced the engine under warranty. New one did the same thing. Turns out that alot of the newer engines do this. The hypereutectic pistons have a greater degree of thermal expansion and can be noisy when the block and pistons are still cold. Good news is it doesn't seem to be causing any problems. I do try to avoid hard acceleration and higher rpm's until it warms up though.

 

Same problem here on KB hyper pistons. The colder it is, the louder and longer it lasts. Usually about 1-5 min and she's silent as a mouse after that.

 

That does make some sense, but I wonder why my high mileage motor would show this sympyom when your motor did with only 3000 or so miles on it? I do follow strict periodic maintenance practices. Just wondering how much I should worry about this.

 

I really don't have a good answer for that other than internal wear. If the noise persisted regardless of engine temp I might suspect a siezed wrist pin. If it became more pronounced after warm-up and/or oil pressure dropped significantly I might be thinking it could be bearing related. But if it's only there during cold weather start-up until the engine begins to warm up I'd be very confident that it's the piston/skirt slap you hear.

I have had two different Ford work vans over the years. The oldest was a 1999. The newest is a 2006. Both had piston slap. The new van has had it on cold winter start-up since new. The older van didn't start doing it until around 120,000 miles on the clock.

 

just went over this last week in class... stock alum pistons will do it some and forged/hyper will do it the most

just luck of the draw its just because the metals cold and hasn't expanded yet nothing to worry about just don't tear *** until it heats up

 

Mine has 140k on it, has done it since i can remember. I just always wait until the noise goes away to start moving. I dont know if that matters or not, but it makes me feel a whole lot better.

 

Sound is normal and harmless, it is piston skirt slap. LS1's are notorious for it.
Here is a good read:
http://www.pistonslap.com/

My new forged Mahle pistons have done it since 300 miles, its more of a scuffing sound than a real knock. It does not even show up on my kncok counts according to the Scanmaster.

 

My 97 z does it a bit (45K miles) but my 2000 Odyssey van does it alot. So much for the myth of Honda engines - the transmissions on that van are garbage too, by the way.

The noise on the Honda only became prominent at around 75K miles and now at 105K miles it is quite noticeable on winter mornings for the first 5 minutes or so. In hindsight I was hearing piston slap probably before 50K miles but it was so slight that I thought it was just in need of a valve adjustment (which they say is required periodically - it didn't need it).

Anyway, on to the potentially useful info. The Honda technician confirmed that it was piston slap by starting the engine cold so that it was exhibiting piston slap and then just disconnected the injector harness leads one at a time to isolate the piston in question.

Obviously this will only be of use if one of your pistons is causing it much more than any of the others. Everything I have heard/read indicates that this is not really too much to be concerned about, as long as it is not excessive.

 

Well stated; best to let the oil come up the temperature before beating on the car.