Cold Start Knock/Clatter

[ChevSub]

Just came over from Pickuptrucks.com and I left a post there.  Dealer told me the other day that some of the 5.3 Chev pick-ups came from the factory with a possible reversed piston in the cylinder. Hence, the cold start knock in the morning (after the truck has been sitting for a long time). Has anybody else heard that from their dealers? I'm a little confused. Why didn't it do it before? Why did the top engine cleaner solve the problem for a few thousand miles? What have you heard?

First it was normal. Second, it was carbon build up. Finally, it's a reversed piston in the cylinder wall. Argh!!

[LT]

Welcome to this forum. I think you will like it here.

I have never heard of a "reversed piston". I'd look elsewere for the problem. Cold plugs, carbon, etc. are more reasonable explanations.

[ChevSub]

Thanks LT.  It's hard to believe that there is a reversed piston. Besides, from what I've heard, the piston design is not unique.  It was designed to have the same tolerances.  Hard to believe it's ALL carbon fouling.  Might try different plugs.  Also, had the top engine cleaner sprayed into the plug holes. Worked great, but only for 3,000 miles and the knock came back.  

Thanks again.

[brax71]

if you're still under warranty, take it from dealership to dealership until you're satisfied...

mine knocks for a few seconds at start-up, but as soon as i put it in gear, it stops immediately.  i've got +50k on my 99 and have had no significant problems despite this noise every morning (knock on wood)!

i've basically just gotten used to it.

[ChevSub]

Checked my post at pickuptrucks.com and have received a few posts.  Chevrolet will be shipping new ceramic coated pistons in December for folks with the knock.  This has been hush, hush so far.  Apparently the pistons are on the small side, prior to reaching temperature.  They are banging inside the cylinder chamber.  Oops!  I think it has to deal with the fact they are aluminum.  Aluminum has a real crappy way of transferring heat.  They are just not like the old cast iron.    

[Roofer]

If the problem is because of aluminum pistons, then they could be in even more trouble in the future. Alot of people already doubt the aluminum heads on the D-max. I hope that they get things handled better than other(Ford) brands.

[Kansas Kid]

For the life of me, I can't figure out why so many people are scared of aluminum.  If something is wrong with a motor and that part happens to be made of aluminum, just blame it on the fact that it is aluminum.   :angry:  Just as a FYI, diesel engines have used aluminum pistons for a very long time, and that means Cummins, Cat, Detroit Diesel, Isuzu, everybody.  And of course, all race engines use aluminum pistons.  Now if aluminum was such a crappy material, why in the he!! would they be used in such extreme applications?  

Now as for this problem, it is very common with hypereutectic pistons.  Most race motors use this type of piston, and most of them knock.  I personally think that GM just needs to do some readjusting on their tolerences to take care of this.

[76BBSub]

The pistons in production automotive engines have been aluminum for a LONG time, cast pistons in modern engines are cast aluminum, not cast iron.  Even my '76 Suburban 454 has factory aluminum pistons.  Hypereutectic pistons are simply aluminum pistons with a slightly different material composition, making them stronger than cast pistons but cheaper than forged pistons.  Hypereutectic pistons are what GM started using in '96 on their Vortec V8s, and they have had problems with piston slap ever since.  It is due to the contraction of the piston when cold.  The alternatives are to use cast pistons and reduce the power output of the engine, or use forged pistons and increase the cost, which we all the GM will NOT do, being GM and all.  I have no idea why some engines with hypereutectic pistons have piston slap and others of the same type don't.  I expect it has something to do with the tolerance stack up, related to the bore of the engine, the piston OD, and the fit-up of the rings.  It is my understanding that mild piston slap is not harmful to the engine, but it is VERY bothersome when you have just spent ?-?K or more for a truck that rattles.

And, actually, aluminum transfers heat VERY efficiently, that's the main reason why engines with aluminum heads can run higher compression ratios.  The aluminum draws more heat out of the combustion chamber, meaning less octane is required to control pre-ignition and detonation (knock).

Jeff

[ChevSub]

Thanks everyone for your information.  I'm gonna start running Mobil 1 and a PF59 filter w/anti-drainback valve.  I've heard it helps.

My dealership has finally admitted to the piston epidemic.  I know not everyone has this problem, but it's good to speak with a few folks about it.  I've talked to lots of folks reference the piston slap.  There have been a few posts at www.pickuptrucks.com and now they have listed a number to call and voice your concerns.  I have documented my case with both Chevrolet and kept all paperwork since day 1 of the pick-up.  I have the local area rep. coming to listen on July 16.  Hope I can get a little insurance on an engine.  I have no doubts, I hope, that the engine will continue to run like a champ.  Maybe it will quiet up once I replace w/ Mobil 1 synthetic, 5w30.

Also, has anyone seen a cut-away of the new engines?  It makes my head dizzy compared to other GM power plants.  No wonder they don't recommend 10w30.  It's too thick.  It would never reach everything.  Keeping my fingers crossed.

P.S.  I can't wait to see what happens once I return at the end of my lease.  Maybe by then, the new iron pistons will be out and I can trade it in for a new truck.

[76BBSub]

There won't be any iron pistons, read my post right before your last post.

They may produce some aluminum hypereutectic pistons with a slightly different chemical composition, but they certainly won't be iron.

Jeff