GM 6.0L Cold Knock?

I'm looking at a truck that is pretty much exactly what I want, except it has a cold knock/tick that goes away within about 30 seconds (at an ambient temp of 85F). I think I've heard about this issue with this engine in the past, but can't remember what the cause is. Can you guys fill me in? If it matters, it's a '00 GMC Sierra 2500.

Sounds like a lifter bleeding down or possibly piston slap. Being a GM, it will probably do that for a long time before it becomes a problem. Sounds like a negotiation too to me. Get them to knock the price down.

I thought there was a known problem with these. I did some research and found two problems. One is carbon buildup on the pistons. This creates detonation and a knock that happens when cold and hot. The other is piston slap due to short piston skirts. This noise happens when cold and goes away when warm. Since that is what happens on this truck, I think that's the problem. GM claims it doesn't effect durability, although I can't see how excessive lateral piston movement relative to the piston wall isn't going to speed up wear. My concerns are that it is clearly on one bank only, and if it does it when it's this hot out, how bad will it be in the winter? I'd like to hear from guys that have this engine, and what their experiences are. Is it possible to get a 100k+ 6.0 that doesn't do this?

Ok. Since I've been abused for suggesting that lots of Neons have piston clatter when they are cold, I won't suggest that many of the LS motors do exactly the same thing. Its obviously a figment of your imagination.

Subarus don't ever have leaky head gaskets either.

When did I say it didn't have piston slap?

i;ve seen the LS truck motors with CSK lasting 300+k.... and they started the noise around 50k.

Mine does it. I'm not worried. It's done it the whole time I've owned the truck (1+ year). It's not getting worse. Mine currently has 150,000 kms on it.

doc_speeder wrote: Mine does it. I'm not worried. It's done it the whole time I've owned the truck (1+ year). It's not getting worse. Mine currently has 150,000 kms on it.

Is it worse in cold weather? It sounds like it's not something to worry about as far as durability, but I'm concerned that it will be much worse in cold weather than it is now.

it is a little louder when it's -10 outside, and it does take about 45 seconds to go away when it's that cold. Dad's 03 has had it since about 75k miles, and he's at 160k right now. It is a little louder when it is below freezing but not that much. Damn does it run good though!

Do you know if they all do it, or if not, an estimate of the percentage of them that do it? IOW, what are my chances of finding one that doesn't do it?

i'm guessing once you climb above 100k the chances of finding one without CSK is slim to none....especially on older trucks (99-03).

My 5.4 Ford has had cold start piston slap since about 60K. On a real cold day after sitting for a week or two it sounds like a tin can full of rocks. It shuts up after a mile or so. I wouldn't worry about it too much. Just don't dog it until it warms up good. Think of it as racing tolerances.

Thanks for the advice, guys. My current old truck has forged pistons and makes essentially the same noise for about the same amount of time, but I have to admit that even though I built the engine, that knock bugs the hell out of me. Anyway, the truck is an '00 and has 118k on it. Sounds like I shouldn't let it stop me from buying it, right? Should I expect it to get worse with age, or does it pretty much stay the same?

Sounds like classic LSx piston slap. The pistons expand a lot as they warm up so they fit loosely when cold. It's noisy but very common and doesn't really affect how it runs.

bravenrace wrote:

doc_speeder wrote: Mine does it. I'm not worried. It's done it the whole time I've owned the truck (1+ year). It's not getting worse. Mine currently has 150,000 kms on it.

Is it worse in cold weather? It sounds like it's not something to worry about as far as durability, but I'm concerned that it will be much worse in cold weather than it is now.

I don't notice much difference in the cold. Oh and when I say cold, I mean -35C cold...

The last thing I did at work today was an oil pan gasket on a 6 liter box truck.

The truck HAD a cold start knock/rattle/low oil pressure issue. It went away with the new oil pan gasket.

The oil feed goes through the oil pan since the filter threads to the oil pan and not the block. The O-rings (metal/rubber composite gasket) were brittle and broken. It'd hold oil pressure when hot and everything was expanded, but when it was cold, the oil would leak down out of the engine, most of it into the oil pan, the rest all over the ground.

Fortunately, at least in the van chassis, it's a very easy fix. Pulling the oil pan in, say, a Corvette, however...

(did that one too...)

It's just like the problem that '83-85 12A RX-7s have, except you don't have to disassemble the engine to repair it.

I think mine's done it since new. 01 2500 HD and I'm at 146 ish and I've owned is since new.

Knurled wrote: The last thing I did at work today was an oil pan gasket on a 6 liter box truck. The truck HAD a cold start knock/rattle/low oil pressure issue. It went away with the new oil pan gasket. The oil feed goes through the oil pan since the filter threads to the oil pan and not the block. The O-rings (metal/rubber composite gasket) were brittle and broken. It'd hold oil pressure when hot and everything was expanded, but when it was cold, the oil would leak down out of the engine, most of it into the oil pan, the rest all over the ground. Fortunately, at least in the van chassis, it's a very easy fix. Pulling the oil pan in, say, a Corvette, however... (did that one too...) It's just like the problem that '83-85 12A RX-7s have, except you don't have to disassemble the engine to repair it.

My understanding is that there are two cold start knock problems. The one you are talking about normally creates a knock all the time, cold or hot, although I can see how it could also be only when cold. It is accompanied by low oil pressure as you state. The other is a result of short skirt pistons combined with excessively wide piston to wall clearance. Apparently GM used to fit each piston to each cylinder, but to save cost at some point changed to not doing that. The result is sometimes there are pistons that are a little too loose, and knock until they come up to temp. This problem doesn't result in low oil pressure, and this truck doesn't have low oil pressure, so I'm thinking the problem it has is the latter of the two. It seems to me that over time this would cause increased piston wear and ultimately oil burning and/or a compression drop, but every source I've found indicates that it has virtually no detrimental effect on engine life.

GM started using really short piston skirts in about 94. The reasoning was less friction, less weight, and a hotter piston for emissions help. It does cause piston slap. My LT1 started slapping at 20k. It now has 127k and it just slaps for 1 minute instead of 30 seconds.