Engine Knock?

[MattDepault]

I've got a 2000 Sierra 1500 with the 4.8 in it, and its been knocking for a while. It has 130,xxx miles on it. I change the oil regularly with valvoline high mileage oil, and it comes out realtively clean and without any metal shavings as far as i can tell. But the first time i start it every day, it makes a fairly loud low-end knocking noise. I'm not an expert, but to me it sounds like a bad rod bearing. Oil pressure gauge is within normal range and it doesnt get much better when warmed up. Any ideas? I'm bringing it to the shop next week to have them look at it, but I'd like to have some idea of what it could be. Hopefully I dont need anything rebuild and its just noisy lifters or something easy and inexpensive. has anyone else had this problem?

 

[ifixedit]

The cold start knock on these engines when they get a few miles on them has been complained about and written about on the internet endlessly. What you are likely actually hearing is what some refer to as piston slap. There's two schools of thought (or a combination of both) that causes this noise. The first one is the pistons with some miles (and wear) on them are now rocking in their bores just a little on cold start-up and it just takes a few seconds for the oil to flow and the pistons to warm up and expand and then it goes away. I've also read this condition is exaggerated by a combination of carbon build up on the top of the pistons and the initial timing and spark curve the computer delivers when the engine is cold. The more miles the engine has, the more you hear it until it begins to warm up, and once it's run maybe 5 minutes depending on outside temps, you don't hear it at all. In either case you are likely to be told it's "normal."

 

I've got a 98 Yukon with the 5.7 and 203,000 rounds on the clock and it does exactly the same thing, first start and I get a fairly loud KNOCK for a few seconds, then gets quieter and is completely gone after about 30 seconds of idling, but if I drive it immediately after this it will continue to knock under acceleration until it has warmed up some. You just need to let it idle and warm up a bit longer than when it was newer before you get on it. I let the old Yukon idle 5 minutes before I drive it in warm temps and at least 10 minutes or so in cold temps and all is fine.

 

Noisy lifters will be more of tick, not a knock, but in any case a good engine flush if you've never done one could be in order and may quiet down the cold start noises some. In my case the engine flush most definitely helped with the noisy lifter ticks on cold start-up, but won't cure the piston knock, that unfortunately is there to stay.

 

Unless the thing is going through oil like it shouldn't and is puffing blue smoke all the time, I'd save the trip (and the $$) to the mechanic, keep doing your routine oil changes like you've been doing and just let it warm up for a few before you take off. It's just getting broke in @ 130,000!

[The Zip]

MattDepault - I would agree with ifixedit's assessment, to include the comment " I'd save the trip (and the $$) to the mechanic and just let it warm up for a few before you take off ". Also, to minimize the warm-up time, I would recommend using a lighter weight (5W) oil so that if flows more easily.

[Colossus]

cold start knock and piston slap is VERY common on the GMT-800s. 10 years ago on here it was quite the topic.

[MattDepault]

I've thought it could be that, but I already use 5W-30 and even when it isn't terribly cold out, after 20 minutes of idling to warm it up, it stays the same. It doesn't get that much quieter, which I feel like it should if it was just because the engine is cold. even when in the regular operating temp range, there is still a noticeable knock from the lower end of the engine. Would it stilll do that if it's just a piston slap? and I have flushed the engine, and it didn't help much.

[servantsalesmen]

I've thought it could be that, but I already use 5W-30 and even when it isn't terribly cold out, after 20 minutes of idling to warm it up, it stays the same. It doesn't get that much quieter, which I feel like it should if it was just because the engine is cold. even when in the regular operating temp range, there is still a noticeable knock from the lower end of the engine. Would it stilll do that if it's just a piston slap? and I have flushed the engine, and it didn't help much.

My money says it's a cracked flexplate. I'd crawl under it and listen right around the bellhousing of the tranny. If its coming right off the tranny, that's a flexplate. My truck has cracked 2 in its lifetime. 6.0 liter on a 4l80e trans in my case but it wouldn't surprise me if it happened to you as well.

[ifixedit]

Yeah, the cold start knock should only last maybe a few minutes at most, if it's still knocking after 20 minutes then it's something else. Unless you've really ran the hell out of it (like you've been drag racing and doing neutral tranny drops ) I'd be hard pressed to say it's a rod or main bearing knock and I'd probably be getting it in to take a look at that flex plate.

[MattDepault]

I checked the flex plate as well, and it isn't cracked and the bolts look tight. I'm probably going to bring it in because at this point I'm not sure what else it could be. I know a mechanic that doesn't charge me much so hopefully they can take a look.

[ifixedit]

Unless you are able to diagnose it yourself, at this point getting it in for a look and another opinion might not be a bad idea. Unless it's just the piston slap mentioned here or a rod or main bearing knock (but then it should be smoking like a steam ship), if the oil pressure is good and holds steady at 30-40 psi when idling, that would indicate the bearings in the engine are fine. A questionable rod or main bearing will (or at least should) cause a knock and will usually result in the oil pressure starting out good and dropping to an unacceptable level at idle (20 PSI or less) as the engine warms up. If one of the bearings is really out to lunch it would cause a knock and result in very low oil pressure right off the get go, but in either case you aren't reporting that. Since you report good oil pressure if the engine is cold or warm, the knock has to be something concerning the transmission as it's the next in line from the engine.

 

The only other rather slim to none possibility I can think of is one of the pistons has cracked and has lost it's skirt, that can happen if you get on a cold engine, especially if you did it when it was really cold. A piston with the skirt missing should certainly cause a condition where the piston continues to rock in it's bore causing the knock that would never go away no matter how warmed up the engine is. Let's hope it's not that.

[00Silverado4x4]

Another check would be for exhaust leaks. That would make your engine pretty loud running as well. These engine also like to snap off exhaust manifold bolt heads, but I'd check the entire system, manifold to tail pipe.

[Desi]

Mine had what I thought was a cracked flex plate. Turned out to be catalytic converter guts were loose. Sounded like a low end engine knock.

[the blur]

My 2011 Escalade does that with 40,000 miles. Sounds like a bearing knock. Probably piston slap.

[ifixedit]

The thing I guess I'm now wondering is if it's really a knock or is it just more of a tick and how loud it is? My 6.0 has been kind of a clickity clacky engine since day one at idle and even if I sit in the driveway in park and bring the engine up to say 2000 RPM things are clicking away under the hood, it's just the way they are and maybe some are put together a bit more loose than others depending on if the engine was built on a Monday or a Friday. The clicks are not loud but I can hear them. The engine has never been what I'd consider even remotely abused, but has seen the red-line a few times and any engine should be able to withstand that for brief periods. Much of the noise I hear is coming from what I suspect are the injectors and the valve train. Unless it's just normal noises for these engines and the flex plate / any kind of tranny issues or any other contributing factor has been ruled out and it really is more of a knock (and it should be a fairly loud knock), I'd be back to questioning if it's possible that one of the pistons has perhaps lost a skirt. These engines do use the hypereutectic pistons which are known for being a bit on the brittle side compared to cast or forged and are more prone to fail under repeated exposure to detonation. At this point it's worth a look to verify I'd say.