94 LT1 Has hollow sounding Knock when cold at idle and fades as engine warms up. Sounds like what you would expect a type of Vac. valve to sound like. Used stethescope all around engine and can't hear anything semilar. Does this sound fimiliar to anyone? Iv'e read that the two piece fly wheel can make a tapping sound, but this is not what I would discribe as a tap. Thanks for any info.

 

What oil do you run?

 

Could be piston slap. The pistons have short skirts on them and until everything expands, they move around a little in the cylinder. This is not uncommon, and usually nothing to worry about. So long as the knock goes away when warm.

 

wasn't piston slap an issue in the late 90's and forward?

 

My vote is for piston slap, too GM 4.3's are known to do this as well - as long as it goes away (usually after the vehicle has run for half a minute or so) it's nothing to be very concerned about. Be sure the vehicle is up to normal operating temp before you 'get on it'.

 

Well, yes more so with the LSx engines because of the aluminum block. They would expand at different rates. But the short skirts of the pistons also present on the LTx motors is the main contributor. You hear about it more from the LSx guys b/c you hear it more clearly on the LSx motors. My LS1 Camaro had it. My LT4 has it but it doesn't last as long and is barely noticeable. In fact, if I didn't know what it sounded like I wouldn't know it was there.

 

From what I had read some time ago, GM was unintentially overboring one cylinder which accounted for the piston slap. It wasn't every cylinder in every LS1. Though I had it in my '99 LS1, I lost the first buyer over it too. I had not ever heard much about it being a genuine issue with LTx engines.

 

I've experienced this with other Chevy's I've had. When it's a cold start I heard the same sound as you desribe then it goes away once the engine warms up. In my experience I know this sound and it's a bearing inside the motor.

Engle1147 hit the nail on the head with asking what oil is in it. It's maybe time for an oil change and a better oil. I've used a product called Lucus oil additive in a few cars I had and our everyday cars we have now. Try that Lucus stuff along with an oil change and filter. I'll bet the mild knocking sound will go away.

 

It has 90K on it. I use mobil 1 10X30. Has less than 500 miles.

 

I use mobil 1 10X30 oil has less than 500 miles on it

 

I run the same stuff.
If the noise is recent with the oil change I'd swap the filter out and maybe the oil too.....try the simple stuff before you go tearing into stuff.

Side note: My vette drinks/burns/evaporates almost an entire quart of every 2,000 miles....she's a high mileage kinda gal. Knowing this I went to Wal-E-Mart one day and picked up 1 quart of Mobile 10W30. I went to pour it in the car the next day and noticed that the locking safety tabs didn't really break when I opened the lid....so I smelled the oil...and it wasn't oil that was in their it was brake fluid instead. I almost poured that crap in my engine Returned it to the store and told the "returns person" what was up. The response I got was that people do that "people return stuff all the time after they use what was in the container they refill it with something else that is less expensive and return it for a full refund of the expensive stuff.....so basically the next customer gets screwed. I have no idea where you bought your oil from but times are tough and people do strange things to save a buck. From then on, I've started checking the lids on the motor oil to ensure that the cap seals are intact or untampered with.

 

Not sure if it's the same problem but my Silverado does this when cold.

My mechanic said most likely a lifter "had lost its prime" which I think basically means it had depressurized somehow and so it was kind of "flat" and not really pumping up properly.

So that we didn't have to tear into he tried some kind of snake oil that is supposed to help the lifter "re-prime" and actually it fixed the problem believe it or not. But it seems to return eventually so I'm just living with it since truck has 187K!

 

I was suspecking the Seagraves car had around 100,000 miles. My Tahoe does the same exact thing. It has that hollow sounding knocking sound when it's cold, it has 119,000 miles on it. I've had 1 or 2 cars do this same thing in the past. It's the bearings a little worn in the engine, more than likely a rod to crank bearing. In my Tahoe once the cold weather went away the knocking stopped. What happens is the oil is so cold it's not circulating all that well, feeding the bearings. Once the engine warms up a bit so does the oil and the knock goes away. On the other 2 cars I had I changed the oil and used the Lucus. In the cold I never heard the knock again. The lucus clings and stays on these parts, thus the bearings have some lube on them.

A colapsted lifter, that's an entirely different sound, it's a valve tap and a loud one. Years ago my new 1979 Blazer did this when it was around 2 years old and had around 25,000 miles on it. I took the valve cover off and with the engine running at idle I could not adjust the nut inside the rocker arm to get rid of this extremely loud valve tap. Before this happened I read a trick in my, How To Hot Rod Big Block Chevy's. The trick is, take the valve cover off, start the engine and let it idle, spray Gumout down the outsides of the push rod. The Gumout frees up the plunger inside the lifter. This may free up the lifters plunger and the sound will go away. So, I tried the Gumout trick, held my breath and honest to God it worked. I adjusted the rocker arms nut for the proper valve lash and it's been fine ever since. My Blazer now has 140,000 miles on it and this lifter has behaved itself ever since that day long ago. The 350 in my Blazer has never been opened up for anything in all this time, except taking that valve cover off.

Oh, I did have both covers off long ago as the valve seals went bad and it smoked oil pretty bad. I put what's called skirts over the valve springs to prevent oil going down the valve stem and burning in the cylinders. No more smoke out of it's tail pipe. These skirts were a very good idea, but they haven't been sold in probaly 20 years. This product just disappeared. My Blazer also has Lucus in the oil. It's engine runs just the same as when it was brand new.